From b261537485c11cd4a8f57651648ed702e6498087 Mon Sep 17 00:00:00 2001
From: Foxtrek_64 <foxtrek64@gmail.com>
Date: Sun, 28 Feb 2021 14:43:30 -0600
Subject: [PATCH 01/14] Fix #20064

---
 .../src/System/Linq/CachedReflection.cs       | 42 ++++++++++
 .../src/System/Linq/Queryable.cs              | 82 +++++++++++++++++++
 .../System.Linq/src/System/Linq/First.cs      | 24 ++++--
 .../System.Linq/src/System/Linq/Last.cs       | 20 +++--
 .../System.Linq/src/System/Linq/Single.cs     | 12 ++-
 5 files changed, 165 insertions(+), 15 deletions(-)

diff --git a/src/libraries/System.Linq.Queryable/src/System/Linq/CachedReflection.cs b/src/libraries/System.Linq.Queryable/src/System/Linq/CachedReflection.cs
index f0eb0f169bf3c0..2d37fe2f53016a 100644
--- a/src/libraries/System.Linq.Queryable/src/System/Linq/CachedReflection.cs
+++ b/src/libraries/System.Linq.Queryable/src/System/Linq/CachedReflection.cs
@@ -284,6 +284,20 @@ public static MethodInfo FirstOrDefault_TSource_2(Type TSource) =>
              (s_FirstOrDefault_TSource_2 ??= new Func<IQueryable<object>, Expression<Func<object, bool>>, object?>(Queryable.FirstOrDefault).GetMethodInfo().GetGenericMethodDefinition())
               .MakeGenericMethod(TSource);
 
+        private static MethodInfo? s_FirstOrDefault_TSource_3;
+
+        public static MethodInfo FirstOrDefault_TSource_3(Type TSource) =>
+            (s_FirstOrDefault_TSource_3 ??
+            (s_FirstOrDefault_TSource_3 = new Func<IQueryable<object>, object?, object?>(Queryable.FirstOrDefault).GetMethodInfo().GetGenericMethodDefinition()))
+            .MakeGenericMethod();
+
+        private static MethodInfo? s_FirstOrDefault_TSource_4;
+
+        public static MethodInfo FirstOrDefault_TSource_4(Type TSource) =>
+            (s_FirstOrDefault_TSource_4 ??
+             (s_FirstOrDefault_TSource_4 = new Func<IQueryable<object>, Expression<Func<object, bool>>, object?, object?>(Queryable.FirstOrDefault).GetMethodInfo().GetGenericMethodDefinition()))
+            .MakeGenericMethod(TSource);
+
         private static MethodInfo? s_GroupBy_TSource_TKey_2;
 
         public static MethodInfo GroupBy_TSource_TKey_2(Type TSource, Type TKey) =>
@@ -392,6 +406,20 @@ public static MethodInfo LastOrDefault_TSource_2(Type TSource) =>
              (s_LastOrDefault_TSource_2 ??= new Func<IQueryable<object>, Expression<Func<object, bool>>, object?>(Queryable.LastOrDefault).GetMethodInfo().GetGenericMethodDefinition())
               .MakeGenericMethod(TSource);
 
+        private static MethodInfo? s_LastOrDefault_TSource_3;
+
+        public static MethodInfo LastOrDefault_TSource_3(Type TSource) =>
+            (s_LastOrDefault_TSource_3 ??
+             (s_LastOrDefault_TSource_3 = new Func<IQueryable<object>, object?, object?>(Queryable.LastOrDefault).GetMethodInfo().GetGenericMethodDefinition()))
+            .MakeGenericMethod(TSource);
+
+        private static MethodInfo? s_LastOrDefault_TSource_4;
+
+        public static MethodInfo LastOrDefault_TSource_4(Type TSource) =>
+            (s_LastOrDefault_TSource_4 ??
+             (s_LastOrDefault_TSource_4 = new Func<IQueryable<object>, Expression<Func<object, bool>>, object?, object?>(Queryable.LastOrDefault).GetMethodInfo().GetGenericMethodDefinition()))
+            .MakeGenericMethod(TSource);
+
         private static MethodInfo? s_LongCount_TSource_1;
 
         public static MethodInfo LongCount_TSource_1(Type TSource) =>
@@ -536,6 +564,20 @@ public static MethodInfo SingleOrDefault_TSource_2(Type TSource) =>
              (s_SingleOrDefault_TSource_2 ??= new Func<IQueryable<object>, Expression<Func<object, bool>>, object?>(Queryable.SingleOrDefault).GetMethodInfo().GetGenericMethodDefinition())
               .MakeGenericMethod(TSource);
 
+        private static MethodInfo? s_SingleOrDefault_TSource_3;
+
+        public static MethodInfo SingleOrDefault_TSource_3(Type TSource) =>
+            (s_SingleOrDefault_TSource_3 ??
+            (s_SingleOrDefault_TSource_3 = new Func<IQueryable<object>, object?, object?>(Queryable.SingleOrDefault).GetMethodInfo().GetGenericMethodDefinition()))
+             .MakeGenericMethod(TSource);
+
+        private static MethodInfo? s_SingleOrDefault_TSource_4;
+
+        public static MethodInfo SingleOrDefault_TSource_4(Type TSource) =>
+            (s_SingleOrDefault_TSource_4 ??
+            (s_SingleOrDefault_TSource_4 = new Func<IQueryable<object>, Expression<Func<object, bool>>, object?, object?>(Queryable.SingleOrDefault).GetMethodInfo().GetGenericMethodDefinition()))
+             .MakeGenericMethod();
+
         private static MethodInfo? s_Skip_TSource_2;
 
         public static MethodInfo Skip_TSource_2(Type TSource) =>
diff --git a/src/libraries/System.Linq.Queryable/src/System/Linq/Queryable.cs b/src/libraries/System.Linq.Queryable/src/System/Linq/Queryable.cs
index c08bcab79af79c..065218f1ba12ce 100644
--- a/src/libraries/System.Linq.Queryable/src/System/Linq/Queryable.cs
+++ b/src/libraries/System.Linq.Queryable/src/System/Linq/Queryable.cs
@@ -864,6 +864,18 @@ public static TSource First<TSource>(this IQueryable<TSource> source, Expression
                     CachedReflectionInfo.FirstOrDefault_TSource_1(typeof(TSource)), source.Expression));
         }
 
+        [DynamicDependency("FirstOrDefault`1", typeof(Enumerable))]
+        public static TSource? FirstOrDefault<TSource>(this IQueryable<TSource> source, TSource? defaultValue)
+        {
+            if (source == null)
+                throw Error.ArgumentNull(nameof(source));
+            return source.Provider.Execute<TSource>(
+                Expression.Call(
+                    null,
+                    CachedReflectionInfo.FirstOrDefault_TSource_3(typeof(TSource)),
+                    source.Expression, Expression.Constant(defaultValue, typeof(TSource))));
+        }
+
         [DynamicDependency("FirstOrDefault`1", typeof(Enumerable))]
         public static TSource? FirstOrDefault<TSource>(this IQueryable<TSource> source, Expression<Func<TSource, bool>> predicate)
         {
@@ -879,6 +891,21 @@ public static TSource First<TSource>(this IQueryable<TSource> source, Expression
                     ));
         }
 
+        [DynamicDependency("FirstOrDefault`1", typeof(Enumerable))]
+        public static TSource? FirstOrDefault<TSource>(this IQueryable<TSource> source, Expression<Func<TSource, bool>> predicate, TSource? defaultValue)
+        {
+            if (source == null)
+                throw Error.ArgumentNull(nameof(source));
+            if (predicate == null)
+                throw Error.ArgumentNull(nameof(predicate));
+            return source.Provider.Execute<TSource>(
+                Expression.Call(
+                    null,
+                    CachedReflectionInfo.FirstOrDefault_TSource_4(typeof(TSource)),
+                    source.Expression, Expression.Quote(predicate), Expression.Constant(defaultValue, typeof(TSource))
+                ));
+        }
+
         [DynamicDependency("Last`1", typeof(Enumerable))]
         public static TSource Last<TSource>(this IQueryable<TSource> source)
         {
@@ -916,6 +943,18 @@ public static TSource Last<TSource>(this IQueryable<TSource> source, Expression<
                     CachedReflectionInfo.LastOrDefault_TSource_1(typeof(TSource)), source.Expression));
         }
 
+        [DynamicDependency("LastOrDefault`1", typeof(Enumerable))]
+        public static TSource? LastOrDefault<TSource>(this IQueryable<TSource> source, TSource? defaultValue)
+        {
+            if (source == null)
+                throw Error.ArgumentNull(nameof(source));
+            return source.Provider.Execute<TSource>(
+                Expression.Call(
+                    null,
+                    CachedReflectionInfo.LastOrDefault_TSource_3(typeof(TSource)),
+                    source.Expression, Expression.Constant(defaultValue, typeof(TSource))));
+        }
+
         [DynamicDependency("LastOrDefault`1", typeof(Enumerable))]
         public static TSource? LastOrDefault<TSource>(this IQueryable<TSource> source, Expression<Func<TSource, bool>> predicate)
         {
@@ -931,6 +970,21 @@ public static TSource Last<TSource>(this IQueryable<TSource> source, Expression<
                     ));
         }
 
+        [DynamicDependency("LastOrDefault`1", typeof(Enumerable))]
+        public static TSource? LastOrDefault<TSource>(this IQueryable<TSource> source, Expression<Func<TSource, bool>> predicate, TSource? defaultValue)
+        {
+            if (source == null)
+                throw Error.ArgumentNull(nameof(source));
+            if (predicate == null)
+                throw Error.ArgumentNull(nameof(predicate));
+            return source.Provider.Execute<TSource>(
+                Expression.Call(
+                    null,
+                    CachedReflectionInfo.LastOrDefault_TSource_4(typeof(TSource)),
+                    source.Expression, Expression.Quote(predicate), Expression.Constant(defaultValue, typeof(TSource))
+                ));
+        }
+
         [DynamicDependency("Single`1", typeof(Enumerable))]
         public static TSource Single<TSource>(this IQueryable<TSource> source)
         {
@@ -968,6 +1022,19 @@ public static TSource Single<TSource>(this IQueryable<TSource> source, Expressio
                     CachedReflectionInfo.SingleOrDefault_TSource_1(typeof(TSource)), source.Expression));
         }
 
+        [DynamicDependency("SingleOrDefault`1", typeof(Enumerable))]
+        public static TSource? SingleOrDefault<TSource>(this IQueryable<TSource> source, TSource? defaultValue)
+        {
+            if (source == null)
+                throw Error.ArgumentNull(nameof(source));
+            return source.Provider.Execute<TSource>(
+                Expression.Call(
+                    null,
+                    CachedReflectionInfo.SingleOrDefault_TSource_3(typeof(TSource)),
+                    source.Expression, Expression.Constant(defaultValue, typeof(TSource))));
+
+        }
+
         [DynamicDependency("SingleOrDefault`1", typeof(Enumerable))]
         public static TSource? SingleOrDefault<TSource>(this IQueryable<TSource> source, Expression<Func<TSource, bool>> predicate)
         {
@@ -983,6 +1050,21 @@ public static TSource Single<TSource>(this IQueryable<TSource> source, Expressio
                     ));
         }
 
+        [DynamicDependency("SingleOrDefault`1", typeof(Enumerable))]
+        public static TSource? SingleOrDefault<TSource>(this IQueryable<TSource> source, Expression<Func<TSource, bool>> predicate, TSource? defaultValue)
+        {
+            if (source == null)
+                throw Error.ArgumentNull(nameof(source));
+            if (predicate == null)
+                throw Error.ArgumentNull(nameof(predicate));
+            return source.Provider.Execute<TSource>(
+                Expression.Call(
+                    null,
+                    CachedReflectionInfo.SingleOrDefault_TSource_4(typeof(TSource)),
+                    source.Expression, Expression.Quote(predicate), Expression.Constant(defaultValue, typeof(TSource))
+                ));
+        }
+
         [DynamicDependency("ElementAt`1", typeof(Enumerable))]
         public static TSource ElementAt<TSource>(this IQueryable<TSource> source, int index)
         {
diff --git a/src/libraries/System.Linq/src/System/Linq/First.cs b/src/libraries/System.Linq/src/System/Linq/First.cs
index 64b89915c259f8..6751a14e9f1550 100644
--- a/src/libraries/System.Linq/src/System/Linq/First.cs
+++ b/src/libraries/System.Linq/src/System/Linq/First.cs
@@ -31,12 +31,21 @@ public static TSource First<TSource>(this IEnumerable<TSource> source, Func<TSou
         }
 
         public static TSource? FirstOrDefault<TSource>(this IEnumerable<TSource> source) =>
-            source.TryGetFirst(out bool _);
+            source.TryGetFirst(out _);
+
+        public static TSource? FirstOrDefault<TSource>(this IEnumerable<TSource> source, TSource? defaultValue) =>
+            source.TryGetFirst(defaultValue, out _);
 
         public static TSource? FirstOrDefault<TSource>(this IEnumerable<TSource> source, Func<TSource, bool> predicate) =>
-            source.TryGetFirst(predicate, out bool _);
+            source.TryGetFirst(predicate, out _);
+
+        public static TSource? FirstOrDefault<TSource>(this IEnumerable<TSource> source, Func<TSource, bool> predicate, TSource? defaultValue) =>
+            source.TryGetFirst(predicate, defaultValue, out _);
 
-        private static TSource? TryGetFirst<TSource>(this IEnumerable<TSource> source, out bool found)
+        private static TSource? TryGetFirst<TSource>(this IEnumerable<TSource> source, out bool found) =>
+            source.TryGetFirst(default(TSource), out found);
+
+        private static TSource? TryGetFirst<TSource>(this IEnumerable<TSource> source, TSource? defaultValue, out bool found)
         {
             if (source == null)
             {
@@ -69,10 +78,13 @@ public static TSource First<TSource>(this IEnumerable<TSource> source, Func<TSou
             }
 
             found = false;
-            return default;
+            return defaultValue;
         }
 
-        private static TSource? TryGetFirst<TSource>(this IEnumerable<TSource> source, Func<TSource, bool> predicate, out bool found)
+        private static TSource? TryGetFirst<TSource>(this IEnumerable<TSource> source, Func<TSource, bool> predicate, out bool found) =>
+            source.TryGetFirst(predicate, default(TSource), out found);
+
+        private static TSource? TryGetFirst<TSource>(this IEnumerable<TSource> source, Func<TSource, bool> predicate, TSource? defaultValue, out bool found)
         {
             if (source == null)
             {
@@ -94,7 +106,7 @@ public static TSource First<TSource>(this IEnumerable<TSource> source, Func<TSou
             }
 
             found = false;
-            return default;
+            return defaultValue;
         }
     }
 }
diff --git a/src/libraries/System.Linq/src/System/Linq/Last.cs b/src/libraries/System.Linq/src/System/Linq/Last.cs
index f7cd74eb694bd4..9d0b67e0a131a4 100644
--- a/src/libraries/System.Linq/src/System/Linq/Last.cs
+++ b/src/libraries/System.Linq/src/System/Linq/Last.cs
@@ -30,13 +30,19 @@ public static TSource Last<TSource>(this IEnumerable<TSource> source, Func<TSour
             return last!;
         }
 
