From 4879802478382e16ee6109e6fe4e218c49545dd5 Mon Sep 17 00:00:00 2001
From: Anders Hejlsberg <andersh@microsoft.com>
Date: Fri, 17 Jan 2020 09:10:00 -0800
Subject: [PATCH 1/3] No covariance default for recursive references in
 variance measurement

---
 src/compiler/checker.ts | 17 +++++++++++++----
 1 file changed, 13 insertions(+), 4 deletions(-)

diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 966e6f4e4c75c..e5f2932aab7f3 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -306,6 +306,7 @@ namespace ts {
 
         const emptySymbols = createSymbolTable();
         const identityMapper: (type: Type) => Type = identity;
+        const arrayVariances = [VarianceFlags.Covariant];
 
         const compilerOptions = host.getCompilerOptions();
         const languageVersion = getEmitScriptTarget(compilerOptions);
@@ -15403,6 +15404,9 @@ namespace ts {
                     source.aliasTypeArguments && source.aliasSymbol === target.aliasSymbol &&
                     !(source.aliasTypeArgumentsContainsMarker || target.aliasTypeArgumentsContainsMarker)) {
                     const variances = getAliasVariances(source.aliasSymbol);
+                    if (variances === emptyArray) {
+                        return Ternary.Maybe;
+                    }
                     const varianceResult = relateVariances(source.aliasTypeArguments, target.aliasTypeArguments, variances, intersectionState);
                     if (varianceResult !== undefined) {
                         return varianceResult;
@@ -15599,6 +15603,12 @@ namespace ts {
                         // type references (which are intended by be compared structurally). Obtain the variance
                         // information for the type parameters and relate the type arguments accordingly.
                         const variances = getVariances((<TypeReference>source).target);
+                        // We return Ternary.Maybe for a recursive invocation of getVariances (signalled by emptyArray). This
+                        // effectively means we measure variance only from type parameter occurrences that aren't nested in
+                        // recursive instantiations of the generic type.
+                        if (variances === emptyArray) {
+                            return Ternary.Maybe;
+                        }
                         const varianceResult = relateVariances(getTypeArguments(<TypeReference>source), getTypeArguments(<TypeReference>target), variances, intersectionState);
                         if (varianceResult !== undefined) {
                             return varianceResult;
@@ -16418,8 +16428,7 @@ namespace ts {
         // a digest of the type comparisons that occur for each type argument when instantiations of the
         // generic type are structurally compared. We infer the variance information by comparing
         // instantiations of the generic type for type arguments with known relations. The function
-        // returns the emptyArray singleton if we're not in strictFunctionTypes mode or if the function
-        // has been invoked recursively for the given generic type.
+        // returns the emptyArray singleton when invoked recursively for the given generic type.
         function getVariancesWorker<TCache extends { variances?: VarianceFlags[] }>(typeParameters: readonly TypeParameter[] = emptyArray, cache: TCache, createMarkerType: (input: TCache, param: TypeParameter, marker: Type) => Type): VarianceFlags[] {
             let variances = cache.variances;
             if (!variances) {
@@ -16462,9 +16471,9 @@ namespace ts {
         }
 
         function getVariances(type: GenericType): VarianceFlags[] {
-            // Arrays and tuples are known to be covariant, no need to spend time computing this (emptyArray implies covariance for all parameters)
+            // Arrays and tuples are known to be covariant, no need to spend time computing this.
             if (type === globalArrayType || type === globalReadonlyArrayType || type.objectFlags & ObjectFlags.Tuple) {
-                return emptyArray;
+                return arrayVariances;
             }
             return getVariancesWorker(type.typeParameters, type, getMarkerTypeReference);
         }

From af7f5b1b48b77793d5368b424d7e78d984ab33b5 Mon Sep 17 00:00:00 2001
From: Anders Hejlsberg <andersh@microsoft.com>
Date: Fri, 17 Jan 2020 09:18:39 -0800
Subject: [PATCH 2/3] Add tests

---
 .../reference/varianceMeasurement.errors.txt  | 122 ++++++++++++
 .../reference/varianceMeasurement.js          |  83 ++++++++
 .../reference/varianceMeasurement.symbols     | 185 ++++++++++++++++++
 .../reference/varianceMeasurement.types       | 133 +++++++++++++
 tests/cases/compiler/varianceMeasurement.ts   |  64 ++++++
 5 files changed, 587 insertions(+)
 create mode 100644 tests/baselines/reference/varianceMeasurement.errors.txt
 create mode 100644 tests/baselines/reference/varianceMeasurement.js
 create mode 100644 tests/baselines/reference/varianceMeasurement.symbols
 create mode 100644 tests/baselines/reference/varianceMeasurement.types
 create mode 100644 tests/cases/compiler/varianceMeasurement.ts

diff --git a/tests/baselines/reference/varianceMeasurement.errors.txt b/tests/baselines/reference/varianceMeasurement.errors.txt
new file mode 100644
index 0000000000000..eb77b28564669
--- /dev/null
+++ b/tests/baselines/reference/varianceMeasurement.errors.txt
@@ -0,0 +1,122 @@
+tests/cases/compiler/varianceMeasurement.ts(10,7): error TS2322: Type 'Foo1<string>' is not assignable to type 'Foo1<"a">'.
