Fix union type parameter inference

src/compiler/checker.ts

@@ -5073,9 +5073,6 @@ namespace ts {
                     }
                     return objectTypeRelatedTo(source, source, target, /*reportErrors*/ false);
                 }
-                if (source.flags & TypeFlags.TypeParameter && target.flags & TypeFlags.TypeParameter) {
-                    return typeParameterIdenticalTo(<TypeParameter>source, <TypeParameter>target);
-                }
                 if (source.flags & TypeFlags.Union && target.flags & TypeFlags.Union ||
                     source.flags & TypeFlags.Intersection && target.flags & TypeFlags.Intersection) {
                     if (result = eachTypeRelatedToSomeType(<UnionOrIntersectionType>source, <UnionOrIntersectionType>target)) {
@@ -5206,17 +5203,6 @@ namespace ts {
                 return result;
             }
 
-            function typeParameterIdenticalTo(source: TypeParameter, target: TypeParameter): Ternary {
-                // covers case when both type parameters does not have constraint (both equal to noConstraintType)
-                if (source.constraint === target.constraint) {
-                    return Ternary.True;
-                }
-                if (source.constraint === noConstraintType || target.constraint === noConstraintType) {
-                    return Ternary.False;
-                }
-                return isIdenticalTo(source.constraint, target.constraint);
-            }
-
             // Determine if two object types are related by structure. First, check if the result is already available in the global cache.
             // Second, check if we have already started a comparison of the given two types in which case we assume the result to be true.
             // Third, check if both types are part of deeply nested chains of generic type instantiations and if so assume the types are
@@ -5765,26 +5751,19 @@ namespace ts {
             if (!(isMatchingSignature(source, target, partialMatch))) {
                 return Ternary.False;
             }
-            let result = Ternary.True;
-            if (source.typeParameters && target.typeParameters) {
-                if (source.typeParameters.length !== target.typeParameters.length) {
-                    return Ternary.False;
-                }
-                for (let i = 0, len = source.typeParameters.length; i < len; ++i) {
-                    const related = compareTypes(source.typeParameters[i], target.typeParameters[i]);
-                    if (!related) {
-                        return Ternary.False;
-                    }
-                    result &= related;
-                }
-            }
-            else if (source.typeParameters || target.typeParameters) {
+            // Check that the two signatures have the same number of type parameters. We might consider
+            // also checking that any type parameter constraints match, but that would require instantiating
+            // the constraints with a common set of type arguments to get relatable entities in places where
+            // type parameters occur in the constraints. The complexity of doing that doesn't seem worthwhile,
+            // particularly as we're comparing erased versions of the signatures below.
+            if ((source.typeParameters ? source.typeParameters.length : 0) !== (target.typeParameters ? target.typeParameters.length : 0)) {
                 return Ternary.False;
             }
             // Spec 1.0 Section 3.8.3 & 3.8.4:
             // M and N (the signatures) are instantiated using type Any as the type argument for all type parameters declared by M and N
             source = getErasedSignature(source);
             target = getErasedSignature(target);
+            let result = Ternary.True;
             const targetLen = target.parameters.length;
             for (let i = 0; i < targetLen; i++) {
                 const s = isRestParameterIndex(source, i) ? getRestTypeOfSignature(source) : getTypeOfSymbol(source.parameters[i]);

tests/baselines/reference/genericSignatureIdentity.js

@@ -0,0 +1,32 @@
+//// [genericSignatureIdentity.ts]
+// This test is here to remind us of our current limits of type identity checking.
+// Ideally all of the below declarations would be considered different (and thus errors)
+// but they aren't because we erase type parameters to type any and don't check that
+// constraints are identical.
+
+var x: {
+    <T extends Date>(x: T): T;
+};
+
+var x: {
+    <T extends number>(x: T): T;
+};
+
+var x: {
+    <T>(x: T): T;
+};
+
+var x: {
+    <T>(x: any): any;
+};
+
+
+//// [genericSignatureIdentity.js]
+// This test is here to remind us of our current limits of type identity checking.
+// Ideally all of the below declarations would be considered different (and thus errors)
+// but they aren't because we erase type parameters to type any and don't check that
+// constraints are identical.
+var x;
+var x;
+var x;
+var x;