-        public static TSource? LastOrDefault<TSource>(this IEnumerable<TSource> source) =>
-            source.TryGetLast(out bool _);
+        public static TSource? LastOrDefault<TSource>(this IEnumerable<TSource> source)
+            => source.TryGetLast(out _);
+        public static TSource? LastOrDefault<TSource>(this IEnumerable<TSource> source, TSource? defaultValue)
+            => source.TryGetLast(defaultValue, out _);
 
-        public static TSource? LastOrDefault<TSource>(this IEnumerable<TSource> source, Func<TSource, bool> predicate) =>
-            source.TryGetLast(predicate, out bool _);
+        public static TSource? LastOrDefault<TSource>(this IEnumerable<TSource> source, Func<TSource, bool> predicate)
+            => source.TryGetLast(predicate, out bool _);
+        public static TSource? LastOrDefault<TSource>(this IEnumerable<TSource> source, Func<TSource, bool> predicate, TSource? defaultValue)
+            => source.TryGetLast(predicate, defaultValue, out bool _);
 
         private static TSource? TryGetLast<TSource>(this IEnumerable<TSource> source, out bool found)
+            => source.TryGetLast(default(TSource?), out found);
+        private static TSource? TryGetLast<TSource>(this IEnumerable<TSource> source, TSource? defaultValue, out bool found)
         {
             if (source == null)
             {
@@ -77,10 +83,12 @@ public static TSource Last<TSource>(this IEnumerable<TSource> source, Func<TSour
             }
 
             found = false;
-            return default;
+            return defaultValue;
         }
 
         private static TSource? TryGetLast<TSource>(this IEnumerable<TSource> source, Func<TSource, bool> predicate, out bool found)
+            => source.TryGetLast(predicate, default(TSource?), out found);
+        private static TSource? TryGetLast<TSource>(this IEnumerable<TSource> source, Func<TSource, bool> predicate, TSource? defaultValue, out bool found)
         {
             if (source == null)
             {
@@ -135,7 +143,7 @@ public static TSource Last<TSource>(this IEnumerable<TSource> source, Func<TSour
             }
 
             found = false;
-            return default;
+            return defaultValue;
         }
     }
 }
diff --git a/src/libraries/System.Linq/src/System/Linq/Single.cs b/src/libraries/System.Linq/src/System/Linq/Single.cs
index 913405d502c2f7..396c1f1aeaee3a 100644
--- a/src/libraries/System.Linq/src/System/Linq/Single.cs
+++ b/src/libraries/System.Linq/src/System/Linq/Single.cs
@@ -84,6 +84,9 @@ public static TSource Single<TSource>(this IEnumerable<TSource> source, Func<TSo
         }
 
         public static TSource? SingleOrDefault<TSource>(this IEnumerable<TSource> source)
+            => source.SingleOrDefault(default(TSource));
+
+        public static TSource? SingleOrDefault<TSource>(this IEnumerable<TSource> source, TSource? defaultValue)
         {
             if (source == null)
             {
@@ -95,7 +98,7 @@ public static TSource Single<TSource>(this IEnumerable<TSource> source, Func<TSo
                 switch (list.Count)
                 {
                     case 0:
-                        return default;
+                        return defaultValue;
                     case 1:
                         return list[0];
                 }
@@ -106,7 +109,7 @@ public static TSource Single<TSource>(this IEnumerable<TSource> source, Func<TSo
                 {
                     if (!e.MoveNext())
                     {
-                        return default;
+                        return defaultValue;
                     }
 
                     TSource result = e.Current;
@@ -122,6 +125,9 @@ public static TSource Single<TSource>(this IEnumerable<TSource> source, Func<TSo
         }
 
         public static TSource? SingleOrDefault<TSource>(this IEnumerable<TSource> source, Func<TSource, bool> predicate)
+            => source.SingleOrDefault(predicate, default);
+
+        public static TSource? SingleOrDefault<TSource>(this IEnumerable<TSource> source, Func<TSource, bool> predicate, TSource? defaultValue)
         {
             if (source == null)
             {
@@ -153,7 +159,7 @@ public static TSource Single<TSource>(this IEnumerable<TSource> source, Func<TSo
                 }
             }
 
-            return default;
+            return defaultValue;
         }
     }
 }

From 41b8091b173b9928256b1243a9435d93c77f634f Mon Sep 17 00:00:00 2001
From: Foxtrek_64 <foxtrek64@gmail.com>
Date: Sun, 28 Feb 2021 16:45:20 -0600
Subject: [PATCH 02/14] Add API to ref assembly

---
 .../System.Linq.Queryable/ref/System.Linq.Queryable.cs      | 6 ++++++
 src/libraries/System.Linq/ref/System.Linq.cs                | 6 ++++++
 2 files changed, 12 insertions(+)

diff --git a/src/libraries/System.Linq.Queryable/ref/System.Linq.Queryable.cs b/src/libraries/System.Linq.Queryable/ref/System.Linq.Queryable.cs
index 692f4bc23ac5df..3635765940eeee 100644
--- a/src/libraries/System.Linq.Queryable/ref/System.Linq.Queryable.cs
+++ b/src/libraries/System.Linq.Queryable/ref/System.Linq.Queryable.cs
@@ -86,7 +86,9 @@ public static partial class Queryable
         public static System.Linq.IQueryable<TSource> Except<TSource>(this System.Linq.IQueryable<TSource> source1, System.Collections.Generic.IEnumerable<TSource> source2) { throw null; }
         public static System.Linq.IQueryable<TSource> Except<TSource>(this System.Linq.IQueryable<TSource> source1, System.Collections.Generic.IEnumerable<TSource> source2, System.Collections.Generic.IEqualityComparer<TSource>? comparer) { throw null; }
         public static TSource? FirstOrDefault<TSource>(this System.Linq.IQueryable<TSource> source) { throw null; }
+        public static TSource? FirstOrDefault<TSource>(this System.Linq.IQueryable<TSource> source, TSource? defaultValue) { throw null; }
         public static TSource? FirstOrDefault<TSource>(this System.Linq.IQueryable<TSource> source, System.Linq.Expressions.Expression<System.Func<TSource, bool>> predicate) { throw null; }
+        public static TSource? FirstOrDefault<TSource>(this System.Linq.IQueryable<TSource> source, System.Linq.Expressions.Expression<System.Func<TSource, bool>> predicate, TSource? defaultValue) { throw null; }
         public static TSource First<TSource>(this System.Linq.IQueryable<TSource> source) { throw null; }
         public static TSource First<TSource>(this System.Linq.IQueryable<TSource> source, System.Linq.Expressions.Expression<System.Func<TSource, bool>> predicate) { throw null; }
         public static System.Linq.IQueryable<System.Linq.IGrouping<TKey, TSource>> GroupBy<TSource, TKey>(this System.Linq.IQueryable<TSource> source, System.Linq.Expressions.Expression<System.Func<TSource, TKey>> keySelector) { throw null; }
@@ -104,7 +106,9 @@ public static partial class Queryable
         public static System.Linq.IQueryable<TResult> Join<TOuter, TInner, TKey, TResult>(this System.Linq.IQueryable<TOuter> outer, System.Collections.Generic.IEnumerable<TInner> inner, System.Linq.Expressions.Expression<System.Func<TOuter, TKey>> outerKeySelector, System.Linq.Expressions.Expression<System.Func<TInner, TKey>> innerKeySelector, System.Linq.Expressions.Expression<System.Func<TOuter, TInner, TResult>> resultSelector) { throw null; }
         public static System.Linq.IQueryable<TResult> Join<TOuter, TInner, TKey, TResult>(this System.Linq.IQueryable<TOuter> outer, System.Collections.Generic.IEnumerable<TInner> inner, System.Linq.Expressions.Expression<System.Func<TOuter, TKey>> outerKeySelector, System.Linq.Expressions.Expression<System.Func<TInner, TKey>> innerKeySelector, System.Linq.Expressions.Expression<System.Func<TOuter, TInner, TResult>> resultSelector, System.Collections.Generic.IEqualityComparer<TKey>? comparer) { throw null; }
         public static TSource? LastOrDefault<TSource>(this System.Linq.IQueryable<TSource> source) { throw null; }
+        public static TSource? LastOrDefault<TSource>(this System.Linq.IQueryable<TSource> source, TSource? defaultValue) { throw null; }
         public static TSource? LastOrDefault<TSource>(this System.Linq.IQueryable<TSource> source, System.Linq.Expressions.Expression<System.Func<TSource, bool>> predicate) { throw null; }
+        public static TSource? LastOrDefault<TSource>(this System.Linq.IQueryable<TSource> source, System.Linq.Expressions.Expression<System.Func<TSource, bool>> predicate, TSource? defaultValue) { throw null; }
         public static TSource Last<TSource>(this System.Linq.IQueryable<TSource> source) { throw null; }
         public static TSource Last<TSource>(this System.Linq.IQueryable<TSource> source, System.Linq.Expressions.Expression<System.Func<TSource, bool>> predicate) { throw null; }
         public static long LongCount<TSource>(this System.Linq.IQueryable<TSource> source) { throw null; }
@@ -129,7 +133,9 @@ public static partial class Queryable
         public static bool SequenceEqual<TSource>(this System.Linq.IQueryable<TSource> source1, System.Collections.Generic.IEnumerable<TSource> source2) { throw null; }
         public static bool SequenceEqual<TSource>(this System.Linq.IQueryable<TSource> source1, System.Collections.Generic.IEnumerable<TSource> source2, System.Collections.Generic.IEqualityComparer<TSource>? comparer) { throw null; }
         public static TSource? SingleOrDefault<TSource>(this System.Linq.IQueryable<TSource> source) { throw null; }
+        public static TSource? SingleOrDefault<TSource>(this System.Linq.IQueryable<TSource> source, TSource? defaultValue) { throw null; }
         public static TSource? SingleOrDefault<TSource>(this System.Linq.IQueryable<TSource> source, System.Linq.Expressions.Expression<System.Func<TSource, bool>> predicate) { throw null; }
+        public static TSource? SingleOrDefault<TSource>(this System.Linq.IQueryable<TSource> source, System.Linq.Expressions.Expression<System.Func<TSource, bool>> predicate, TSource? defaultValue) { throw null; }
         public static TSource Single<TSource>(this System.Linq.IQueryable<TSource> source) { throw null; }
         public static TSource Single<TSource>(this System.Linq.IQueryable<TSource> source, System.Linq.Expressions.Expression<System.Func<TSource, bool>> predicate) { throw null; }
         public static System.Linq.IQueryable<TSource> SkipLast<TSource>(this System.Linq.IQueryable<TSource> source, int count) { throw null; }
diff --git a/src/libraries/System.Linq/ref/System.Linq.cs b/src/libraries/System.Linq/ref/System.Linq.cs
index 7a86417711aa74..716fb0ba8ed9e1 100644
--- a/src/libraries/System.Linq/ref/System.Linq.cs
+++ b/src/libraries/System.Linq/ref/System.Linq.cs
@@ -59,7 +59,9 @@ public static System.Collections.Generic.IEnumerable<
         public static System.Collections.Generic.IEnumerable<TSource> Except<TSource>(this System.Collections.Generic.IEnumerable<TSource> first, System.Collections.Generic.IEnumerable<TSource> second) { throw null; }
         public static System.Collections.Generic.IEnumerable<TSource> Except<TSource>(this System.Collections.Generic.IEnumerable<TSource> first, System.Collections.Generic.IEnumerable<TSource> second, System.Collections.Generic.IEqualityComparer<TSource>? comparer) { throw null; }
         public static TSource? FirstOrDefault<TSource>(this System.Collections.Generic.IEnumerable<TSource> source) { throw null; }
+        public static TSource? FirstOrDefault<TSource>(this System.Collections.Generic.IEnumerable<TSource> source, TSource? defaultValue) { throw null; }
         public static TSource? FirstOrDefault<TSource>(this System.Collections.Generic.IEnumerable<TSource> source, System.Func<TSource, bool> predicate) { throw null; }
+        public static TSource? FirstOrDefault<TSource>(this System.Collections.Generic.IEnumerable<TSource> source, System.Func<TSource, bool> predicate, TSource? defaultValue) { throw null; }
         public static TSource First<TSource>(this System.Collections.Generic.IEnumerable<TSource> source) { throw null; }
         public static TSource First<TSource>(this System.Collections.Generic.IEnumerable<TSource> source, System.Func<TSource, bool> predicate) { throw null; }
         public static System.Collections.Generic.IEnumerable<System.Linq.IGrouping<TKey, TSource>> GroupBy<TSource, TKey>(this System.Collections.Generic.IEnumerable<TSource> source, System.Func<TSource, TKey> keySelector) { throw null; }
@@ -77,7 +79,9 @@ public static System.Collections.Generic.IEnumerable<
         public static System.Collections.Generic.IEnumerable<TResult> Join<TOuter, TInner, TKey, TResult>(this System.Collections.Generic.IEnumerable<TOuter> outer, System.Collections.Generic.IEnumerable<TInner> inner, System.Func<TOuter, TKey> outerKeySelector, System.Func<TInner, TKey> innerKeySelector, System.Func<TOuter, TInner, TResult> resultSelector) { throw null; }
         public static System.Collections.Generic.IEnumerable<TResult> Join<TOuter, TInner, TKey, TResult>(this System.Collections.Generic.IEnumerable<TOuter> outer, System.Collections.Generic.IEnumerable<TInner> inner, System.Func<TOuter, TKey> outerKeySelector, System.Func<TInner, TKey> innerKeySelector, System.Func<TOuter, TInner, TResult> resultSelector, System.Collections.Generic.IEqualityComparer<TKey>? comparer) { throw null; }
         public static TSource? LastOrDefault<TSource>(this System.Collections.Generic.IEnumerable<TSource> source) { throw null; }
+        public static TSource? LastOrDefault<TSource>(this System.Collections.Generic.IEnumerable<TSource> source, TSource? defaultValue) { throw null; }
         public static TSource? LastOrDefault<TSource>(this System.Collections.Generic.IEnumerable<TSource> source, System.Func<TSource, bool> predicate) { throw null; }
+        public static TSource? LastOrDefault<TSource>(this System.Collections.Generic.IEnumerable<TSource> source, System.Func<TSource, bool> predicate, TSource? defaultValue) { throw null; }
         public static TSource Last<TSource>(this System.Collections.Generic.IEnumerable<TSource> source) { throw null; }
         public static TSource Last<TSource>(this System.Collections.Generic.IEnumerable<TSource> source, System.Func<TSource, bool> predicate) { throw null; }
         public static long LongCount<TSource>(this System.Collections.Generic.IEnumerable<TSource> source) { throw null; }
@@ -144,7 +148,9 @@ public static System.Collections.Generic.IEnumerable<
         public static bool SequenceEqual<TSource>(this System.Collections.Generic.IEnumerable<TSource> first, System.Collections.Generic.IEnumerable<TSource> second) { throw null; }
         public static bool SequenceEqual<TSource>(this System.Collections.Generic.IEnumerable<TSource> first, System.Collections.Generic.IEnumerable<TSource> second, System.Collections.Generic.IEqualityComparer<TSource>? comparer) { throw null; }
         public static TSource? SingleOrDefault<TSource>(this System.Collections.Generic.IEnumerable<TSource> source) { throw null; }
+        public static TSource? SingleOrDefault<TSource>(this System.Collections.Generic.IEnumerable<TSource> source, TSource? defaultValue) { throw null; }
         public static TSource? SingleOrDefault<TSource>(this System.Collections.Generic.IEnumerable<TSource> source, System.Func<TSource, bool> predicate) { throw null; }
+        public static TSource? SingleOrDefault<TSource>(this System.Collections.Generic.IEnumerable<TSource> source, System.Func<TSource, bool> predicate, TSource? defaultValue) { throw null; }
         public static TSource Single<TSource>(this System.Collections.Generic.IEnumerable<TSource> source) { throw null; }
         public static TSource Single<TSource>(this System.Collections.Generic.IEnumerable<TSource> source, System.Func<TSource, bool> predicate) { throw null; }
         public static System.Collections.Generic.IEnumerable<TSource> SkipLast<TSource>(this System.Collections.Generic.IEnumerable<TSource> source, int count) { throw null; }