+  Type 'string' is not assignable to type '"a"'.
+tests/cases/compiler/varianceMeasurement.ts(21,7): error TS2322: Type 'Foo2<string>' is not assignable to type 'Foo2<"a">'.
+  Types of property 'x' are incompatible.
+    Type 'string' is not assignable to type '"a"'.
+tests/cases/compiler/varianceMeasurement.ts(22,7): error TS2322: Type 'Foo2<string>' is not assignable to type 'Foo2<unknown>'.
+  The types of 'y.x' are incompatible between these types.
+    Type '(arg: string) => void' is not assignable to type '(arg: unknown) => void'.
+      Types of parameters 'arg' and 'arg' are incompatible.
+        Type 'unknown' is not assignable to type 'string'.
+tests/cases/compiler/varianceMeasurement.ts(33,7): error TS2322: Type 'Foo3<string>' is not assignable to type 'Foo3<"a">'.
+  Type 'string' is not assignable to type '"a"'.
+tests/cases/compiler/varianceMeasurement.ts(44,7): error TS2322: Type 'Foo4<string>' is not assignable to type 'Foo4<"a">'.
+  Types of property 'x' are incompatible.
+    Type 'string' is not assignable to type '"a"'.
+tests/cases/compiler/varianceMeasurement.ts(45,7): error TS2322: Type 'Foo4<string>' is not assignable to type 'Foo4<unknown>'.
+  The types of 'y.x' are incompatible between these types.
+    Type '(arg: string) => void' is not assignable to type '(arg: unknown) => void'.
+      Types of parameters 'arg' and 'arg' are incompatible.
+        Type 'unknown' is not assignable to type 'string'.
+tests/cases/compiler/varianceMeasurement.ts(57,7): error TS2322: Type 'Fn<string, number>' is not assignable to type 'Fn<unknown, number>'.
+  Type 'unknown' is not assignable to type 'string'.
+tests/cases/compiler/varianceMeasurement.ts(62,7): error TS2322: Type 'Fn<string, number>' is not assignable to type 'Fn<string, 0>'.
+  Type 'number' is not assignable to type '0'.
+
+
+==== tests/cases/compiler/varianceMeasurement.ts (8 errors) ====
+    // The type below should be invariant in T but is measured as covariant because
+    // we don't analyze recursive references.
+    
+    interface Foo1<T> {
+      x: T;
+      y: Foo1<(arg: T) => void>;
+    }
+    
+    declare const f10: Foo1<string>;
+    const f11: Foo1<'a'> = f10;
+          ~~~
+!!! error TS2322: Type 'Foo1<string>' is not assignable to type 'Foo1<"a">'.
+!!! error TS2322:   Type 'string' is not assignable to type '"a"'.
+    const f12: Foo1<unknown> = f10;
+    
+    // The type below is invariant in T and is measured as such.
+    
+    interface Foo2<T> {
+      x: T;
+      y: { x: (arg: T) => void, y: Foo2<(arg: T) => void>; }
+    }
+    
+    declare const f20: Foo2<string>;
+    const f21: Foo2<'a'> = f20;
+          ~~~
+!!! error TS2322: Type 'Foo2<string>' is not assignable to type 'Foo2<"a">'.
+!!! error TS2322:   Types of property 'x' are incompatible.
+!!! error TS2322:     Type 'string' is not assignable to type '"a"'.
+    const f22: Foo2<unknown> = f20;
+          ~~~
+!!! error TS2322: Type 'Foo2<string>' is not assignable to type 'Foo2<unknown>'.
+!!! error TS2322:   The types of 'y.x' are incompatible between these types.
+!!! error TS2322:     Type '(arg: string) => void' is not assignable to type '(arg: unknown) => void'.
+!!! error TS2322:       Types of parameters 'arg' and 'arg' are incompatible.
+!!! error TS2322:         Type 'unknown' is not assignable to type 'string'.
+    
+    // The type below should be invariant in T but is measured as covariant because
+    // we don't analyze recursive references.