tests/baselines/reference/genericSignatureIdentity.symbols

@@ -0,0 +1,49 @@
+=== tests/cases/compiler/genericSignatureIdentity.ts ===
+// This test is here to remind us of our current limits of type identity checking.
+// Ideally all of the below declarations would be considered different (and thus errors)
+// but they aren't because we erase type parameters to type any and don't check that
+// constraints are identical.
+
+var x: {
+>x : Symbol(x, Decl(genericSignatureIdentity.ts, 5, 3), Decl(genericSignatureIdentity.ts, 9, 3), Decl(genericSignatureIdentity.ts, 13, 3), Decl(genericSignatureIdentity.ts, 17, 3))
+
+    <T extends Date>(x: T): T;
+>T : Symbol(T, Decl(genericSignatureIdentity.ts, 6, 5))
+>Date : Symbol(Date, Decl(lib.d.ts, --, --), Decl(lib.d.ts, --, --))
+>x : Symbol(x, Decl(genericSignatureIdentity.ts, 6, 21))
+>T : Symbol(T, Decl(genericSignatureIdentity.ts, 6, 5))
+>T : Symbol(T, Decl(genericSignatureIdentity.ts, 6, 5))
+
+};
+
+var x: {
+>x : Symbol(x, Decl(genericSignatureIdentity.ts, 5, 3), Decl(genericSignatureIdentity.ts, 9, 3), Decl(genericSignatureIdentity.ts, 13, 3), Decl(genericSignatureIdentity.ts, 17, 3))
+
+    <T extends number>(x: T): T;
+>T : Symbol(T, Decl(genericSignatureIdentity.ts, 10, 5))
+>x : Symbol(x, Decl(genericSignatureIdentity.ts, 10, 23))
+>T : Symbol(T, Decl(genericSignatureIdentity.ts, 10, 5))
+>T : Symbol(T, Decl(genericSignatureIdentity.ts, 10, 5))
+
+};
+
+var x: {
+>x : Symbol(x, Decl(genericSignatureIdentity.ts, 5, 3), Decl(genericSignatureIdentity.ts, 9, 3), Decl(genericSignatureIdentity.ts, 13, 3), Decl(genericSignatureIdentity.ts, 17, 3))
+
+    <T>(x: T): T;
+>T : Symbol(T, Decl(genericSignatureIdentity.ts, 14, 5))
+>x : Symbol(x, Decl(genericSignatureIdentity.ts, 14, 8))
+>T : Symbol(T, Decl(genericSignatureIdentity.ts, 14, 5))
+>T : Symbol(T, Decl(genericSignatureIdentity.ts, 14, 5))
+
+};
+
+var x: {
+>x : Symbol(x, Decl(genericSignatureIdentity.ts, 5, 3), Decl(genericSignatureIdentity.ts, 9, 3), Decl(genericSignatureIdentity.ts, 13, 3), Decl(genericSignatureIdentity.ts, 17, 3))
+
+    <T>(x: any): any;
+>T : Symbol(T, Decl(genericSignatureIdentity.ts, 18, 5))
+>x : Symbol(x, Decl(genericSignatureIdentity.ts, 18, 8))
+
+};
+

tests/baselines/reference/genericSignatureIdentity.types

@@ -0,0 +1,49 @@
+=== tests/cases/compiler/genericSignatureIdentity.ts ===
+// This test is here to remind us of our current limits of type identity checking.
+// Ideally all of the below declarations would be considered different (and thus errors)
+// but they aren't because we erase type parameters to type any and don't check that
+// constraints are identical.
+
+var x: {
+>x : <T extends Date>(x: T) => T
+
+    <T extends Date>(x: T): T;
+>T : T
+>Date : Date
+>x : T
+>T : T
+>T : T
+
+};
+
+var x: {
+>x : <T extends Date>(x: T) => T
+
+    <T extends number>(x: T): T;
+>T : T
+>x : T
+>T : T
+>T : T
+
+};
+
+var x: {
+>x : <T extends Date>(x: T) => T
+
+    <T>(x: T): T;
+>T : T
+>x : T
+>T : T
+>T : T
+
+};
+
+var x: {
+>x : <T extends Date>(x: T) => T
+
+    <T>(x: any): any;
+>T : T
+>x : any
+
+};
+

tests/baselines/reference/unionTypeParameterInference.js

@@ -0,0 +1,17 @@
+//// [unionTypeParameterInference.ts]
+// Regression test for #5861
+
+interface Foo<T> { prop: T; }
+
+declare function lift<U>(value: U | Foo<U>): Foo<U>;
+
+function unlift<U>(value: U | Foo<U>): U {
+    return lift(value).prop;
+}
+
+
+//// [unionTypeParameterInference.js]
+// Regression test for #5861
+function unlift(value) {
+    return lift(value).prop;
+}