From d7e120bdb21e3912f232bf0f7739f4f8f744d591 Mon Sep 17 00:00:00 2001
From: Foxtrek_64 <foxtrek64@gmail.com>
Date: Mon, 1 Mar 2021 19:05:08 -0600
Subject: [PATCH 03/14] Make overloads with defaultValue not nullable

---
 .../ref/System.Linq.Queryable.cs                   | 12 ++++++------
 .../src/System/Linq/CachedReflection.cs            |  6 +++---
 .../src/System/Linq/Queryable.cs                   | 12 ++++++------
 src/libraries/System.Linq/ref/System.Linq.cs       | 12 ++++++------
 src/libraries/System.Linq/src/System/Linq/First.cs | 12 ++++++------
 src/libraries/System.Linq/src/System/Linq/Last.cs  | 14 +++++++-------
 .../System.Linq/src/System/Linq/Single.cs          |  2 +-
 7 files changed, 35 insertions(+), 35 deletions(-)

diff --git a/src/libraries/System.Linq.Queryable/ref/System.Linq.Queryable.cs b/src/libraries/System.Linq.Queryable/ref/System.Linq.Queryable.cs
index 3635765940eeee..bd3497d53c6580 100644
--- a/src/libraries/System.Linq.Queryable/ref/System.Linq.Queryable.cs
+++ b/src/libraries/System.Linq.Queryable/ref/System.Linq.Queryable.cs
@@ -86,9 +86,9 @@ public static partial class Queryable
         public static System.Linq.IQueryable<TSource> Except<TSource>(this System.Linq.IQueryable<TSource> source1, System.Collections.Generic.IEnumerable<TSource> source2) { throw null; }
         public static System.Linq.IQueryable<TSource> Except<TSource>(this System.Linq.IQueryable<TSource> source1, System.Collections.Generic.IEnumerable<TSource> source2, System.Collections.Generic.IEqualityComparer<TSource>? comparer) { throw null; }
         public static TSource? FirstOrDefault<TSource>(this System.Linq.IQueryable<TSource> source) { throw null; }
-        public static TSource? FirstOrDefault<TSource>(this System.Linq.IQueryable<TSource> source, TSource? defaultValue) { throw null; }
+        public static TSource FirstOrDefault<TSource>(this System.Linq.IQueryable<TSource> source, TSource defaultValue) { throw null; }
         public static TSource? FirstOrDefault<TSource>(this System.Linq.IQueryable<TSource> source, System.Linq.Expressions.Expression<System.Func<TSource, bool>> predicate) { throw null; }
-        public static TSource? FirstOrDefault<TSource>(this System.Linq.IQueryable<TSource> source, System.Linq.Expressions.Expression<System.Func<TSource, bool>> predicate, TSource? defaultValue) { throw null; }
+        public static TSource FirstOrDefault<TSource>(this System.Linq.IQueryable<TSource> source, System.Linq.Expressions.Expression<System.Func<TSource, bool>> predicate, TSource defaultValue) { throw null; }
         public static TSource First<TSource>(this System.Linq.IQueryable<TSource> source) { throw null; }
         public static TSource First<TSource>(this System.Linq.IQueryable<TSource> source, System.Linq.Expressions.Expression<System.Func<TSource, bool>> predicate) { throw null; }
         public static System.Linq.IQueryable<System.Linq.IGrouping<TKey, TSource>> GroupBy<TSource, TKey>(this System.Linq.IQueryable<TSource> source, System.Linq.Expressions.Expression<System.Func<TSource, TKey>> keySelector) { throw null; }
@@ -106,9 +106,9 @@ public static partial class Queryable
         public static System.Linq.IQueryable<TResult> Join<TOuter, TInner, TKey, TResult>(this System.Linq.IQueryable<TOuter> outer, System.Collections.Generic.IEnumerable<TInner> inner, System.Linq.Expressions.Expression<System.Func<TOuter, TKey>> outerKeySelector, System.Linq.Expressions.Expression<System.Func<TInner, TKey>> innerKeySelector, System.Linq.Expressions.Expression<System.Func<TOuter, TInner, TResult>> resultSelector) { throw null; }
         public static System.Linq.IQueryable<TResult> Join<TOuter, TInner, TKey, TResult>(this System.Linq.IQueryable<TOuter> outer, System.Collections.Generic.IEnumerable<TInner> inner, System.Linq.Expressions.Expression<System.Func<TOuter, TKey>> outerKeySelector, System.Linq.Expressions.Expression<System.Func<TInner, TKey>> innerKeySelector, System.Linq.Expressions.Expression<System.Func<TOuter, TInner, TResult>> resultSelector, System.Collections.Generic.IEqualityComparer<TKey>? comparer) { throw null; }
         public static TSource? LastOrDefault<TSource>(this System.Linq.IQueryable<TSource> source) { throw null; }
-        public static TSource? LastOrDefault<TSource>(this System.Linq.IQueryable<TSource> source, TSource? defaultValue) { throw null; }
+        public static TSource LastOrDefault<TSource>(this System.Linq.IQueryable<TSource> source, TSource defaultValue) { throw null; }
         public static TSource? LastOrDefault<TSource>(this System.Linq.IQueryable<TSource> source, System.Linq.Expressions.Expression<System.Func<TSource, bool>> predicate) { throw null; }
-        public static TSource? LastOrDefault<TSource>(this System.Linq.IQueryable<TSource> source, System.Linq.Expressions.Expression<System.Func<TSource, bool>> predicate, TSource? defaultValue) { throw null; }
+        public static TSource LastOrDefault<TSource>(this System.Linq.IQueryable<TSource> source, System.Linq.Expressions.Expression<System.Func<TSource, bool>> predicate, TSource defaultValue) { throw null; }
         public static TSource Last<TSource>(this System.Linq.IQueryable<TSource> source) { throw null; }
         public static TSource Last<TSource>(this System.Linq.IQueryable<TSource> source, System.Linq.Expressions.Expression<System.Func<TSource, bool>> predicate) { throw null; }
         public static long LongCount<TSource>(this System.Linq.IQueryable<TSource> source) { throw null; }
@@ -133,9 +133,9 @@ public static partial class Queryable
         public static bool SequenceEqual<TSource>(this System.Linq.IQueryable<TSource> source1, System.Collections.Generic.IEnumerable<TSource> source2) { throw null; }
         public static bool SequenceEqual<TSource>(this System.Linq.IQueryable<TSource> source1, System.Collections.Generic.IEnumerable<TSource> source2, System.Collections.Generic.IEqualityComparer<TSource>? comparer) { throw null; }
         public static TSource? SingleOrDefault<TSource>(this System.Linq.IQueryable<TSource> source) { throw null; }
-        public static TSource? SingleOrDefault<TSource>(this System.Linq.IQueryable<TSource> source, TSource? defaultValue) { throw null; }
+        public static TSource SingleOrDefault<TSource>(this System.Linq.IQueryable<TSource> source, TSource defaultValue) { throw null; }
         public static TSource? SingleOrDefault<TSource>(this System.Linq.IQueryable<TSource> source, System.Linq.Expressions.Expression<System.Func<TSource, bool>> predicate) { throw null; }
-        public static TSource? SingleOrDefault<TSource>(this System.Linq.IQueryable<TSource> source, System.Linq.Expressions.Expression<System.Func<TSource, bool>> predicate, TSource? defaultValue) { throw null; }
+        public static TSource SingleOrDefault<TSource>(this System.Linq.IQueryable<TSource> source, System.Linq.Expressions.Expression<System.Func<TSource, bool>> predicate, TSource defaultValue) { throw null; }
         public static TSource Single<TSource>(this System.Linq.IQueryable<TSource> source) { throw null; }
         public static TSource Single<TSource>(this System.Linq.IQueryable<TSource> source, System.Linq.Expressions.Expression<System.Func<TSource, bool>> predicate) { throw null; }
         public static System.Linq.IQueryable<TSource> SkipLast<TSource>(this System.Linq.IQueryable<TSource> source, int count) { throw null; }
diff --git a/src/libraries/System.Linq.Queryable/src/System/Linq/CachedReflection.cs b/src/libraries/System.Linq.Queryable/src/System/Linq/CachedReflection.cs
index 2d37fe2f53016a..37ac202c0f45bc 100644
--- a/src/libraries/System.Linq.Queryable/src/System/Linq/CachedReflection.cs
+++ b/src/libraries/System.Linq.Queryable/src/System/Linq/CachedReflection.cs
@@ -295,7 +295,7 @@ public static MethodInfo FirstOrDefault_TSource_3(Type TSource) =>
 
         public static MethodInfo FirstOrDefault_TSource_4(Type TSource) =>
             (s_FirstOrDefault_TSource_4 ??
-             (s_FirstOrDefault_TSource_4 = new Func<IQueryable<object>, Expression<Func<object, bool>>, object?, object?>(Queryable.FirstOrDefault).GetMethodInfo().GetGenericMethodDefinition()))
+             (s_FirstOrDefault_TSource_4 = new Func<IQueryable<object>, Expression<Func<object, bool>>, object, object?>(Queryable.FirstOrDefault).GetMethodInfo().GetGenericMethodDefinition()))
             .MakeGenericMethod(TSource);
 
         private static MethodInfo? s_GroupBy_TSource_TKey_2;
@@ -417,7 +417,7 @@ public static MethodInfo LastOrDefault_TSource_3(Type TSource) =>
 
         public static MethodInfo LastOrDefault_TSource_4(Type TSource) =>
             (s_LastOrDefault_TSource_4 ??
-             (s_LastOrDefault_TSource_4 = new Func<IQueryable<object>, Expression<Func<object, bool>>, object?, object?>(Queryable.LastOrDefault).GetMethodInfo().GetGenericMethodDefinition()))
+             (s_LastOrDefault_TSource_4 = new Func<IQueryable<object>, Expression<Func<object, bool>>, object, object?>(Queryable.LastOrDefault).GetMethodInfo().GetGenericMethodDefinition()))
             .MakeGenericMethod(TSource);
 
         private static MethodInfo? s_LongCount_TSource_1;
@@ -575,7 +575,7 @@ public static MethodInfo SingleOrDefault_TSource_3(Type TSource) =>
 
         public static MethodInfo SingleOrDefault_TSource_4(Type TSource) =>
             (s_SingleOrDefault_TSource_4 ??
-            (s_SingleOrDefault_TSource_4 = new Func<IQueryable<object>, Expression<Func<object, bool>>, object?, object?>(Queryable.SingleOrDefault).GetMethodInfo().GetGenericMethodDefinition()))
+            (s_SingleOrDefault_TSource_4 = new Func<IQueryable<object>, Expression<Func<object, bool>>, object, object?>(Queryable.SingleOrDefault).GetMethodInfo().GetGenericMethodDefinition()))
              .MakeGenericMethod();
 
         private static MethodInfo? s_Skip_TSource_2;
diff --git a/src/libraries/System.Linq.Queryable/src/System/Linq/Queryable.cs b/src/libraries/System.Linq.Queryable/src/System/Linq/Queryable.cs
index 065218f1ba12ce..36a7789d464f84 100644
--- a/src/libraries/System.Linq.Queryable/src/System/Linq/Queryable.cs
+++ b/src/libraries/System.Linq.Queryable/src/System/Linq/Queryable.cs
@@ -865,7 +865,7 @@ public static TSource First<TSource>(this IQueryable<TSource> source, Expression
         }
 
         [DynamicDependency("FirstOrDefault`1", typeof(Enumerable))]
-        public static TSource? FirstOrDefault<TSource>(this IQueryable<TSource> source, TSource? defaultValue)
+        public static TSource FirstOrDefault<TSource>(this IQueryable<TSource> source, TSource defaultValue)
         {
             if (source == null)
                 throw Error.ArgumentNull(nameof(source));
@@ -892,7 +892,7 @@ public static TSource First<TSource>(this IQueryable<TSource> source, Expression
         }
 
         [DynamicDependency("FirstOrDefault`1", typeof(Enumerable))]
-        public static TSource? FirstOrDefault<TSource>(this IQueryable<TSource> source, Expression<Func<TSource, bool>> predicate, TSource? defaultValue)
+        public static TSource FirstOrDefault<TSource>(this IQueryable<TSource> source, Expression<Func<TSource, bool>> predicate, TSource defaultValue)
         {
             if (source == null)
                 throw Error.ArgumentNull(nameof(source));
@@ -944,7 +944,7 @@ public static TSource Last<TSource>(this IQueryable<TSource> source, Expression<
         }
 
         [DynamicDependency("LastOrDefault`1", typeof(Enumerable))]
-        public static TSource? LastOrDefault<TSource>(this IQueryable<TSource> source, TSource? defaultValue)
+        public static TSource LastOrDefault<TSource>(this IQueryable<TSource> source, TSource defaultValue)
         {
             if (source == null)
                 throw Error.ArgumentNull(nameof(source));
@@ -971,7 +971,7 @@ public static TSource Last<TSource>(this IQueryable<TSource> source, Expression<
         }
 
         [DynamicDependency("LastOrDefault`1", typeof(Enumerable))]
-        public static TSource? LastOrDefault<TSource>(this IQueryable<TSource> source, Expression<Func<TSource, bool>> predicate, TSource? defaultValue)
+        public static TSource LastOrDefault<TSource>(this IQueryable<TSource> source, Expression<Func<TSource, bool>> predicate, TSource defaultValue)
         {
             if (source == null)
                 throw Error.ArgumentNull(nameof(source));
@@ -1023,7 +1023,7 @@ public static TSource Single<TSource>(this IQueryable<TSource> source, Expressio
         }
 