+    
+    type Foo3<T> = {
+      x: T;
+      y: Foo3<(arg: T) => void>;
+    }
+    
+    declare const f30: Foo3<string>;
+    const f31: Foo3<'a'> = f30;
+          ~~~
+!!! error TS2322: Type 'Foo3<string>' is not assignable to type 'Foo3<"a">'.
+!!! error TS2322:   Type 'string' is not assignable to type '"a"'.
+    const f32: Foo3<unknown> = f30;
+    
+    // The type below is invariant in T and is measured as such.
+    
+    type Foo4<T> = {
+      x: T;
+      y: { x: (arg: T) => void, y: Foo4<(arg: T) => void>; }
+    }
+    
+    declare const f40: Foo4<string>;
+    const f41: Foo4<'a'> = f40;
+          ~~~
+!!! error TS2322: Type 'Foo4<string>' is not assignable to type 'Foo4<"a">'.
+!!! error TS2322:   Types of property 'x' are incompatible.
+!!! error TS2322:     Type 'string' is not assignable to type '"a"'.
+    const f42: Foo4<unknown> = f40;
+          ~~~
+!!! error TS2322: Type 'Foo4<string>' is not assignable to type 'Foo4<unknown>'.
+!!! error TS2322:   The types of 'y.x' are incompatible between these types.
+!!! error TS2322:     Type '(arg: string) => void' is not assignable to type '(arg: unknown) => void'.
+!!! error TS2322:       Types of parameters 'arg' and 'arg' are incompatible.
+!!! error TS2322:         Type 'unknown' is not assignable to type 'string'.
+    
+    // Repro from #3580
+    
+    interface Fn<A, B> {
+      (a: A): B;
+      then<C>(next: Fn<B, C>): Fn<A, C>;
+    }
+    
+    declare const fn: Fn<string, number>;
+    
+    // Contravariant in A
+    const fn1: Fn<unknown, number> = fn;  // Error
+          ~~~
+!!! error TS2322: Type 'Fn<string, number>' is not assignable to type 'Fn<unknown, number>'.
+!!! error TS2322:   Type 'unknown' is not assignable to type 'string'.
+    const fn2: Fn<'a', number> = fn;
+    
+    // Covariant in B
+    const fn3: Fn<string, unknown> = fn;
+    const fn4: Fn<string, 0> = fn;  // Error
+          ~~~
+!!! error TS2322: Type 'Fn<string, number>' is not assignable to type 'Fn<string, 0>'.
+!!! error TS2322:   Type 'number' is not assignable to type '0'.
+    
\ No newline at end of file
diff --git a/tests/baselines/reference/varianceMeasurement.js b/tests/baselines/reference/varianceMeasurement.js
new file mode 100644
index 0000000000000..b597e5d24639b
--- /dev/null
+++ b/tests/baselines/reference/varianceMeasurement.js
@@ -0,0 +1,83 @@
+//// [varianceMeasurement.ts]
+// The type below should be invariant in T but is measured as covariant because
+// we don't analyze recursive references.
+
+interface Foo1<T> {
+  x: T;
+  y: Foo1<(arg: T) => void>;
+}
+
+declare const f10: Foo1<string>;
+const f11: Foo1<'a'> = f10;
+const f12: Foo1<unknown> = f10;
+
+// The type below is invariant in T and is measured as such.
+
+interface Foo2<T> {
+  x: T;
+  y: { x: (arg: T) => void, y: Foo2<(arg: T) => void>; }
+}
+
+declare const f20: Foo2<string>;
+const f21: Foo2<'a'> = f20;
+const f22: Foo2<unknown> = f20;
+
+// The type below should be invariant in T but is measured as covariant because
+// we don't analyze recursive references.
+
+type Foo3<T> = {
+  x: T;
+  y: Foo3<(arg: T) => void>;
+}
+
+declare const f30: Foo3<string>;
+const f31: Foo3<'a'> = f30;
+const f32: Foo3<unknown> = f30;
+
+// The type below is invariant in T and is measured as such.
+
+type Foo4<T> = {
+  x: T;
+  y: { x: (arg: T) => void, y: Foo4<(arg: T) => void>; }
+}
+
+declare const f40: Foo4<string>;
+const f41: Foo4<'a'> = f40;
+const f42: Foo4<unknown> = f40;
+
+// Repro from #3580
+
+interface Fn<A, B> {
+  (a: A): B;
+  then<C>(next: Fn<B, C>): Fn<A, C>;
+}
+
+declare const fn: Fn<string, number>;
+
+// Contravariant in A
+const fn1: Fn<unknown, number> = fn;  // Error
+const fn2: Fn<'a', number> = fn;
+
+// Covariant in B
+const fn3: Fn<string, unknown> = fn;
+const fn4: Fn<string, 0> = fn;  // Error
+
+
+//// [varianceMeasurement.js]
+"use strict";
+// The type below should be invariant in T but is measured as covariant because
+// we don't analyze recursive references.