tests/baselines/reference/unionTypeParameterInference.symbols

@@ -0,0 +1,35 @@
+=== tests/cases/compiler/unionTypeParameterInference.ts ===
+// Regression test for #5861
+
+interface Foo<T> { prop: T; }
+>Foo : Symbol(Foo, Decl(unionTypeParameterInference.ts, 0, 0))
+>T : Symbol(T, Decl(unionTypeParameterInference.ts, 2, 14))
+>prop : Symbol(prop, Decl(unionTypeParameterInference.ts, 2, 18))
+>T : Symbol(T, Decl(unionTypeParameterInference.ts, 2, 14))
+
+declare function lift<U>(value: U | Foo<U>): Foo<U>;
+>lift : Symbol(lift, Decl(unionTypeParameterInference.ts, 2, 29))
+>U : Symbol(U, Decl(unionTypeParameterInference.ts, 4, 22))
+>value : Symbol(value, Decl(unionTypeParameterInference.ts, 4, 25))
+>U : Symbol(U, Decl(unionTypeParameterInference.ts, 4, 22))
+>Foo : Symbol(Foo, Decl(unionTypeParameterInference.ts, 0, 0))
+>U : Symbol(U, Decl(unionTypeParameterInference.ts, 4, 22))
+>Foo : Symbol(Foo, Decl(unionTypeParameterInference.ts, 0, 0))
+>U : Symbol(U, Decl(unionTypeParameterInference.ts, 4, 22))
+
+function unlift<U>(value: U | Foo<U>): U {
+>unlift : Symbol(unlift, Decl(unionTypeParameterInference.ts, 4, 52))
+>U : Symbol(U, Decl(unionTypeParameterInference.ts, 6, 16))
+>value : Symbol(value, Decl(unionTypeParameterInference.ts, 6, 19))
+>U : Symbol(U, Decl(unionTypeParameterInference.ts, 6, 16))
+>Foo : Symbol(Foo, Decl(unionTypeParameterInference.ts, 0, 0))
+>U : Symbol(U, Decl(unionTypeParameterInference.ts, 6, 16))
+>U : Symbol(U, Decl(unionTypeParameterInference.ts, 6, 16))
+
+    return lift(value).prop;
+>lift(value).prop : Symbol(Foo.prop, Decl(unionTypeParameterInference.ts, 2, 18))
+>lift : Symbol(lift, Decl(unionTypeParameterInference.ts, 2, 29))
+>value : Symbol(value, Decl(unionTypeParameterInference.ts, 6, 19))
+>prop : Symbol(Foo.prop, Decl(unionTypeParameterInference.ts, 2, 18))
+}
+

tests/baselines/reference/unionTypeParameterInference.types

@@ -0,0 +1,36 @@
+=== tests/cases/compiler/unionTypeParameterInference.ts ===
+// Regression test for #5861
+
+interface Foo<T> { prop: T; }
+>Foo : Foo<T>
+>T : T
+>prop : T
+>T : T
+
+declare function lift<U>(value: U | Foo<U>): Foo<U>;
+>lift : <U>(value: U | Foo<U>) => Foo<U>
+>U : U
+>value : U | Foo<U>
+>U : U
+>Foo : Foo<T>
+>U : U
+>Foo : Foo<T>
+>U : U
+
+function unlift<U>(value: U | Foo<U>): U {
+>unlift : <U>(value: U | Foo<U>) => U
+>U : U
+>value : U | Foo<U>
+>U : U
+>Foo : Foo<T>
+>U : U
+>U : U
+
+    return lift(value).prop;
+>lift(value).prop : U
+>lift(value) : Foo<U>
+>lift : <U>(value: U | Foo<U>) => Foo<U>
+>value : U | Foo<U>
+>prop : U
+}
+

tests/cases/compiler/genericSignatureIdentity.ts

@@ -0,0 +1,20 @@
+// This test is here to remind us of our current limits of type identity checking.
+// Ideally all of the below declarations would be considered different (and thus errors)
+// but they aren't because we erase type parameters to type any and don't check that
+// constraints are identical.
+
+var x: {
+    <T extends Date>(x: T): T;
+};
+
+var x: {
+    <T extends number>(x: T): T;
+};
+
+var x: {
+    <T>(x: T): T;
+};
+
+var x: {
+    <T>(x: any): any;
+};

tests/cases/compiler/unionTypeParameterInference.ts

@@ -0,0 +1,9 @@
+// Regression test for #5861
+
+interface Foo<T> { prop: T; }
+
+declare function lift<U>(value: U | Foo<U>): Foo<U>;
+
+function unlift<U>(value: U | Foo<U>): U {
+    return lift(value).prop;
+}