         [DynamicDependency("SingleOrDefault`1", typeof(Enumerable))]
-        public static TSource? SingleOrDefault<TSource>(this IQueryable<TSource> source, TSource? defaultValue)
+        public static TSource SingleOrDefault<TSource>(this IQueryable<TSource> source, TSource defaultValue)
         {
             if (source == null)
                 throw Error.ArgumentNull(nameof(source));
@@ -1051,7 +1051,7 @@ public static TSource Single<TSource>(this IQueryable<TSource> source, Expressio
         }
 
         [DynamicDependency("SingleOrDefault`1", typeof(Enumerable))]
-        public static TSource? SingleOrDefault<TSource>(this IQueryable<TSource> source, Expression<Func<TSource, bool>> predicate, TSource? defaultValue)
+        public static TSource SingleOrDefault<TSource>(this IQueryable<TSource> source, Expression<Func<TSource, bool>> predicate, TSource defaultValue)
         {
             if (source == null)
                 throw Error.ArgumentNull(nameof(source));
diff --git a/src/libraries/System.Linq/ref/System.Linq.cs b/src/libraries/System.Linq/ref/System.Linq.cs
index 716fb0ba8ed9e1..0f09240a0249de 100644
--- a/src/libraries/System.Linq/ref/System.Linq.cs
+++ b/src/libraries/System.Linq/ref/System.Linq.cs
@@ -59,9 +59,9 @@ public static System.Collections.Generic.IEnumerable<
         public static System.Collections.Generic.IEnumerable<TSource> Except<TSource>(this System.Collections.Generic.IEnumerable<TSource> first, System.Collections.Generic.IEnumerable<TSource> second) { throw null; }
         public static System.Collections.Generic.IEnumerable<TSource> Except<TSource>(this System.Collections.Generic.IEnumerable<TSource> first, System.Collections.Generic.IEnumerable<TSource> second, System.Collections.Generic.IEqualityComparer<TSource>? comparer) { throw null; }
         public static TSource? FirstOrDefault<TSource>(this System.Collections.Generic.IEnumerable<TSource> source) { throw null; }
-        public static TSource? FirstOrDefault<TSource>(this System.Collections.Generic.IEnumerable<TSource> source, TSource? defaultValue) { throw null; }
+        public static TSource FirstOrDefault<TSource>(this System.Collections.Generic.IEnumerable<TSource> source, TSource defaultValue) { throw null; }
         public static TSource? FirstOrDefault<TSource>(this System.Collections.Generic.IEnumerable<TSource> source, System.Func<TSource, bool> predicate) { throw null; }
-        public static TSource? FirstOrDefault<TSource>(this System.Collections.Generic.IEnumerable<TSource> source, System.Func<TSource, bool> predicate, TSource? defaultValue) { throw null; }
+        public static TSource FirstOrDefault<TSource>(this System.Collections.Generic.IEnumerable<TSource> source, System.Func<TSource, bool> predicate, TSource defaultValue) { throw null; }
         public static TSource First<TSource>(this System.Collections.Generic.IEnumerable<TSource> source) { throw null; }
         public static TSource First<TSource>(this System.Collections.Generic.IEnumerable<TSource> source, System.Func<TSource, bool> predicate) { throw null; }
         public static System.Collections.Generic.IEnumerable<System.Linq.IGrouping<TKey, TSource>> GroupBy<TSource, TKey>(this System.Collections.Generic.IEnumerable<TSource> source, System.Func<TSource, TKey> keySelector) { throw null; }
@@ -79,9 +79,9 @@ public static System.Collections.Generic.IEnumerable<
         public static System.Collections.Generic.IEnumerable<TResult> Join<TOuter, TInner, TKey, TResult>(this System.Collections.Generic.IEnumerable<TOuter> outer, System.Collections.Generic.IEnumerable<TInner> inner, System.Func<TOuter, TKey> outerKeySelector, System.Func<TInner, TKey> innerKeySelector, System.Func<TOuter, TInner, TResult> resultSelector) { throw null; }
         public static System.Collections.Generic.IEnumerable<TResult> Join<TOuter, TInner, TKey, TResult>(this System.Collections.Generic.IEnumerable<TOuter> outer, System.Collections.Generic.IEnumerable<TInner> inner, System.Func<TOuter, TKey> outerKeySelector, System.Func<TInner, TKey> innerKeySelector, System.Func<TOuter, TInner, TResult> resultSelector, System.Collections.Generic.IEqualityComparer<TKey>? comparer) { throw null; }
         public static TSource? LastOrDefault<TSource>(this System.Collections.Generic.IEnumerable<TSource> source) { throw null; }
-        public static TSource? LastOrDefault<TSource>(this System.Collections.Generic.IEnumerable<TSource> source, TSource? defaultValue) { throw null; }
+        public static TSource LastOrDefault<TSource>(this System.Collections.Generic.IEnumerable<TSource> source, TSource defaultValue) { throw null; }
         public static TSource? LastOrDefault<TSource>(this System.Collections.Generic.IEnumerable<TSource> source, System.Func<TSource, bool> predicate) { throw null; }
-        public static TSource? LastOrDefault<TSource>(this System.Collections.Generic.IEnumerable<TSource> source, System.Func<TSource, bool> predicate, TSource? defaultValue) { throw null; }
+        public static TSource LastOrDefault<TSource>(this System.Collections.Generic.IEnumerable<TSource> source, System.Func<TSource, bool> predicate, TSource defaultValue) { throw null; }
         public static TSource Last<TSource>(this System.Collections.Generic.IEnumerable<TSource> source) { throw null; }
         public static TSource Last<TSource>(this System.Collections.Generic.IEnumerable<TSource> source, System.Func<TSource, bool> predicate) { throw null; }
         public static long LongCount<TSource>(this System.Collections.Generic.IEnumerable<TSource> source) { throw null; }
@@ -148,9 +148,9 @@ public static System.Collections.Generic.IEnumerable<
         public static bool SequenceEqual<TSource>(this System.Collections.Generic.IEnumerable<TSource> first, System.Collections.Generic.IEnumerable<TSource> second) { throw null; }
         public static bool SequenceEqual<TSource>(this System.Collections.Generic.IEnumerable<TSource> first, System.Collections.Generic.IEnumerable<TSource> second, System.Collections.Generic.IEqualityComparer<TSource>? comparer) { throw null; }
         public static TSource? SingleOrDefault<TSource>(this System.Collections.Generic.IEnumerable<TSource> source) { throw null; }
-        public static TSource? SingleOrDefault<TSource>(this System.Collections.Generic.IEnumerable<TSource> source, TSource? defaultValue) { throw null; }
+        public static TSource SingleOrDefault<TSource>(this System.Collections.Generic.IEnumerable<TSource> source, TSource defaultValue) { throw null; }
         public static TSource? SingleOrDefault<TSource>(this System.Collections.Generic.IEnumerable<TSource> source, System.Func<TSource, bool> predicate) { throw null; }
-        public static TSource? SingleOrDefault<TSource>(this System.Collections.Generic.IEnumerable<TSource> source, System.Func<TSource, bool> predicate, TSource? defaultValue) { throw null; }
+        public static TSource SingleOrDefault<TSource>(this System.Collections.Generic.IEnumerable<TSource> source, System.Func<TSource, bool> predicate, TSource defaultValue) { throw null; }
         public static TSource Single<TSource>(this System.Collections.Generic.IEnumerable<TSource> source) { throw null; }
         public static TSource Single<TSource>(this System.Collections.Generic.IEnumerable<TSource> source, System.Func<TSource, bool> predicate) { throw null; }
         public static System.Collections.Generic.IEnumerable<TSource> SkipLast<TSource>(this System.Collections.Generic.IEnumerable<TSource> source, int count) { throw null; }
diff --git a/src/libraries/System.Linq/src/System/Linq/First.cs b/src/libraries/System.Linq/src/System/Linq/First.cs
index 6751a14e9f1550..dcb410c9e54300 100644
--- a/src/libraries/System.Linq/src/System/Linq/First.cs
+++ b/src/libraries/System.Linq/src/System/Linq/First.cs
@@ -33,19 +33,19 @@ public static TSource First<TSource>(this IEnumerable<TSource> source, Func<TSou
         public static TSource? FirstOrDefault<TSource>(this IEnumerable<TSource> source) =>
             source.TryGetFirst(out _);
 
-        public static TSource? FirstOrDefault<TSource>(this IEnumerable<TSource> source, TSource? defaultValue) =>
-            source.TryGetFirst(defaultValue, out _);
+        public static TSource FirstOrDefault<TSource>(this IEnumerable<TSource> source, TSource defaultValue) =>
+            source.TryGetFirst(defaultValue, out _)!;
 
         public static TSource? FirstOrDefault<TSource>(this IEnumerable<TSource> source, Func<TSource, bool> predicate) =>
             source.TryGetFirst(predicate, out _);
 
-        public static TSource? FirstOrDefault<TSource>(this IEnumerable<TSource> source, Func<TSource, bool> predicate, TSource? defaultValue) =>
-            source.TryGetFirst(predicate, defaultValue, out _);
+        public static TSource FirstOrDefault<TSource>(this IEnumerable<TSource> source, Func<TSource, bool> predicate, TSource defaultValue) =>
+            source.TryGetFirst(predicate, defaultValue, out _)!;
 
         private static TSource? TryGetFirst<TSource>(this IEnumerable<TSource> source, out bool found) =>
             source.TryGetFirst(default(TSource), out found);
 
-        private static TSource? TryGetFirst<TSource>(this IEnumerable<TSource> source, TSource? defaultValue, out bool found)
+        private static TSource? TryGetFirst<TSource>(this IEnumerable<TSource> source, TSource defaultValue, out bool found)
         {
             if (source == null)
             {
@@ -82,7 +82,7 @@ public static TSource First<TSource>(this IEnumerable<TSource> source, Func<TSou
         }
 
         private static TSource? TryGetFirst<TSource>(this IEnumerable<TSource> source, Func<TSource, bool> predicate, out bool found) =>
-            source.TryGetFirst(predicate, default(TSource), out found);
+            source.TryGetFirst(predicate, default, out found);
 
         private static TSource? TryGetFirst<TSource>(this IEnumerable<TSource> source, Func<TSource, bool> predicate, TSource? defaultValue, out bool found)
         {
diff --git a/src/libraries/System.Linq/src/System/Linq/Last.cs b/src/libraries/System.Linq/src/System/Linq/Last.cs
index 9d0b67e0a131a4..9a83a0a59bd0e3 100644
--- a/src/libraries/System.Linq/src/System/Linq/Last.cs
+++ b/src/libraries/System.Linq/src/System/Linq/Last.cs
@@ -32,17 +32,17 @@ public static TSource Last<TSource>(this IEnumerable<TSource> source, Func<TSour
 
         public static TSource? LastOrDefault<TSource>(this IEnumerable<TSource> source)
             => source.TryGetLast(out _);
-        public static TSource? LastOrDefault<TSource>(this IEnumerable<TSource> source, TSource? defaultValue)
-            => source.TryGetLast(defaultValue, out _);
+        public static TSource LastOrDefault<TSource>(this IEnumerable<TSource> source, TSource defaultValue)
+            => source.TryGetLast(defaultValue, out _)!;
 
         public static TSource? LastOrDefault<TSource>(this IEnumerable<TSource> source, Func<TSource, bool> predicate)
             => source.TryGetLast(predicate, out bool _);
-        public static TSource? LastOrDefault<TSource>(this IEnumerable<TSource> source, Func<TSource, bool> predicate, TSource? defaultValue)
-            => source.TryGetLast(predicate, defaultValue, out bool _);
+        public static TSource LastOrDefault<TSource>(this IEnumerable<TSource> source, Func<TSource, bool> predicate, TSource defaultValue)
+            => source.TryGetLast(predicate, defaultValue, out bool _)!;
 
         private static TSource? TryGetLast<TSource>(this IEnumerable<TSource> source, out bool found)
-            => source.TryGetLast(default(TSource?), out found);
-        private static TSource? TryGetLast<TSource>(this IEnumerable<TSource> source, TSource? defaultValue, out bool found)
+            => source.TryGetLast(default(TSource), out found);
+        private static TSource? TryGetLast<TSource>(this IEnumerable<TSource> source, TSource defaultValue, out bool found)
         {
             if (source == null)
             {
@@ -87,7 +87,7 @@ public static TSource Last<TSource>(this IEnumerable<TSource> source, Func<TSour
         }
 
         private static TSource? TryGetLast<TSource>(this IEnumerable<TSource> source, Func<TSource, bool> predicate, out bool found)
-            => source.TryGetLast(predicate, default(TSource?), out found);
+            => source.TryGetLast(predicate, default, out found);
         private static TSource? TryGetLast<TSource>(this IEnumerable<TSource> source, Func<TSource, bool> predicate, TSource? defaultValue, out bool found)
         {
             if (source == null)
diff --git a/src/libraries/System.Linq/src/System/Linq/Single.cs b/src/libraries/System.Linq/src/System/Linq/Single.cs
index 396c1f1aeaee3a..d8f5f236b44614 100644
--- a/src/libraries/System.Linq/src/System/Linq/Single.cs
+++ b/src/libraries/System.Linq/src/System/Linq/Single.cs
@@ -86,7 +86,7 @@ public static TSource Single<TSource>(this IEnumerable<TSource> source, Func<TSo
         public static TSource? SingleOrDefault<TSource>(this IEnumerable<TSource> source)
             => source.SingleOrDefault(default(TSource));
 
-        public static TSource? SingleOrDefault<TSource>(this IEnumerable<TSource> source, TSource? defaultValue)
+        public static TSource SingleOrDefault<TSource>(this IEnumerable<TSource> source, TSource defaultValue)
         {
             if (source == null)
             {

From 2ff34cd65d749072d53ac428d6a03778d85ce8c2 Mon Sep 17 00:00:00 2001
From: Foxtrek_64 <foxtrek64@gmail.com>
Date: Wed, 10 Mar 2021 18:29:28 -0600
Subject: [PATCH 04/14] Add unit tests, simplify implementation