+var f11 = f10;
+var f12 = f10;
+var f21 = f20;
+var f22 = f20;
+var f31 = f30;
+var f32 = f30;
+var f41 = f40;
+var f42 = f40;
+// Contravariant in A
+var fn1 = fn; // Error
+var fn2 = fn;
+// Covariant in B
+var fn3 = fn;
+var fn4 = fn; // Error
diff --git a/tests/baselines/reference/varianceMeasurement.symbols b/tests/baselines/reference/varianceMeasurement.symbols
new file mode 100644
index 0000000000000..98d36fe16f91d
--- /dev/null
+++ b/tests/baselines/reference/varianceMeasurement.symbols
@@ -0,0 +1,185 @@
+=== tests/cases/compiler/varianceMeasurement.ts ===
+// The type below should be invariant in T but is measured as covariant because
+// we don't analyze recursive references.
+
+interface Foo1<T> {
+>Foo1 : Symbol(Foo1, Decl(varianceMeasurement.ts, 0, 0))
+>T : Symbol(T, Decl(varianceMeasurement.ts, 3, 15))
+
+  x: T;
+>x : Symbol(Foo1.x, Decl(varianceMeasurement.ts, 3, 19))
+>T : Symbol(T, Decl(varianceMeasurement.ts, 3, 15))
+
+  y: Foo1<(arg: T) => void>;
+>y : Symbol(Foo1.y, Decl(varianceMeasurement.ts, 4, 7))
+>Foo1 : Symbol(Foo1, Decl(varianceMeasurement.ts, 0, 0))
+>arg : Symbol(arg, Decl(varianceMeasurement.ts, 5, 11))
+>T : Symbol(T, Decl(varianceMeasurement.ts, 3, 15))
+}
+
+declare const f10: Foo1<string>;
+>f10 : Symbol(f10, Decl(varianceMeasurement.ts, 8, 13))
+>Foo1 : Symbol(Foo1, Decl(varianceMeasurement.ts, 0, 0))
+
+const f11: Foo1<'a'> = f10;
+>f11 : Symbol(f11, Decl(varianceMeasurement.ts, 9, 5))
+>Foo1 : Symbol(Foo1, Decl(varianceMeasurement.ts, 0, 0))
+>f10 : Symbol(f10, Decl(varianceMeasurement.ts, 8, 13))
+
+const f12: Foo1<unknown> = f10;
+>f12 : Symbol(f12, Decl(varianceMeasurement.ts, 10, 5))
+>Foo1 : Symbol(Foo1, Decl(varianceMeasurement.ts, 0, 0))
+>f10 : Symbol(f10, Decl(varianceMeasurement.ts, 8, 13))
+
+// The type below is invariant in T and is measured as such.
+
+interface Foo2<T> {
+>Foo2 : Symbol(Foo2, Decl(varianceMeasurement.ts, 10, 31))
+>T : Symbol(T, Decl(varianceMeasurement.ts, 14, 15))
+
+  x: T;
+>x : Symbol(Foo2.x, Decl(varianceMeasurement.ts, 14, 19))
+>T : Symbol(T, Decl(varianceMeasurement.ts, 14, 15))
+
+  y: { x: (arg: T) => void, y: Foo2<(arg: T) => void>; }
+>y : Symbol(Foo2.y, Decl(varianceMeasurement.ts, 15, 7))
+>x : Symbol(x, Decl(varianceMeasurement.ts, 16, 6))
+>arg : Symbol(arg, Decl(varianceMeasurement.ts, 16, 11))
+>T : Symbol(T, Decl(varianceMeasurement.ts, 14, 15))
+>y : Symbol(y, Decl(varianceMeasurement.ts, 16, 27))
+>Foo2 : Symbol(Foo2, Decl(varianceMeasurement.ts, 10, 31))
+>arg : Symbol(arg, Decl(varianceMeasurement.ts, 16, 37))
+>T : Symbol(T, Decl(varianceMeasurement.ts, 14, 15))
+}
+
+declare const f20: Foo2<string>;
+>f20 : Symbol(f20, Decl(varianceMeasurement.ts, 19, 13))
+>Foo2 : Symbol(Foo2, Decl(varianceMeasurement.ts, 10, 31))
+
+const f21: Foo2<'a'> = f20;
+>f21 : Symbol(f21, Decl(varianceMeasurement.ts, 20, 5))
+>Foo2 : Symbol(Foo2, Decl(varianceMeasurement.ts, 10, 31))
+>f20 : Symbol(f20, Decl(varianceMeasurement.ts, 19, 13))
+
+const f22: Foo2<unknown> = f20;
+>f22 : Symbol(f22, Decl(varianceMeasurement.ts, 21, 5))
+>Foo2 : Symbol(Foo2, Decl(varianceMeasurement.ts, 10, 31))
+>f20 : Symbol(f20, Decl(varianceMeasurement.ts, 19, 13))
+
+// The type below should be invariant in T but is measured as covariant because
+// we don't analyze recursive references.
