From 77c3efb2db41802338d05da15cbb65a015e07be5 Mon Sep 17 00:00:00 2001
From: Som Snytt <som.snytt@gmail.com>
Date: Fri, 7 Mar 2025 13:06:55 -0800
Subject: [PATCH] Ignore params to default arg getters

---
 .../tools/dotc/transform/CheckUnused.scala    |  6 ++++--
 tests/warn/i22746.scala                       | 21 +++++++++++++++++++
 2 files changed, 25 insertions(+), 2 deletions(-)
 create mode 100644 tests/warn/i22746.scala

diff --git a/compiler/src/dotty/tools/dotc/transform/CheckUnused.scala b/compiler/src/dotty/tools/dotc/transform/CheckUnused.scala
index 4dfaea70cb50..37eee35fdb39 100644
--- a/compiler/src/dotty/tools/dotc/transform/CheckUnused.scala
+++ b/compiler/src/dotty/tools/dotc/transform/CheckUnused.scala
@@ -8,7 +8,8 @@ import dotty.tools.dotc.core.Contexts.*
 import dotty.tools.dotc.core.Flags.*
 import dotty.tools.dotc.core.Names.{Name, SimpleName, DerivedName, TermName, termName}
 import dotty.tools.dotc.core.NameOps.{isAnonymousFunctionName, isReplWrapperName}
-import dotty.tools.dotc.core.NameKinds.{BodyRetainerName, ContextBoundParamName, ContextFunctionParamName, WildcardParamName}
+import dotty.tools.dotc.core.NameKinds.{
+  BodyRetainerName, ContextBoundParamName, ContextFunctionParamName, DefaultGetterName, WildcardParamName}
 import dotty.tools.dotc.core.StdNames.nme
 import dotty.tools.dotc.core.Symbols.{ClassSymbol, NoSymbol, Symbol, defn, isDeprecated, requiredClass, requiredModule}
 import dotty.tools.dotc.core.Types.*
@@ -175,7 +176,8 @@ class CheckUnused private (phaseMode: PhaseMode, suffix: String) extends MiniPha
   override def prepareForDefDef(tree: DefDef)(using Context): Context =
     def trivial = tree.symbol.is(Deferred) || isUnconsuming(tree.rhs)
     def nontrivial = tree.symbol.isConstructor || tree.symbol.isAnonymousFunction
-    if !nontrivial && trivial then
+    def isDefault = tree.symbol.name.is(DefaultGetterName)
+    if !nontrivial && trivial || isDefault then
       refInfos.skip.addOne(tree.symbol)
     if tree.symbol.is(Inline) then
       refInfos.inliners += 1
diff --git a/tests/warn/i22746.scala b/tests/warn/i22746.scala
new file mode 100644
index 000000000000..79576f5924be
--- /dev/null
+++ b/tests/warn/i22746.scala
@@ -0,0 +1,21 @@
+
+//> using options -Wunused:all -Werror
+
+import java.time.ZonedDateTime
+
+trait Foo[A] {
+  def apply(a: A, t: ZonedDateTime): A
+}
+
+extension [A](a: A)(using f: Foo[A]) {
+  def foo(t: ZonedDateTime = ZonedDateTime.now): A = f(a, t)
+}
+
+def test[I, A](in: I)(
+  run: I => Either[Throwable, A],
+  onErr: Throwable => Throwable = identity[Throwable]
+): Either[Throwable, A] =
+  run(in) match {
+    case Left(t) => Left(onErr(t))
+    case r @ Right(_) => r
+  }