---
 .../System.Linq/src/System/Linq/First.cs      | 14 ++-----
 .../System.Linq/tests/FirstOrDefaultTests.cs  | 42 +++++++++++++++++++
 .../System.Linq/tests/LastOrDefaultTests.cs   | 17 ++++++++
 .../System.Linq/tests/SingleOrDefaultTests.cs | 18 ++++++++
 4 files changed, 81 insertions(+), 10 deletions(-)

diff --git a/src/libraries/System.Linq/src/System/Linq/First.cs b/src/libraries/System.Linq/src/System/Linq/First.cs
index dcb410c9e54300..45c06ac64d3396 100644
--- a/src/libraries/System.Linq/src/System/Linq/First.cs
+++ b/src/libraries/System.Linq/src/System/Linq/First.cs
@@ -10,7 +10,7 @@ public static partial class Enumerable
     {
         public static TSource First<TSource>(this IEnumerable<TSource> source)
         {
-            TSource? first = source.TryGetFirst(out bool found);
+            TSource? first = source.TryGetFirst(default, out bool found);
             if (!found)
             {
                 ThrowHelper.ThrowNoElementsException();
@@ -21,7 +21,7 @@ public static TSource First<TSource>(this IEnumerable<TSource> source)
 
         public static TSource First<TSource>(this IEnumerable<TSource> source, Func<TSource, bool> predicate)
         {
-            TSource? first = source.TryGetFirst(predicate, out bool found);
+            TSource? first = source.TryGetFirst(predicate, default, out bool found);
             if (!found)
             {
                 ThrowHelper.ThrowNoMatchException();
@@ -31,20 +31,17 @@ public static TSource First<TSource>(this IEnumerable<TSource> source, Func<TSou
         }
 
         public static TSource? FirstOrDefault<TSource>(this IEnumerable<TSource> source) =>
-            source.TryGetFirst(out _);
+            source.TryGetFirst(default, out _);
 
         public static TSource FirstOrDefault<TSource>(this IEnumerable<TSource> source, TSource defaultValue) =>
             source.TryGetFirst(defaultValue, out _)!;
 
         public static TSource? FirstOrDefault<TSource>(this IEnumerable<TSource> source, Func<TSource, bool> predicate) =>
-            source.TryGetFirst(predicate, out _);
+            source.TryGetFirst(predicate, default, out _);
 
         public static TSource FirstOrDefault<TSource>(this IEnumerable<TSource> source, Func<TSource, bool> predicate, TSource defaultValue) =>
             source.TryGetFirst(predicate, defaultValue, out _)!;
 
-        private static TSource? TryGetFirst<TSource>(this IEnumerable<TSource> source, out bool found) =>
-            source.TryGetFirst(default(TSource), out found);
-
         private static TSource? TryGetFirst<TSource>(this IEnumerable<TSource> source, TSource defaultValue, out bool found)
         {
             if (source == null)
@@ -81,9 +78,6 @@ public static TSource FirstOrDefault<TSource>(this IEnumerable<TSource> source,
             return defaultValue;
         }
 
-        private static TSource? TryGetFirst<TSource>(this IEnumerable<TSource> source, Func<TSource, bool> predicate, out bool found) =>
-            source.TryGetFirst(predicate, default, out found);
-
         private static TSource? TryGetFirst<TSource>(this IEnumerable<TSource> source, Func<TSource, bool> predicate, TSource? defaultValue, out bool found)
         {
             if (source == null)
diff --git a/src/libraries/System.Linq/tests/FirstOrDefaultTests.cs b/src/libraries/System.Linq/tests/FirstOrDefaultTests.cs
index f0642c3e8165ab..0e6f27dc9f515b 100644
--- a/src/libraries/System.Linq/tests/FirstOrDefaultTests.cs
+++ b/src/libraries/System.Linq/tests/FirstOrDefaultTests.cs
@@ -39,6 +39,15 @@ private static void TestEmptyIList<T>()
             Assert.Equal(expected, source.RunOnce().FirstOrDefault());
         }
 
+        private static void TestEmptyIListDefault<T>(T defaultValue)
+        {
+            T[] source = { };
+
+            Assert.IsAssignableFrom<IList<T>>(source);
+
+            Assert.Equal(defaultValue, source.RunOnce().FirstOrDefault(defaultValue));
+        }
+
         [Fact]
         public void EmptyIListT()
         {
@@ -48,6 +57,14 @@ public void EmptyIListT()
             TestEmptyIList<FirstOrDefaultTests>();
         }
 
+        [Fact]
+        public void EmptyIListDefault()
+        {
+            TestEmptyIListDefault(5); // int
+            TestEmptyIListDefault("Hello"); // string
+            TestEmptyIListDefault(DateTime.UnixEpoch); //DateTime
+        }
+
         [Fact]
         public void IListTOneElement()
         {
@@ -59,6 +76,17 @@ public void IListTOneElement()
             Assert.Equal(expected, source.FirstOrDefault());
         }
 
+        [Fact]
+        public void IListOneElementDefault()
+        {
+            int[] source = { 5 };
+            int expected = 5;
+
+            Assert.IsAssignableFrom<IList<int>>(source);
+
+            Assert.Equal(expected, source.FirstOrDefault(3));
+        }
+
         [Fact]
         public void IListTManyElementsFirstIsDefault()
         {
@@ -96,6 +124,20 @@ static IEnumerable<T1> EmptySource<T1>()
             Assert.Equal(expected, source.RunOnce().FirstOrDefault());
         }
 
+        private static void TestEmptyNotIListDefault<T>(T defaultValue)
+        {
+            static IEnumerable<T1> EmptySource<T1>()
+            {
+                yield break;
+            }
+
+            var source = EmptySource<T>();
+
+            Assert.Null(source as IList<T>);
+
+            Assert.Equal(defaultValue, source.RunOnce().FirstOrDefault(defaultValue));
+        }
+
         [Fact]
         public void EmptyNotIListT()
         {
diff --git a/src/libraries/System.Linq/tests/LastOrDefaultTests.cs b/src/libraries/System.Linq/tests/LastOrDefaultTests.cs
index ec8d6c40b62448..5d5fe5e49d6190 100644
--- a/src/libraries/System.Linq/tests/LastOrDefaultTests.cs
+++ b/src/libraries/System.Linq/tests/LastOrDefaultTests.cs
@@ -39,6 +39,15 @@ private static void TestEmptyIList<T>()
             Assert.Equal(expected, source.RunOnce().LastOrDefault());
         }
 
+        private static void TestEmptyIListDefault<T>(T defaultValue)
+        {
+            T[] source = { };
+
+            Assert.IsAssignableFrom<IList<T>>(source);
+
+            Assert.Equal(defaultValue, source.RunOnce().LastOrDefault(defaultValue));
+        }
+
         [Fact]
         public void EmptyIListT()
         {
@@ -48,6 +57,14 @@ public void EmptyIListT()
             TestEmptyIList<LastOrDefaultTests>();
         }
 
+        [Fact]
+        public void EmptyIList()
+        {
+            TestEmptyIListDefault(5); // int
+            TestEmptyIListDefault("Hello"); // string
+            TestEmptyIListDefault(DateTime.UnixEpoch);
+        }
+
         [Fact]
         public void IListTOneElement()
         {
diff --git a/src/libraries/System.Linq/tests/SingleOrDefaultTests.cs b/src/libraries/System.Linq/tests/SingleOrDefaultTests.cs
index 1dfca02713808e..8660edb944828c 100644
--- a/src/libraries/System.Linq/tests/SingleOrDefaultTests.cs
+++ b/src/libraries/System.Linq/tests/SingleOrDefaultTests.cs
@@ -36,6 +36,15 @@ public void EmptyIList()
             Assert.Equal(expected, source.SingleOrDefault());
         }
 
+        [Fact]
+        public void EmptyIListDefault()
+        {
+            int?[] source = { };
+            int expected = 5;
+
+            Assert.Equal(expected, source.SingleOrDefault(5));
+        }
+
         [Fact]
         public void SingleElementIList()
         {
@@ -45,6 +54,15 @@ public void SingleElementIList()
             Assert.Equal(expected, source.SingleOrDefault());
         }
 
+        [Fact]
+        public void SingleElementIListDefault()
+        {
+            int[] source = { 4 };
+            int expected = 4;
+
+            Assert.Equal(expected, source.SingleOrDefault(5));
+        }
+
         [Fact]
         public void ManyElementIList()
         {

From b54edb40e3d20572d709725cd2012cb5222bead3 Mon Sep 17 00:00:00 2001
From: Foxtrek_64 <foxtrek64@gmail.com>
Date: Wed, 10 Mar 2021 18:30:01 -0600
Subject: [PATCH 05/14] Add LastOrDefault tests

---
 src/libraries/System.Linq/tests/LastOrDefaultTests.cs | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/src/libraries/System.Linq/tests/LastOrDefaultTests.cs b/src/libraries/System.Linq/tests/LastOrDefaultTests.cs
index 5d5fe5e49d6190..dff8fb5843df17 100644
--- a/src/libraries/System.Linq/tests/LastOrDefaultTests.cs
+++ b/src/libraries/System.Linq/tests/LastOrDefaultTests.cs
@@ -76,6 +76,17 @@ public void IListTOneElement()
             Assert.Equal(expected, source.LastOrDefault());
         }
 
+        [Fact]
+        public void IListTOneElementDefault()
+        {
+            int[] source = { 5 };
+            int expected = 5;
+
+            Assert.IsAssignableFrom<IList<int>>(source);
+
+            Assert.Equal(expected, source.LastOrDefault(4));
+        }
+
 
         [Fact]
         public void IListTManyElementsLastIsDefault()

From ed55a24b58af76787abda4e998dd75e5392cb5a1 Mon Sep 17 00:00:00 2001
From: Foxtrek_64 <foxtrek64@gmail.com>
Date: Thu, 11 Mar 2021 08:31:28 -0600
Subject: [PATCH 06/14] Add Queryable tests

---
 .../tests/FirstOrDefaultTests.cs                 | 16 ++++++++++++++++
 .../tests/LastOrDefaultTests.cs                  |  8 ++++++++
 .../tests/SingleOrDefaultTests.cs                | 14 ++++++++++++++
 3 files changed, 38 insertions(+)

diff --git a/src/libraries/System.Linq.Queryable/tests/FirstOrDefaultTests.cs b/src/libraries/System.Linq.Queryable/tests/FirstOrDefaultTests.cs
index 95093c0e8596af..ff13e3bcfe04cb 100644
--- a/src/libraries/System.Linq.Queryable/tests/FirstOrDefaultTests.cs
+++ b/src/libraries/System.Linq.Queryable/tests/FirstOrDefaultTests.cs
@@ -16,6 +16,15 @@ public void Empty()
             Assert.Equal(0, source.AsQueryable().FirstOrDefault());
         }
 
+        [Fact]
+        public void EmptyDefault()
+        {
+            int[] source = { };
+            int defaultValue = 5;
+
+            Assert.Equal(defaultValue, source.AsQueryable().FirstOrDefault(defaultValue));
+        }
+
         [Fact]
         public void ManyElementsFirstIsDefault()
         {
@@ -37,6 +46,13 @@ public void OneElementTruePredicate()
             Assert.Equal(4, source.AsQueryable().FirstOrDefault(i => i % 2 == 0));
         }
 
+        [Fact]
+        public void OneElementFalsePredicate()
+        {
+            int[] source = { 3 };
+            Assert.Equal(5, source.AsQueryable().FirstOrDefault(i => i % 2 == 0, 5));
+        }
+
         [Fact]
         public void ManyElementsPredicateFalseForAll()
         {
diff --git a/src/libraries/System.Linq.Queryable/tests/LastOrDefaultTests.cs b/src/libraries/System.Linq.Queryable/tests/LastOrDefaultTests.cs
index 62bfe0a9d29f2c..8083c77738bb01 100644
--- a/src/libraries/System.Linq.Queryable/tests/LastOrDefaultTests.cs
+++ b/src/libraries/System.Linq.Queryable/tests/LastOrDefaultTests.cs
@@ -14,6 +14,14 @@ public void Empty()
             Assert.Null(Enumerable.Empty<int?>().AsQueryable().LastOrDefault());
         }
 
+        [Fact]
+        public void EmptyDefault()
+        {
+            int[] source = { };
+            int defaultValue = 5;
+            Assert.Equal(defaultValue, source.AsQueryable().LastOrDefault(defaultValue));
+        }
+
         [Fact]
         public void OneElement()
         {
diff --git a/src/libraries/System.Linq.Queryable/tests/SingleOrDefaultTests.cs b/src/libraries/System.Linq.Queryable/tests/SingleOrDefaultTests.cs
index 032ca44ba83ce5..f34edae8b9f84d 100644
--- a/src/libraries/System.Linq.Queryable/tests/SingleOrDefaultTests.cs
+++ b/src/libraries/System.Linq.Queryable/tests/SingleOrDefaultTests.cs
@@ -22,12 +22,26 @@ public void Empty()
             Assert.Null(Enumerable.Empty<int?>().AsQueryable().SingleOrDefault());
         }
 
+        [Fact]
+        public void EmptyDefault()
+        {
+            int[] source = { };
+            int defaultValue = 5;
+            Assert.Equal(defaultValue, source.AsQueryable().SingleOrDefault(5));
+        }
+
         [Fact]
         public void EmptySourceWithPredicate()
         {
             Assert.Null(Enumerable.Empty<int?>().AsQueryable().SingleOrDefault(i => i % 2 == 0));
         }
 
+        [Fact]
+        public void EmptySourceWithPredicateDefault()
+        {
+            Assert.Equal(5, Enumerable.Empty<int?>().AsQueryable().SingleOrDefault(i => i % 2 == 0, 5));
+        }
+
         [Theory]
         [InlineData(1, 100)]
         [InlineData(42, 100)]

From fa5bc806f022f2e2f092685fc747ae95c8cdbdc2 Mon Sep 17 00:00:00 2001
From: Foxtrek_64 <foxtrek64@gmail.com>
Date: Thu, 11 Mar 2021 17:59:16 -0600
Subject: [PATCH 07/14] Additional tests. Reformatting TryGet Methods.