+
+type Foo3<T> = {
+>Foo3 : Symbol(Foo3, Decl(varianceMeasurement.ts, 21, 31))
+>T : Symbol(T, Decl(varianceMeasurement.ts, 26, 10))
+
+  x: T;
+>x : Symbol(x, Decl(varianceMeasurement.ts, 26, 16))
+>T : Symbol(T, Decl(varianceMeasurement.ts, 26, 10))
+
+  y: Foo3<(arg: T) => void>;
+>y : Symbol(y, Decl(varianceMeasurement.ts, 27, 7))
+>Foo3 : Symbol(Foo3, Decl(varianceMeasurement.ts, 21, 31))
+>arg : Symbol(arg, Decl(varianceMeasurement.ts, 28, 11))
+>T : Symbol(T, Decl(varianceMeasurement.ts, 26, 10))
+}
+
+declare const f30: Foo3<string>;
+>f30 : Symbol(f30, Decl(varianceMeasurement.ts, 31, 13))
+>Foo3 : Symbol(Foo3, Decl(varianceMeasurement.ts, 21, 31))
+
+const f31: Foo3<'a'> = f30;
+>f31 : Symbol(f31, Decl(varianceMeasurement.ts, 32, 5))
+>Foo3 : Symbol(Foo3, Decl(varianceMeasurement.ts, 21, 31))
+>f30 : Symbol(f30, Decl(varianceMeasurement.ts, 31, 13))
+
+const f32: Foo3<unknown> = f30;
+>f32 : Symbol(f32, Decl(varianceMeasurement.ts, 33, 5))
+>Foo3 : Symbol(Foo3, Decl(varianceMeasurement.ts, 21, 31))
+>f30 : Symbol(f30, Decl(varianceMeasurement.ts, 31, 13))
+
+// The type below is invariant in T and is measured as such.
+
+type Foo4<T> = {
+>Foo4 : Symbol(Foo4, Decl(varianceMeasurement.ts, 33, 31))
+>T : Symbol(T, Decl(varianceMeasurement.ts, 37, 10))
+
+  x: T;
+>x : Symbol(x, Decl(varianceMeasurement.ts, 37, 16))
+>T : Symbol(T, Decl(varianceMeasurement.ts, 37, 10))
+
+  y: { x: (arg: T) => void, y: Foo4<(arg: T) => void>; }
+>y : Symbol(y, Decl(varianceMeasurement.ts, 38, 7))
+>x : Symbol(x, Decl(varianceMeasurement.ts, 39, 6))
+>arg : Symbol(arg, Decl(varianceMeasurement.ts, 39, 11))
+>T : Symbol(T, Decl(varianceMeasurement.ts, 37, 10))
+>y : Symbol(y, Decl(varianceMeasurement.ts, 39, 27))
+>Foo4 : Symbol(Foo4, Decl(varianceMeasurement.ts, 33, 31))
+>arg : Symbol(arg, Decl(varianceMeasurement.ts, 39, 37))
+>T : Symbol(T, Decl(varianceMeasurement.ts, 37, 10))
+}
+
+declare const f40: Foo4<string>;
+>f40 : Symbol(f40, Decl(varianceMeasurement.ts, 42, 13))
+>Foo4 : Symbol(Foo4, Decl(varianceMeasurement.ts, 33, 31))
+
+const f41: Foo4<'a'> = f40;
+>f41 : Symbol(f41, Decl(varianceMeasurement.ts, 43, 5))
+>Foo4 : Symbol(Foo4, Decl(varianceMeasurement.ts, 33, 31))
+>f40 : Symbol(f40, Decl(varianceMeasurement.ts, 42, 13))
+
+const f42: Foo4<unknown> = f40;
+>f42 : Symbol(f42, Decl(varianceMeasurement.ts, 44, 5))
+>Foo4 : Symbol(Foo4, Decl(varianceMeasurement.ts, 33, 31))
+>f40 : Symbol(f40, Decl(varianceMeasurement.ts, 42, 13))
+
+// Repro from #3580
+
+interface Fn<A, B> {
+>Fn : Symbol(Fn, Decl(varianceMeasurement.ts, 44, 31))
+>A : Symbol(A, Decl(varianceMeasurement.ts, 48, 13))
+>B : Symbol(B, Decl(varianceMeasurement.ts, 48, 15))
+
+  (a: A): B;
+>a : Symbol(a, Decl(varianceMeasurement.ts, 49, 3))
+>A : Symbol(A, Decl(varianceMeasurement.ts, 48, 13))
+>B : Symbol(B, Decl(varianceMeasurement.ts, 48, 15))
+
+  then<C>(next: Fn<B, C>): Fn<A, C>;
+>then : Symbol(Fn.then, Decl(varianceMeasurement.ts, 49, 12))
+>C : Symbol(C, Decl(varianceMeasurement.ts, 50, 7))
+>next : Symbol(next, Decl(varianceMeasurement.ts, 50, 10))
+>Fn : Symbol(Fn, Decl(varianceMeasurement.ts, 44, 31))
+>B : Symbol(B, Decl(varianceMeasurement.ts, 48, 15))
+>C : Symbol(C, Decl(varianceMeasurement.ts, 50, 7))
+>Fn : Symbol(Fn, Decl(varianceMeasurement.ts, 44, 31))
+>A : Symbol(A, Decl(varianceMeasurement.ts, 48, 13))
+>C : Symbol(C, Decl(varianceMeasurement.ts, 50, 7))
+}
+
+declare const fn: Fn<string, number>;
+>fn : Symbol(fn, Decl(varianceMeasurement.ts, 53, 13))
+>Fn : Symbol(Fn, Decl(varianceMeasurement.ts, 44, 31))
+
+// Contravariant in A
+const fn1: Fn<unknown, number> = fn;  // Error
+>fn1 : Symbol(fn1, Decl(varianceMeasurement.ts, 56, 5))
+>Fn : Symbol(Fn, Decl(varianceMeasurement.ts, 44, 31))
+>fn : Symbol(fn, Decl(varianceMeasurement.ts, 53, 13))
+
+const fn2: Fn<'a', number> = fn;
+>fn2 : Symbol(fn2, Decl(varianceMeasurement.ts, 57, 5))
+>Fn : Symbol(Fn, Decl(varianceMeasurement.ts, 44, 31))
+>fn : Symbol(fn, Decl(varianceMeasurement.ts, 53, 13))
+
+// Covariant in B
+const fn3: Fn<string, unknown> = fn;
+>fn3 : Symbol(fn3, Decl(varianceMeasurement.ts, 60, 5))
+>Fn : Symbol(Fn, Decl(varianceMeasurement.ts, 44, 31))
+>fn : Symbol(fn, Decl(varianceMeasurement.ts, 53, 13))
+
+const fn4: Fn<string, 0> = fn;  // Error
+>fn4 : Symbol(fn4, Decl(varianceMeasurement.ts, 61, 5))
+>Fn : Symbol(Fn, Decl(varianceMeasurement.ts, 44, 31))
+>fn : Symbol(fn, Decl(varianceMeasurement.ts, 53, 13))
+
diff --git a/tests/baselines/reference/varianceMeasurement.types b/tests/baselines/reference/varianceMeasurement.types
new file mode 100644
index 0000000000000..83be34759f1ed
--- /dev/null
+++ b/tests/baselines/reference/varianceMeasurement.types
@@ -0,0 +1,133 @@
+=== tests/cases/compiler/varianceMeasurement.ts ===
+// The type below should be invariant in T but is measured as covariant because
+// we don't analyze recursive references.
+
+interface Foo1<T> {
+  x: T;
+>x : T
+
+  y: Foo1<(arg: T) => void>;
+>y : Foo1<(arg: T) => void>
+>arg : T
+}
+
+declare const f10: Foo1<string>;
+>f10 : Foo1<string>
+
+const f11: Foo1<'a'> = f10;
+>f11 : Foo1<"a">
+>f10 : Foo1<string>
+
+const f12: Foo1<unknown> = f10;
+>f12 : Foo1<unknown>
+>f10 : Foo1<string>
+
+// The type below is invariant in T and is measured as such.
+
+interface Foo2<T> {
+  x: T;
+>x : T
+
+  y: { x: (arg: T) => void, y: Foo2<(arg: T) => void>; }
+>y : { x: (arg: T) => void; y: Foo2<(arg: T) => void>; }
+>x : (arg: T) => void
+>arg : T
+>y : Foo2<(arg: T) => void>
+>arg : T
+}
+
+declare const f20: Foo2<string>;
+>f20 : Foo2<string>
+
+const f21: Foo2<'a'> = f20;
+>f21 : Foo2<"a">
+>f20 : Foo2<string>
+
+const f22: Foo2<unknown> = f20;
+>f22 : Foo2<unknown>
+>f20 : Foo2<string>
+
+// The type below should be invariant in T but is measured as covariant because
+// we don't analyze recursive references.