---
 .../tests/FirstOrDefaultTests.cs              |  30 +++++
 .../tests/LastOrDefaultTests.cs               |   7 ++
 .../System.Linq/src/System/Linq/First.cs      |  31 +++--
 .../System.Linq/src/System/Linq/Last.cs       |  19 +--
 .../System.Linq/src/System/Linq/Single.cs     | 108 ++++++------------
 .../System.Linq/tests/FirstOrDefaultTests.cs  |  43 +++++++
 .../System.Linq/tests/LastOrDefaultTests.cs   |  42 +++++++
 .../System.Linq/tests/SingleOrDefaultTests.cs |  77 +++++++++++++
 8 files changed, 264 insertions(+), 93 deletions(-)

diff --git a/src/libraries/System.Linq.Queryable/tests/FirstOrDefaultTests.cs b/src/libraries/System.Linq.Queryable/tests/FirstOrDefaultTests.cs
index ff13e3bcfe04cb..3b02a7ac8f2241 100644
--- a/src/libraries/System.Linq.Queryable/tests/FirstOrDefaultTests.cs
+++ b/src/libraries/System.Linq.Queryable/tests/FirstOrDefaultTests.cs
@@ -46,8 +46,22 @@ public void OneElementTruePredicate()
             Assert.Equal(4, source.AsQueryable().FirstOrDefault(i => i % 2 == 0));
         }
 
+        [Fact]
+        public void OneElementTruePredicateDefault()
+        {
+            int[] source = { 4 };
+            Assert.Equal(4, source.AsQueryable().FirstOrDefault(i => i % 2 == 0, 5));
+        }
+
         [Fact]
         public void OneElementFalsePredicate()
+        {
+            int[] source = { 3 };
+            Assert.Equal(0, source.AsQueryable().FirstOrDefault(i => i % 2 == 0));
+        }
+
+        [Fact]
+        public void OneElementFalsePredicateDefault()
         {
             int[] source = { 3 };
             Assert.Equal(5, source.AsQueryable().FirstOrDefault(i => i % 2 == 0, 5));
@@ -60,23 +74,38 @@ public void ManyElementsPredicateFalseForAll()
             Assert.Equal(0, source.AsQueryable().FirstOrDefault(i => i % 2 == 0));
         }
 
+        [Fact]
+        public void ManyElementsPredicateFalseForAllDefault()
+        {
+            int[] source = { 9, 5, 1, 3, 17, 21 };
+            Assert.Equal(2, source.AsQueryable().FirstOrDefault(i => i % 2 == 0), 2);
+        }
+
         [Fact]
         public void PredicateTrueForSome()
         {
             int[] source = { 3, 7, 10, 7, 9, 2, 11, 17, 13, 8 };
             Assert.Equal(10, source.AsQueryable().FirstOrDefault(i => i % 2 == 0));
         }
+        [Fact]
+        public void PredicateTrueForSomeDefault()
+        {
+            int[] source = { 3, 7, 10, 7, 9, 2, 11, 17, 13, 8 };
+            Assert.Equal(10, source.AsQueryable().FirstOrDefault(i => i % 2 == 0, 5));
+        }
 
         [Fact]
         public void NullSource()
         {
             AssertExtensions.Throws<ArgumentNullException>("source", () => ((IQueryable<int>)null).FirstOrDefault());
+            AssertExtensions.Throws<ArgumentNullException>("source", () => ((IQueryable<int>)null).FirstOrDefault(5));
         }
 
         [Fact]
         public void NullSourcePredicateUsed()
         {
             AssertExtensions.Throws<ArgumentNullException>("source", () => ((IQueryable<int>)null).FirstOrDefault(i => i != 2));
+            AssertExtensions.Throws<ArgumentNullException>("source", () => ((IQueryable<int>)null).FirstOrDefault(i => i != 2, 5));
         }
 
         [Fact]
@@ -84,6 +113,7 @@ public void NullPredicate()
         {
             Expression<Func<int, bool>> predicate = null;
             AssertExtensions.Throws<ArgumentNullException>("predicate", () => Enumerable.Range(0, 3).AsQueryable().FirstOrDefault(predicate));
+            AssertExtensions.Throws<ArgumentNullException>("predicate", () => Enumerable.Range(0, 3).AsQueryable().FirstOrDefault(predicate, 5));
         }
 
         [Fact]
diff --git a/src/libraries/System.Linq.Queryable/tests/LastOrDefaultTests.cs b/src/libraries/System.Linq.Queryable/tests/LastOrDefaultTests.cs
index 8083c77738bb01..c66c42fce4a4fe 100644
--- a/src/libraries/System.Linq.Queryable/tests/LastOrDefaultTests.cs
+++ b/src/libraries/System.Linq.Queryable/tests/LastOrDefaultTests.cs
@@ -29,6 +29,13 @@ public void OneElement()
             Assert.Equal(5, source.AsQueryable().LastOrDefault());
         }
 
+        [Fact]
+        public void OneElementFalsePredicate()
+        {
+            int[] source = { 3 };
+            Assert.Equal(5, source.AsQueryable().LastOrDefault(i => i % 2 == 0, 5));
+        }
+
         [Fact]
         public void ManyElementsLastIsDefault()
         {
diff --git a/src/libraries/System.Linq/src/System/Linq/First.cs b/src/libraries/System.Linq/src/System/Linq/First.cs
index 45c06ac64d3396..89e56f8803f141 100644
--- a/src/libraries/System.Linq/src/System/Linq/First.cs
+++ b/src/libraries/System.Linq/src/System/Linq/First.cs
@@ -10,7 +10,7 @@ public static partial class Enumerable
     {
         public static TSource First<TSource>(this IEnumerable<TSource> source)
         {
-            TSource? first = source.TryGetFirst(default, out bool found);
+            TSource? first = source.TryGetFirst(out bool found);
             if (!found)
             {
                 ThrowHelper.ThrowNoElementsException();
@@ -21,7 +21,7 @@ public static TSource First<TSource>(this IEnumerable<TSource> source)
 
         public static TSource First<TSource>(this IEnumerable<TSource> source, Func<TSource, bool> predicate)
         {
-            TSource? first = source.TryGetFirst(predicate, default, out bool found);
+            TSource? first = source.TryGetFirst(predicate, out bool found);
             if (!found)
             {
                 ThrowHelper.ThrowNoMatchException();
@@ -31,18 +31,25 @@ public static TSource First<TSource>(this IEnumerable<TSource> source, Func<TSou
         }
 
         public static TSource? FirstOrDefault<TSource>(this IEnumerable<TSource> source) =>
-            source.TryGetFirst(default, out _);
+            source.TryGetFirst(out _);
 
-        public static TSource FirstOrDefault<TSource>(this IEnumerable<TSource> source, TSource defaultValue) =>
-            source.TryGetFirst(defaultValue, out _)!;
+        public static TSource FirstOrDefault<TSource>(this IEnumerable<TSource> source, TSource defaultValue)
+        {
+            var first = source.TryGetFirst(out bool found);
+            return found ? first! : defaultValue;
+        }
 
         public static TSource? FirstOrDefault<TSource>(this IEnumerable<TSource> source, Func<TSource, bool> predicate) =>
-            source.TryGetFirst(predicate, default, out _);
+            source.TryGetFirst(predicate, out _);
+
+        public static TSource FirstOrDefault<TSource>(this IEnumerable<TSource> source, Func<TSource, bool> predicate, TSource defaultValue)
+        {
+            var first = source.TryGetFirst(predicate, out bool found);
+            return found ? first! : defaultValue;
+        }
 
-        public static TSource FirstOrDefault<TSource>(this IEnumerable<TSource> source, Func<TSource, bool> predicate, TSource defaultValue) =>
-            source.TryGetFirst(predicate, defaultValue, out _)!;
 
-        private static TSource? TryGetFirst<TSource>(this IEnumerable<TSource> source, TSource defaultValue, out bool found)
+        private static TSource? TryGetFirst<TSource>(this IEnumerable<TSource> source, out bool found)
         {
             if (source == null)
             {
@@ -75,10 +82,10 @@ public static TSource FirstOrDefault<TSource>(this IEnumerable<TSource> source,
             }
 
             found = false;
-            return defaultValue;
+            return default;
         }
 
-        private static TSource? TryGetFirst<TSource>(this IEnumerable<TSource> source, Func<TSource, bool> predicate, TSource? defaultValue, out bool found)
+        private static TSource? TryGetFirst<TSource>(this IEnumerable<TSource> source, Func<TSource, bool> predicate, out bool found)
         {
             if (source == null)
             {
@@ -100,7 +107,7 @@ public static TSource FirstOrDefault<TSource>(this IEnumerable<TSource> source,
             }
 
             found = false;
-            return defaultValue;
+            return default;
         }
     }
 }
diff --git a/src/libraries/System.Linq/src/System/Linq/Last.cs b/src/libraries/System.Linq/src/System/Linq/Last.cs
index 9a83a0a59bd0e3..300838d48c7c9f 100644
--- a/src/libraries/System.Linq/src/System/Linq/Last.cs
+++ b/src/libraries/System.Linq/src/System/Linq/Last.cs
@@ -33,16 +33,20 @@ public static TSource Last<TSource>(this IEnumerable<TSource> source, Func<TSour
         public static TSource? LastOrDefault<TSource>(this IEnumerable<TSource> source)
             => source.TryGetLast(out _);
         public static TSource LastOrDefault<TSource>(this IEnumerable<TSource> source, TSource defaultValue)
-            => source.TryGetLast(defaultValue, out _)!;
+        {
+            var last = source.TryGetLast(out bool found);
+            return found ? last! : defaultValue;
+        }
 
         public static TSource? LastOrDefault<TSource>(this IEnumerable<TSource> source, Func<TSource, bool> predicate)
             => source.TryGetLast(predicate, out bool _);
         public static TSource LastOrDefault<TSource>(this IEnumerable<TSource> source, Func<TSource, bool> predicate, TSource defaultValue)
-            => source.TryGetLast(predicate, defaultValue, out bool _)!;
+        {
+            var last = source.TryGetLast(out bool found);
+            return found ? last! : defaultValue;
+        }
 
         private static TSource? TryGetLast<TSource>(this IEnumerable<TSource> source, out bool found)
-            => source.TryGetLast(default(TSource), out found);
-        private static TSource? TryGetLast<TSource>(this IEnumerable<TSource> source, TSource defaultValue, out bool found)
         {
             if (source == null)
             {
@@ -83,12 +87,9 @@ public static TSource LastOrDefault<TSource>(this IEnumerable<TSource> source, F
             }
 
             found = false;
-            return defaultValue;
+            return default;
         }
-
         private static TSource? TryGetLast<TSource>(this IEnumerable<TSource> source, Func<TSource, bool> predicate, out bool found)
-            => source.TryGetLast(predicate, default, out found);
-        private static TSource? TryGetLast<TSource>(this IEnumerable<TSource> source, Func<TSource, bool> predicate, TSource? defaultValue, out bool found)
         {
             if (source == null)
             {
@@ -143,7 +144,7 @@ public static TSource LastOrDefault<TSource>(this IEnumerable<TSource> source, F
             }
 
             found = false;
-            return defaultValue;
+            return default;
         }
     }
 }
diff --git a/src/libraries/System.Linq/src/System/Linq/Single.cs b/src/libraries/System.Linq/src/System/Linq/Single.cs
index d8f5f236b44614..e84c0f9c9cbffd 100644
--- a/src/libraries/System.Linq/src/System/Linq/Single.cs
+++ b/src/libraries/System.Linq/src/System/Linq/Single.cs
@@ -10,85 +10,46 @@ public static partial class Enumerable
     {
         public static TSource Single<TSource>(this IEnumerable<TSource> source)
         {
-            if (source == null)
+            var single = source.TryGetSingle(out bool found);
+            if (!found)
             {
-                ThrowHelper.ThrowArgumentNullException(ExceptionArgument.source);
+                ThrowHelper.ThrowNoElementsException();
             }
 
-            if (source is IList<TSource> list)
-            {
-                switch (list.Count)
-                {
-                    case 0:
-                        ThrowHelper.ThrowNoElementsException();
-                        return default;
-                    case 1:
-                        return list[0];
-                }
-            }
-            else
-            {
-                using (IEnumerator<TSource> e = source.GetEnumerator())
-                {
-                    if (!e.MoveNext())
-                    {
-                        ThrowHelper.ThrowNoElementsException();
-                    }
-
-                    TSource result = e.Current;
-                    if (!e.MoveNext())
-                    {
-                        return result;
-                    }
-                }
-            }
-
-            ThrowHelper.ThrowMoreThanOneElementException();
-            return default;
+            return single!;
         }
-
         public static TSource Single<TSource>(this IEnumerable<TSource> source, Func<TSource, bool> predicate)
         {
-            if (source == null)
-            {
-                ThrowHelper.ThrowArgumentNullException(ExceptionArgument.source);
-            }
-
-            if (predicate == null)
+            var single = source.TryGetSingle(predicate, out bool found);
+            if (!found)
             {
-                ThrowHelper.ThrowArgumentNullException(ExceptionArgument.predicate);
+                ThrowHelper.ThrowNoElementsException();
             }
 
-            using (IEnumerator<TSource> e = source.GetEnumerator())
-            {
-                while (e.MoveNext())
-                {
-                    TSource result = e.Current;
-                    if (predicate(result))
-                    {
-                        while (e.MoveNext())
-                        {
-                            if (predicate(e.Current))
-                            {
-                                ThrowHelper.ThrowMoreThanOneMatchException();
-                            }
-                        }
-
-                        return result;
-                    }
-                }
-            }
-
-            ThrowHelper.ThrowNoMatchException();
-            return default;
+            return single!;
         }
 
         public static TSource? SingleOrDefault<TSource>(this IEnumerable<TSource> source)
-            => source.SingleOrDefault(default(TSource));
+            => source.TryGetSingle(out _);
 
         public static TSource SingleOrDefault<TSource>(this IEnumerable<TSource> source, TSource defaultValue)
         {
-            if (source == null)
+            var single = source.TryGetSingle(out bool found);
+            return found ? single! : defaultValue;
+        }
+
+        public static TSource? SingleOrDefault<TSource>(this IEnumerable<TSource> source, Func<TSource, bool> predicate)
+            => source.TryGetSingle(predicate, out _);
+
+        public static TSource SingleOrDefault<TSource>(this IEnumerable<TSource> source, Func<TSource, bool> predicate, TSource defaultValue)
+        {
+            var single = source.TryGetSingle(predicate, out bool found);
+            return found ? single! : defaultValue;
+        }
+
+        private static TSource? TryGetSingle<TSource>(this IEnumerable<TSource> source, out bool found)
+        {
+            if (source is null)
             {
                 ThrowHelper.ThrowArgumentNullException(ExceptionArgument.source);
             }
@@ -98,8 +59,10 @@ public static TSource SingleOrDefault<TSource>(this IEnumerable<TSource> source,
                 switch (list.Count)
                 {
                     case 0:
-                        return defaultValue;
+                        found = false;
+                        return default;
                     case 1:
+                        found = true;
                         return list[0];
                 }
             }
@@ -109,25 +72,25 @@ public static TSource SingleOrDefault<TSource>(this IEnumerable<TSource> source,
                 {
                     if (!e.MoveNext())
                     {
-                        return defaultValue;
+                        found = false;
+                        return default;
                     }
 
                     TSource result = e.Current;
                     if (!e.MoveNext())
                     {
+                        found = true;
                         return result;
                     }
                 }
             }
 
+            found = false;
             ThrowHelper.ThrowMoreThanOneElementException();
             return default;
         }
 
-        public static TSource? SingleOrDefault<TSource>(this IEnumerable<TSource> source, Func<TSource, bool> predicate)
-            => source.SingleOrDefault(predicate, default);
-
-        public static TSource? SingleOrDefault<TSource>(this IEnumerable<TSource> source, Func<TSource, bool> predicate, TSource? defaultValue)
+        private static TSource? TryGetSingle<TSource>(this IEnumerable<TSource> source, Func<TSource, bool> predicate, out bool found)
         {
             if (source == null)
             {
@@ -153,13 +116,14 @@ public static TSource SingleOrDefault<TSource>(this IEnumerable<TSource> source,
                                 ThrowHelper.ThrowMoreThanOneMatchException();
                             }
                         }
-
+                        found = true;
                         return result;
                     }
                 }
             }
 
-            return defaultValue;
+            found = false;
+            return default;
         }
     }
 }
diff --git a/src/libraries/System.Linq/tests/FirstOrDefaultTests.cs b/src/libraries/System.Linq/tests/FirstOrDefaultTests.cs
index 0e6f27dc9f515b..b0fe384389dcba 100644
--- a/src/libraries/System.Linq/tests/FirstOrDefaultTests.cs
+++ b/src/libraries/System.Linq/tests/FirstOrDefaultTests.cs
@@ -188,6 +188,16 @@ public void OneElementTruePredicate()
             Assert.Equal(expected, source.FirstOrDefault(predicate));
         }
 