+
+type Foo3<T> = {
+>Foo3 : Foo3<T>
+
+  x: T;
+>x : T
+
+  y: Foo3<(arg: T) => void>;
+>y : Foo3<(arg: T) => void>
+>arg : T
+}
+
+declare const f30: Foo3<string>;
+>f30 : Foo3<string>
+
+const f31: Foo3<'a'> = f30;
+>f31 : Foo3<"a">
+>f30 : Foo3<string>
+
+const f32: Foo3<unknown> = f30;
+>f32 : Foo3<unknown>
+>f30 : Foo3<string>
+
+// The type below is invariant in T and is measured as such.
+
+type Foo4<T> = {
+>Foo4 : Foo4<T>
+
+  x: T;
+>x : T
+
+  y: { x: (arg: T) => void, y: Foo4<(arg: T) => void>; }
+>y : { x: (arg: T) => void; y: Foo4<(arg: T) => void>; }
+>x : (arg: T) => void
+>arg : T
+>y : Foo4<(arg: T) => void>
+>arg : T
+}
+
+declare const f40: Foo4<string>;
+>f40 : Foo4<string>
+
+const f41: Foo4<'a'> = f40;
+>f41 : Foo4<"a">
+>f40 : Foo4<string>
+
+const f42: Foo4<unknown> = f40;
+>f42 : Foo4<unknown>
+>f40 : Foo4<string>
+
+// Repro from #3580
+
+interface Fn<A, B> {
+  (a: A): B;
+>a : A
+
+  then<C>(next: Fn<B, C>): Fn<A, C>;
+>then : <C>(next: Fn<B, C>) => Fn<A, C>
+>next : Fn<B, C>
+}
+
+declare const fn: Fn<string, number>;
+>fn : Fn<string, number>
+
+// Contravariant in A
+const fn1: Fn<unknown, number> = fn;  // Error
+>fn1 : Fn<unknown, number>
+>fn : Fn<string, number>
+
+const fn2: Fn<'a', number> = fn;
+>fn2 : Fn<"a", number>
+>fn : Fn<string, number>
+
+// Covariant in B
+const fn3: Fn<string, unknown> = fn;
+>fn3 : Fn<string, unknown>
+>fn : Fn<string, number>
+
+const fn4: Fn<string, 0> = fn;  // Error
+>fn4 : Fn<string, 0>
+>fn : Fn<string, number>
+
diff --git a/tests/cases/compiler/varianceMeasurement.ts b/tests/cases/compiler/varianceMeasurement.ts
new file mode 100644
index 0000000000000..6c327291d97a2
--- /dev/null
+++ b/tests/cases/compiler/varianceMeasurement.ts
@@ -0,0 +1,64 @@
+// @strict: true
+
+// The type below should be invariant in T but is measured as covariant because
+// we don't analyze recursive references.
+
+interface Foo1<T> {
+  x: T;
+  y: Foo1<(arg: T) => void>;
+}
+
+declare const f10: Foo1<string>;
+const f11: Foo1<'a'> = f10;
+const f12: Foo1<unknown> = f10;
+
+// The type below is invariant in T and is measured as such.
+
+interface Foo2<T> {
+  x: T;
+  y: { x: (arg: T) => void, y: Foo2<(arg: T) => void>; }
+}
+
+declare const f20: Foo2<string>;
+const f21: Foo2<'a'> = f20;
+const f22: Foo2<unknown> = f20;
+
+// The type below should be invariant in T but is measured as covariant because
+// we don't analyze recursive references.
+
+type Foo3<T> = {
+  x: T;
+  y: Foo3<(arg: T) => void>;
+}
+
+declare const f30: Foo3<string>;
+const f31: Foo3<'a'> = f30;
+const f32: Foo3<unknown> = f30;
+
+// The type below is invariant in T and is measured as such.
+
+type Foo4<T> = {
+  x: T;
+  y: { x: (arg: T) => void, y: Foo4<(arg: T) => void>; }
+}
+
+declare const f40: Foo4<string>;
+const f41: Foo4<'a'> = f40;
+const f42: Foo4<unknown> = f40;
+
+// Repro from #3580
+
+interface Fn<A, B> {
+  (a: A): B;
+  then<C>(next: Fn<B, C>): Fn<A, C>;
+}
+
+declare const fn: Fn<string, number>;
+
+// Contravariant in A
+const fn1: Fn<unknown, number> = fn;  // Error
+const fn2: Fn<'a', number> = fn;
+
+// Covariant in B
+const fn3: Fn<string, unknown> = fn;
+const fn4: Fn<string, 0> = fn;  // Error

From b4536062ec1f5011242ef0db4a3f4cb35e8e6853 Mon Sep 17 00:00:00 2001
From: Anders Hejlsberg <andersh@microsoft.com>
Date: Fri, 17 Jan 2020 09:27:13 -0800
Subject: [PATCH 3/3] Accept new baselines

---
 ...eckInfiniteExpansionTermination.errors.txt | 32 -------------------
 .../recursiveTypeComparison.errors.txt        | 27 ----------------
 2 files changed, 59 deletions(-)
 delete mode 100644 tests/baselines/reference/checkInfiniteExpansionTermination.errors.txt
 delete mode 100644 tests/baselines/reference/recursiveTypeComparison.errors.txt

diff --git a/tests/baselines/reference/checkInfiniteExpansionTermination.errors.txt b/tests/baselines/reference/checkInfiniteExpansionTermination.errors.txt
deleted file mode 100644
index 014087ff6275e..0000000000000
--- a/tests/baselines/reference/checkInfiniteExpansionTermination.errors.txt
+++ /dev/null
@@ -1,32 +0,0 @@
-tests/cases/compiler/checkInfiniteExpansionTermination.ts(16,1): error TS2322: Type 'ISubject<Bar>' is not assignable to type 'IObservable<Foo>'.
-  Types of property 'n' are incompatible.
-    Type 'IObservable<Bar[]>' is not assignable to type 'IObservable<Foo[]>'.
-      Type 'Bar[]' is not assignable to type 'Foo[]'.
-        Property 'x' is missing in type 'Bar' but required in type 'Foo'.
-
-
-==== tests/cases/compiler/checkInfiniteExpansionTermination.ts (1 errors) ====
-    // Regression test for #1002
-    // Before fix this code would cause infinite loop
-    
-    interface IObservable<T> {
-        n: IObservable<T[]>; // Needed, must be T[]
-    }
-    
-    // Needed
-    interface ISubject<T> extends IObservable<T> { }
-    
-    interface Foo { x }
-    interface Bar { y }
-    
-    var values: IObservable<Foo>;
-    var values2: ISubject<Bar>;
-    values = values2;
-    ~~~~~~
-!!! error TS2322: Type 'ISubject<Bar>' is not assignable to type 'IObservable<Foo>'.
-!!! error TS2322:   Types of property 'n' are incompatible.
-!!! error TS2322:     Type 'IObservable<Bar[]>' is not assignable to type 'IObservable<Foo[]>'.
-!!! error TS2322:       Type 'Bar[]' is not assignable to type 'Foo[]'.
-!!! error TS2322:         Property 'x' is missing in type 'Bar' but required in type 'Foo'.
-!!! related TS2728 tests/cases/compiler/checkInfiniteExpansionTermination.ts:11:17: 'x' is declared here.
-    
\ No newline at end of file
diff --git a/tests/baselines/reference/recursiveTypeComparison.errors.txt b/tests/baselines/reference/recursiveTypeComparison.errors.txt
deleted file mode 100644
index f647763b2e275..0000000000000
--- a/tests/baselines/reference/recursiveTypeComparison.errors.txt
+++ /dev/null
@@ -1,27 +0,0 @@
-tests/cases/compiler/recursiveTypeComparison.ts(14,5): error TS2322: Type 'Observable<{}>' is not assignable to type 'Property<number>'.
-  Types of property 'needThisOne' are incompatible.
-    Type 'Observable<{}>' is not assignable to type 'Observable<number>'.
-      Type '{}' is not assignable to type 'number'.
-
-
-==== tests/cases/compiler/recursiveTypeComparison.ts (1 errors) ====
-    // Before fix this would take an exceeding long time to complete (#1170)
-    
-    interface Observable<T> {
-        // This member can't be of type T, Property<T>, or Observable<anything but T>
-        needThisOne: Observable<T>;
-        // Add more to make it slower
-        expo1: Property<T[]>; //  0.31 seconds in check
-        expo2: Property<T[]>; //  3.11 seconds
-        expo3: Property<T[]>; // 82.28 seconds
-    }
-    interface Property<T> extends Observable<T> { }
-    
-    var p: Observable<{}>;
-    var stuck: Property<number> = p;
-        ~~~~~
-!!! error TS2322: Type 'Observable<{}>' is not assignable to type 'Property<number>'.
-!!! error TS2322:   Types of property 'needThisOne' are incompatible.
-!!! error TS2322:     Type 'Observable<{}>' is not assignable to type 'Observable<number>'.
-!!! error TS2322:       Type '{}' is not assignable to type 'number'.
-    
\ No newline at end of file