+        [Fact]
+        public void OneElementTruePredicateDefault()
+        {
+            int[] source = { 4 };
+            Func<int, bool> predicate = IsEven;
+            int expected = 4;
+
+            Assert.Equal(expected, source.FirstOrDefault(predicate, 5));
+        }
+
         [Fact]
         public void ManyElementsPredicateFalseForAll()
         {
@@ -198,6 +208,16 @@ public void ManyElementsPredicateFalseForAll()
             Assert.Equal(expected, source.FirstOrDefault(predicate));
         }
 
+        [Fact]
+        public void ManyElementsPredicateFalseForAllDefault()
+        {
+            int[] source = { 9, 5, 1, 3, 17, 21 };
+            Func<int, bool> predicate = IsEven;
+            int expected = 5;
+
+            Assert.Equal(expected, source.FirstOrDefault(predicate, 5));
+        }
+
         [Fact]
         public void PredicateTrueOnlyForLast()
         {
@@ -208,6 +228,16 @@ public void PredicateTrueOnlyForLast()
             Assert.Equal(expected, source.FirstOrDefault(predicate));
         }
 
+        [Fact]
+        public void PredicateTrueOnlyForLastDefault()
+        {
+            int[] source = { 9, 5, 1, 3, 17, 21, 50 };
+            Func<int, bool> predicate = IsEven;
+            int expected = 50;
+
+            Assert.Equal(expected, source.FirstOrDefault(predicate, 5));
+        }
+
         [Fact]
         public void PredicateTrueForSome()
         {
@@ -218,6 +248,16 @@ public void PredicateTrueForSome()
             Assert.Equal(expected, source.FirstOrDefault(predicate));
         }
 
+        [Fact]
+        public void PredicateTrueForSomeDefault()
+        {
+            int[] source = { 3, 7, 10, 7, 9, 2, 11, 17, 13, 8 };
+            Func<int, bool> predicate = IsEven;
+            int expected = 10;
+
+            Assert.Equal(expected, source.FirstOrDefault(predicate, 5));
+        }
+
         [Fact]
         public void PredicateTrueForSomeRunOnce()
         {
@@ -232,12 +272,14 @@ public void PredicateTrueForSomeRunOnce()
         public void NullSource()
         {
             AssertExtensions.Throws<ArgumentNullException>("source", () => ((IEnumerable<int>)null).FirstOrDefault());
+            AssertExtensions.Throws<ArgumentNullException>("source", () => ((IEnumerable<int>)null).FirstOrDefault(5));
         }
 
         [Fact]
         public void NullSourcePredicateUsed()
         {
             AssertExtensions.Throws<ArgumentNullException>("source", () => ((IEnumerable<int>)null).FirstOrDefault(i => i != 2));
+            AssertExtensions.Throws<ArgumentNullException>("source", () => ((IEnumerable<int>)null).FirstOrDefault(i => i != 2, 5));
         }
 
         [Fact]
@@ -245,6 +287,7 @@ public void NullPredicate()
         {
             Func<int, bool> predicate = null;
             AssertExtensions.Throws<ArgumentNullException>("predicate", () => Enumerable.Range(0, 3).FirstOrDefault(predicate));
+            AssertExtensions.Throws<ArgumentNullException>("predicate", () => Enumerable.Range(0, 3).FirstOrDefault(predicate, 5));
         }
     }
 }
diff --git a/src/libraries/System.Linq/tests/LastOrDefaultTests.cs b/src/libraries/System.Linq/tests/LastOrDefaultTests.cs
index dff8fb5843df17..ab3a3fa1ad8a1b 100644
--- a/src/libraries/System.Linq/tests/LastOrDefaultTests.cs
+++ b/src/libraries/System.Linq/tests/LastOrDefaultTests.cs
@@ -110,6 +110,28 @@ public void IListTManyElementsLastIsNotDefault()
             Assert.Equal(expected, source.LastOrDefault());
         }
 
+        [Fact]
+        public void IListTManyElementsLastHasDefault()
+        {
+            int?[] source = { -10, 2, 4, 3, 0, 2, null };
+            int? expected = null;
+
+            Assert.IsAssignableFrom<IList<int?>>(source);
+
+            Assert.Equal(expected, source.LastOrDefault(5));
+        }
+
+        [Fact]
+        public void IListTManyElementsLastIsHasDefault()
+        {
+            int?[] source = { -10, 2, 4, 3, 0, 2, null, 19 };
+            int? expected = 19;
+
+            Assert.IsAssignableFrom<IList<int?>>(source);
+
+            Assert.Equal(expected, source.LastOrDefault(5));
+        }
+
         private static IEnumerable<T> EmptySource<T>()
         {
             yield break;
@@ -175,6 +197,16 @@ public void OneElementIListTruePredicate()
             Assert.Equal(expected, source.LastOrDefault(predicate));
         }
 
+        [Fact]
+        public void OneElementIListTruePredicateDefault()
+        {
+            int[] source = { 4 };
+            Func<int, bool> predicate = IsEven;
+            int expected = 4;
+
+            Assert.Equal(expected, source.LastOrDefault(predicate, 5));
+        }
+
         [Fact]
         public void ManyElementsIListPredicateFalseForAll()
         {
@@ -185,6 +217,16 @@ public void ManyElementsIListPredicateFalseForAll()
             Assert.Equal(expected, source.LastOrDefault(predicate));
         }
 
+        [Fact]
+        public void ManyElementsIListPredicateFalseForAllDefault()
+        {
+            int[] source = { 9, 5, 1, 3, 17, 21 };
+            Func<int, bool> predicate = IsEven;
+            int expected = 5;
+
+            Assert.Equal(expected, source.LastOrDefault(predicate, 5));
+        }
+
         [Fact]
         public void IListPredicateTrueOnlyForLast()
         {
diff --git a/src/libraries/System.Linq/tests/SingleOrDefaultTests.cs b/src/libraries/System.Linq/tests/SingleOrDefaultTests.cs
index 8660edb944828c..9ac26e76a8fa23 100644
--- a/src/libraries/System.Linq/tests/SingleOrDefaultTests.cs
+++ b/src/libraries/System.Linq/tests/SingleOrDefaultTests.cs
@@ -71,6 +71,14 @@ public void ManyElementIList()
             Assert.Throws<InvalidOperationException>(() => source.SingleOrDefault());
         }
 
+        [Fact]
+        public void ManyElementIListDefault()
+        {
+            int[] source = { 4, 4, 4, 4, 4 };
+
+            Assert.Throws<InvalidOperationException>(() => source.SingleOrDefault(5));
+        }
+
         [Fact]
         public void EmptyNotIList()
         {
@@ -106,6 +114,15 @@ public void EmptySourceWithPredicate()
             Assert.Equal(expected, source.SingleOrDefault(i => i % 2 == 0));
         }
 
+        [Fact]
+        public void EmptySourceWithPredicateDefault()
+        {
+            int[] source = { };
+            int expected = 5;
+
+            Assert.Equal(expected, source.SingleOrDefault(i => i % 2 == 0, 5));
+        }
+
         [Fact]
         public void SingleElementPredicateTrue()
         {
@@ -115,6 +132,15 @@ public void SingleElementPredicateTrue()
             Assert.Equal(expected, source.SingleOrDefault(i => i % 2 == 0));
         }
 
+        [Fact]
+        public void SingleElementPredicateTrueDefault()
+        {
+            int[] source = { 4 };
+            int expected = 4;
+
+            Assert.Equal(expected, source.SingleOrDefault(i => i % 2 == 0, 5));
+        }
+
         [Fact]
         public void SingleElementPredicateFalse()
         {
@@ -124,6 +150,15 @@ public void SingleElementPredicateFalse()
             Assert.Equal(expected, source.SingleOrDefault(i => i % 2 == 0));
         }
 
+        [Fact]
+        public void SingleElementPredicateFalseDefault()
+        {
+            int[] source = { 3 };
+            int expected = 5;
+
+            Assert.Equal(expected, source.SingleOrDefault(i => i % 2 == 0, 5));
+        }
+
         [Fact]
         public void ManyElementsPredicateFalseForAll()
         {
@@ -133,6 +168,15 @@ public void ManyElementsPredicateFalseForAll()
             Assert.Equal(expected, source.SingleOrDefault(i => i % 2 == 0));
         }
 
+        [Fact]
+        public void ManyElementsPredicateFalseForAllDefault()
+        {
+            int[] source = { 3, 1, 7, 9, 13, 19 };
+            int expected = 5;
+
+            Assert.Equal(expected, source.SingleOrDefault(i => i % 2 == 0, 5));
+        }
+
         [Fact]
         public void ManyElementsPredicateTrueForLast()
         {
@@ -142,6 +186,15 @@ public void ManyElementsPredicateTrueForLast()
             Assert.Equal(expected, source.SingleOrDefault(i => i % 2 == 0));
         }
 
+        [Fact]
+        public void ManyElementsPredicateTrueForLastDefault()
+        {
+            int[] source = { 3, 1, 7, 9, 13, 19, 20 };
+            int expected = 20;
+
+            Assert.Equal(expected, source.SingleOrDefault(i => i % 2 == 0, 5));
+        }
+
         [Fact]
         public void ManyElementsPredicateTrueForFirstAndFifth()
         {
@@ -150,6 +203,14 @@ public void ManyElementsPredicateTrueForFirstAndFifth()
             Assert.Throws<InvalidOperationException>(() => source.SingleOrDefault(i => i % 2 == 0));
         }
 
+        [Fact]
+        public void ManyElementsPredicateTrueForFirstAndFifthDefault()
+        {
+            int[] source = { 2, 3, 1, 7, 10, 13, 19, 9 };
+
+            Assert.Throws<InvalidOperationException>(() => source.SingleOrDefault(i => i % 2 == 0, 5));
+        }
+
         [Theory]
         [InlineData(1, 100)]
         [InlineData(42, 100)]
@@ -174,6 +235,14 @@ public void ThrowsOnNullSource()
             AssertExtensions.Throws<ArgumentNullException>("source", () => source.SingleOrDefault(i => i % 2 == 0));
         }
 
+        [Fact]
+        public void ThrowsOnNullSourceDefault()
+        {
+            int[] source = null;
+            AssertExtensions.Throws<ArgumentNullException>("source", () => source.SingleOrDefault(5));
+            AssertExtensions.Throws<ArgumentNullException>("source", () => source.SingleOrDefault(i => i % 2 == 0, 5));
+        }
+
         [Fact]
         public void ThrowsOnNullPredicate()
         {
@@ -181,5 +250,13 @@ public void ThrowsOnNullPredicate()
             Func<int, bool> nullPredicate = null;
             AssertExtensions.Throws<ArgumentNullException>("predicate", () => source.SingleOrDefault(nullPredicate));
         }
+
+        [Fact]
+        public void ThrowsOnNullPredicateDefault()
+        {
+            int[] source = { };
+            Func<int, bool> nullPredicate = null;
+            AssertExtensions.Throws<ArgumentNullException>("predicate", () => source.SingleOrDefault(nullPredicate, 5));
+        }
     }
 }

From 808245921b756177502efb4f96357dd52cd985c9 Mon Sep 17 00:00:00 2001
From: Eirik Tsarpalis <eirik.tsarpalis@gmail.com>
Date: Fri, 12 Mar 2021 13:02:13 +0000
Subject: [PATCH 08/14] Update
 src/libraries/System.Linq.Queryable/tests/FirstOrDefaultTests.cs

---
 .../System.Linq.Queryable/tests/FirstOrDefaultTests.cs          | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/libraries/System.Linq.Queryable/tests/FirstOrDefaultTests.cs b/src/libraries/System.Linq.Queryable/tests/FirstOrDefaultTests.cs
index 3b02a7ac8f2241..ac74ca5bd68365 100644
--- a/src/libraries/System.Linq.Queryable/tests/FirstOrDefaultTests.cs
+++ b/src/libraries/System.Linq.Queryable/tests/FirstOrDefaultTests.cs
@@ -78,7 +78,7 @@ public void ManyElementsPredicateFalseForAll()
         public void ManyElementsPredicateFalseForAllDefault()
         {
             int[] source = { 9, 5, 1, 3, 17, 21 };
-            Assert.Equal(2, source.AsQueryable().FirstOrDefault(i => i % 2 == 0), 2);
+            Assert.Equal(2, source.AsQueryable().FirstOrDefault(i => i % 2 == 0, 2));
         }
 
         [Fact]

From 24cf263ad2845bcdc3f89489a522c1582e4b64f8 Mon Sep 17 00:00:00 2001
From: Foxtrek_64 <Foxtrek64@users.noreply.github.com>
Date: Sun, 14 Mar 2021 00:04:10 +0000
Subject: [PATCH 09/14] Apply suggestions from code review

Co-authored-by: Eirik Tsarpalis <eirik.tsarpalis@gmail.com>
---
 src/libraries/System.Linq/src/System/Linq/First.cs  | 4 ++--
 src/libraries/System.Linq/src/System/Linq/Last.cs   | 8 ++++++--
 src/libraries/System.Linq/src/System/Linq/Single.cs | 6 +++---
 3 files changed, 11 insertions(+), 7 deletions(-)

diff --git a/src/libraries/System.Linq/src/System/Linq/First.cs b/src/libraries/System.Linq/src/System/Linq/First.cs
index 89e56f8803f141..7f59d1c63b5872 100644
--- a/src/libraries/System.Linq/src/System/Linq/First.cs
+++ b/src/libraries/System.Linq/src/System/Linq/First.cs
@@ -35,7 +35,7 @@ public static TSource First<TSource>(this IEnumerable<TSource> source, Func<TSou
 
         public static TSource FirstOrDefault<TSource>(this IEnumerable<TSource> source, TSource defaultValue)
         {
-            var first = source.TryGetFirst(out bool found);
+            TSource? first = source.TryGetFirst(out bool found);
             return found ? first! : defaultValue;
         }
 
@@ -44,7 +44,7 @@ public static TSource FirstOrDefault<TSource>(this IEnumerable<TSource> source,
 
         public static TSource FirstOrDefault<TSource>(this IEnumerable<TSource> source, Func<TSource, bool> predicate, TSource defaultValue)
         {
-            var first = source.TryGetFirst(predicate, out bool found);
+            TSource? first = source.TryGetFirst(predicate, out bool found);
             return found ? first! : defaultValue;
         }
 
diff --git a/src/libraries/System.Linq/src/System/Linq/Last.cs b/src/libraries/System.Linq/src/System/Linq/Last.cs
index 300838d48c7c9f..de730f1c30d26a 100644
--- a/src/libraries/System.Linq/src/System/Linq/Last.cs
+++ b/src/libraries/System.Linq/src/System/Linq/Last.cs
@@ -32,14 +32,17 @@ public static TSource Last<TSource>(this IEnumerable<TSource> source, Func<TSour
 
         public static TSource? LastOrDefault<TSource>(this IEnumerable<TSource> source)
             => source.TryGetLast(out _);
+
+
         public static TSource LastOrDefault<TSource>(this IEnumerable<TSource> source, TSource defaultValue)
         {
-            var last = source.TryGetLast(out bool found);
+            TSource? last = source.TryGetLast(out bool found);
             return found ? last! : defaultValue;
         }
 
         public static TSource? LastOrDefault<TSource>(this IEnumerable<TSource> source, Func<TSource, bool> predicate)
-            => source.TryGetLast(predicate, out bool _);
+            => source.TryGetLast(predicate, out _);
+
         public static TSource LastOrDefault<TSource>(this IEnumerable<TSource> source, Func<TSource, bool> predicate, TSource defaultValue)
         {
             var last = source.TryGetLast(out bool found);
@@ -89,6 +92,7 @@ public static TSource LastOrDefault<TSource>(this IEnumerable<TSource> source, F
             found = false;
             return default;
         }
+
         private static TSource? TryGetLast<TSource>(this IEnumerable<TSource> source, Func<TSource, bool> predicate, out bool found)
         {
             if (source == null)
diff --git a/src/libraries/System.Linq/src/System/Linq/Single.cs b/src/libraries/System.Linq/src/System/Linq/Single.cs
index e84c0f9c9cbffd..6455a841c9c46f 100644
--- a/src/libraries/System.Linq/src/System/Linq/Single.cs
+++ b/src/libraries/System.Linq/src/System/Linq/Single.cs
@@ -10,7 +10,7 @@ public static partial class Enumerable
     {
         public static TSource Single<TSource>(this IEnumerable<TSource> source)
         {
-            var single = source.TryGetSingle(out bool found);
+            TSource? single = source.TryGetSingle(out bool found);
             if (!found)
             {
                 ThrowHelper.ThrowNoElementsException();
@@ -20,10 +20,10 @@ public static TSource Single<TSource>(this IEnumerable<TSource> source)
         }
         public static TSource Single<TSource>(this IEnumerable<TSource> source, Func<TSource, bool> predicate)
         {
-            var single = source.TryGetSingle(predicate, out bool found);
+            TSource? single = source.TryGetSingle(predicate, out bool found);
             if (!found)
             {
-                ThrowHelper.ThrowNoElementsException();
+                ThrowHelper.ThrowNoMatchException();
             }
 
             return single!;

From ed9728dbdb0e87fdc0b1da1d5f602fb2e840ee6b Mon Sep 17 00:00:00 2001
From: Foxtrek_64 <foxtrek64@gmail.com>
Date: Mon, 15 Mar 2021 17:57:14 -0500
Subject: [PATCH 10/14] Fix ref methods

---
 .../System.Linq.Queryable/src/System/Linq/CachedReflection.cs | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/src/libraries/System.Linq.Queryable/src/System/Linq/CachedReflection.cs b/src/libraries/System.Linq.Queryable/src/System/Linq/CachedReflection.cs
index 37ac202c0f45bc..e7bfc2f28b5100 100644
--- a/src/libraries/System.Linq.Queryable/src/System/Linq/CachedReflection.cs
+++ b/src/libraries/System.Linq.Queryable/src/System/Linq/CachedReflection.cs
@@ -568,7 +568,7 @@ public static MethodInfo SingleOrDefault_TSource_2(Type TSource) =>
 
         public static MethodInfo SingleOrDefault_TSource_3(Type TSource) =>
             (s_SingleOrDefault_TSource_3 ??
-            (s_SingleOrDefault_TSource_3 = new Func<IQueryable<object>, object?, object?>(Queryable.SingleOrDefault).GetMethodInfo().GetGenericMethodDefinition()))
+            (s_SingleOrDefault_TSource_3 = new Func<IQueryable<object>, object, object?>(Queryable.SingleOrDefault).GetMethodInfo().GetGenericMethodDefinition()))
              .MakeGenericMethod(TSource);
 
         private static MethodInfo? s_SingleOrDefault_TSource_4;
@@ -576,7 +576,7 @@ public static MethodInfo SingleOrDefault_TSource_3(Type TSource) =>
         public static MethodInfo SingleOrDefault_TSource_4(Type TSource) =>
             (s_SingleOrDefault_TSource_4 ??
             (s_SingleOrDefault_TSource_4 = new Func<IQueryable<object>, Expression<Func<object, bool>>, object, object?>(Queryable.SingleOrDefault).GetMethodInfo().GetGenericMethodDefinition()))
-             .MakeGenericMethod();
+             .MakeGenericMethod(TSource);
 
         private static MethodInfo? s_Skip_TSource_2;
 

From 604ee5983e1a42aca4265752647b7ecee6dd78aa Mon Sep 17 00:00:00 2001
From: Foxtrek_64 <foxtrek64@gmail.com>
Date: Mon, 15 Mar 2021 22:36:35 -0500
Subject: [PATCH 11/14] Further adjust nullability

---
 .../System.Linq.Queryable/src/System/Linq/CachedReflection.cs | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/src/libraries/System.Linq.Queryable/src/System/Linq/CachedReflection.cs b/src/libraries/System.Linq.Queryable/src/System/Linq/CachedReflection.cs
index e7bfc2f28b5100..1fe10e814fc13f 100644
--- a/src/libraries/System.Linq.Queryable/src/System/Linq/CachedReflection.cs
+++ b/src/libraries/System.Linq.Queryable/src/System/Linq/CachedReflection.cs
@@ -568,14 +568,14 @@ public static MethodInfo SingleOrDefault_TSource_2(Type TSource) =>
 
         public static MethodInfo SingleOrDefault_TSource_3(Type TSource) =>
             (s_SingleOrDefault_TSource_3 ??
-            (s_SingleOrDefault_TSource_3 = new Func<IQueryable<object>, object, object?>(Queryable.SingleOrDefault).GetMethodInfo().GetGenericMethodDefinition()))
+            (s_SingleOrDefault_TSource_3 = new Func<IQueryable<object>, object, object>(Queryable.SingleOrDefault).GetMethodInfo().GetGenericMethodDefinition()))
              .MakeGenericMethod(TSource);
 
         private static MethodInfo? s_SingleOrDefault_TSource_4;
 
         public static MethodInfo SingleOrDefault_TSource_4(Type TSource) =>
             (s_SingleOrDefault_TSource_4 ??
-            (s_SingleOrDefault_TSource_4 = new Func<IQueryable<object>, Expression<Func<object, bool>>, object, object?>(Queryable.SingleOrDefault).GetMethodInfo().GetGenericMethodDefinition()))
+            (s_SingleOrDefault_TSource_4 = new Func<IQueryable<object>, Expression<Func<object, bool>>, object, object>(Queryable.SingleOrDefault).GetMethodInfo().GetGenericMethodDefinition()))
              .MakeGenericMethod(TSource);
 
         private static MethodInfo? s_Skip_TSource_2;

From 5a05802504542b655c83d5df7f1c29825b9c2332 Mon Sep 17 00:00:00 2001
From: Foxtrek_64 <foxtrek64@gmail.com>
Date: Tue, 16 Mar 2021 00:56:10 -0500
Subject: [PATCH 12/14] Fix more nullables

---
 .../src/System/Linq/CachedReflection.cs                   | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/src/libraries/System.Linq.Queryable/src/System/Linq/CachedReflection.cs b/src/libraries/System.Linq.Queryable/src/System/Linq/CachedReflection.cs
index 1fe10e814fc13f..2d11a128a33b08 100644
--- a/src/libraries/System.Linq.Queryable/src/System/Linq/CachedReflection.cs
+++ b/src/libraries/System.Linq.Queryable/src/System/Linq/CachedReflection.cs
@@ -288,14 +288,14 @@ public static MethodInfo FirstOrDefault_TSource_2(Type TSource) =>
 
         public static MethodInfo FirstOrDefault_TSource_3(Type TSource) =>
             (s_FirstOrDefault_TSource_3 ??
-            (s_FirstOrDefault_TSource_3 = new Func<IQueryable<object>, object?, object?>(Queryable.FirstOrDefault).GetMethodInfo().GetGenericMethodDefinition()))
+            (s_FirstOrDefault_TSource_3 = new Func<IQueryable<object>, object, object>(Queryable.FirstOrDefault).GetMethodInfo().GetGenericMethodDefinition()))
             .MakeGenericMethod();
 
         private static MethodInfo? s_FirstOrDefault_TSource_4;
 
         public static MethodInfo FirstOrDefault_TSource_4(Type TSource) =>
             (s_FirstOrDefault_TSource_4 ??
-             (s_FirstOrDefault_TSource_4 = new Func<IQueryable<object>, Expression<Func<object, bool>>, object, object?>(Queryable.FirstOrDefault).GetMethodInfo().GetGenericMethodDefinition()))
+            (s_FirstOrDefault_TSource_4 = new Func<IQueryable<object>, Expression<Func<object, bool>>, object, object>(Queryable.FirstOrDefault).GetMethodInfo().GetGenericMethodDefinition()))
             .MakeGenericMethod(TSource);
 
         private static MethodInfo? s_GroupBy_TSource_TKey_2;
@@ -410,14 +410,14 @@ public static MethodInfo LastOrDefault_TSource_2(Type TSource) =>
 
         public static MethodInfo LastOrDefault_TSource_3(Type TSource) =>
             (s_LastOrDefault_TSource_3 ??
-             (s_LastOrDefault_TSource_3 = new Func<IQueryable<object>, object?, object?>(Queryable.LastOrDefault).GetMethodInfo().GetGenericMethodDefinition()))
+             (s_LastOrDefault_TSource_3 = new Func<IQueryable<object>, object, object>(Queryable.LastOrDefault).GetMethodInfo().GetGenericMethodDefinition()))
             .MakeGenericMethod(TSource);
 
         private static MethodInfo? s_LastOrDefault_TSource_4;
 
         public static MethodInfo LastOrDefault_TSource_4(Type TSource) =>
             (s_LastOrDefault_TSource_4 ??
-             (s_LastOrDefault_TSource_4 = new Func<IQueryable<object>, Expression<Func<object, bool>>, object, object?>(Queryable.LastOrDefault).GetMethodInfo().GetGenericMethodDefinition()))
+             (s_LastOrDefault_TSource_4 = new Func<IQueryable<object>, Expression<Func<object, bool>>, object, object>(Queryable.LastOrDefault).GetMethodInfo().GetGenericMethodDefinition()))
             .MakeGenericMethod(TSource);
 
         private static MethodInfo? s_LongCount_TSource_1;

From 9e8699e5ec06ff368581c0b346703703a56e51ad Mon Sep 17 00:00:00 2001
From: Eirik Tsarpalis <eirik.tsarpalis@gmail.com>
Date: Wed, 17 Mar 2021 15:53:54 +0000
Subject: [PATCH 13/14] fix failing tests

---
 .../System.Linq.Queryable/src/System/Linq/CachedReflection.cs   | 2 +-
 .../System.Linq.Queryable/tests/TrimCompatibilityTests.cs       | 2 +-
 src/libraries/System.Linq/src/System/Linq/Last.cs               | 2 +-
 3 files changed, 3 insertions(+), 3 deletions(-)

diff --git a/src/libraries/System.Linq.Queryable/src/System/Linq/CachedReflection.cs b/src/libraries/System.Linq.Queryable/src/System/Linq/CachedReflection.cs
index 2d11a128a33b08..be11c326812cb5 100644
--- a/src/libraries/System.Linq.Queryable/src/System/Linq/CachedReflection.cs
+++ b/src/libraries/System.Linq.Queryable/src/System/Linq/CachedReflection.cs
@@ -289,7 +289,7 @@ public static MethodInfo FirstOrDefault_TSource_2(Type TSource) =>
         public static MethodInfo FirstOrDefault_TSource_3(Type TSource) =>
             (s_FirstOrDefault_TSource_3 ??
             (s_FirstOrDefault_TSource_3 = new Func<IQueryable<object>, object, object>(Queryable.FirstOrDefault).GetMethodInfo().GetGenericMethodDefinition()))
-            .MakeGenericMethod();
+            .MakeGenericMethod(TSource);
 
         private static MethodInfo? s_FirstOrDefault_TSource_4;
 
diff --git a/src/libraries/System.Linq.Queryable/tests/TrimCompatibilityTests.cs b/src/libraries/System.Linq.Queryable/tests/TrimCompatibilityTests.cs
index 3d02b20ae0ec13..b1463804c69086 100644
--- a/src/libraries/System.Linq.Queryable/tests/TrimCompatibilityTests.cs
+++ b/src/libraries/System.Linq.Queryable/tests/TrimCompatibilityTests.cs
@@ -61,7 +61,7 @@ public static void CachedReflectionInfoMethodsNoAnnotations()
                     .Where(m => m.GetParameters().Length > 0);
 
             // If you are adding a new method to this class, ensure the method meets these requirements
-            Assert.Equal(111, methods.Count());
+            Assert.Equal(117, methods.Count());
             foreach (MethodInfo method in methods)
             {
                 ParameterInfo[] parameters = method.GetParameters();
diff --git a/src/libraries/System.Linq/src/System/Linq/Last.cs b/src/libraries/System.Linq/src/System/Linq/Last.cs
index de730f1c30d26a..b28235bd48b336 100644
--- a/src/libraries/System.Linq/src/System/Linq/Last.cs
+++ b/src/libraries/System.Linq/src/System/Linq/Last.cs
@@ -45,7 +45,7 @@ public static TSource LastOrDefault<TSource>(this IEnumerable<TSource> source, T
 
         public static TSource LastOrDefault<TSource>(this IEnumerable<TSource> source, Func<TSource, bool> predicate, TSource defaultValue)
         {
-            var last = source.TryGetLast(out bool found);
+            var last = source.TryGetLast(predicate, out bool found);
             return found ? last! : defaultValue;
         }
 

From 9499df695ac73d299422a1dc6e1aa4a12d216c18 Mon Sep 17 00:00:00 2001
From: Foxtrek_64 <Foxtrek64@users.noreply.github.com>
Date: Wed, 17 Mar 2021 16:07:10 +0000
Subject: [PATCH 14/14] Restore coding style

---
 src/libraries/System.Linq/src/System/Linq/Last.cs | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/src/libraries/System.Linq/src/System/Linq/Last.cs b/src/libraries/System.Linq/src/System/Linq/Last.cs
index b28235bd48b336..16318de2574ccc 100644
--- a/src/libraries/System.Linq/src/System/Linq/Last.cs
+++ b/src/libraries/System.Linq/src/System/Linq/Last.cs
@@ -30,8 +30,8 @@ public static TSource Last<TSource>(this IEnumerable<TSource> source, Func<TSour
             return last!;
         }
 
-        public static TSource? LastOrDefault<TSource>(this IEnumerable<TSource> source)
-            => source.TryGetLast(out _);
+        public static TSource? LastOrDefault<TSource>(this IEnumerable<TSource> source) =>
+            source.TryGetLast(out _);
 
 
         public static TSource LastOrDefault<TSource>(this IEnumerable<TSource> source, TSource defaultValue)