Don't eagerly simplify reducible generic union index types

src/compiler/checker.ts

@@ -814,6 +814,8 @@ namespace ts {
 
         const restrictiveMapper: TypeMapper = makeFunctionTypeMapper(t => t.flags & TypeFlags.TypeParameter ? getRestrictiveTypeParameter(t as TypeParameter) : t);
         const permissiveMapper: TypeMapper = makeFunctionTypeMapper(t => t.flags & TypeFlags.TypeParameter ? wildcardType : t);
+        const uniqueLiteralType = createIntrinsicType(TypeFlags.Never, "never"); // `uniqueLiteralType` is a special `never` flagged by union reduction to behave as a literal
+        const uniqueLiteralMapper: TypeMapper = makeFunctionTypeMapper(t => t.flags & TypeFlags.TypeParameter ? uniqueLiteralType : t); // replace all type parameters with the unique literal type (disregarding constraints)
 
         const emptyObjectType = createAnonymousType(undefined, emptySymbols, emptyArray, emptyArray, emptyArray);
         const emptyJsxObjectType = createAnonymousType(undefined, emptySymbols, emptyArray, emptyArray, emptyArray);
@@ -12367,10 +12369,10 @@ namespace ts {
                 else if (type !== firstType) {
                     checkFlags |= CheckFlags.HasNonUniformType;
                 }
-                if (isLiteralType(type) || isPatternLiteralType(type)) {
+                if (isLiteralType(type) || isPatternLiteralType(type) || type === uniqueLiteralType) {
                     checkFlags |= CheckFlags.HasLiteralType;
                 }
-                if (type.flags & TypeFlags.Never) {
+                if (type.flags & TypeFlags.Never && type !== uniqueLiteralType) {
                     checkFlags |= CheckFlags.HasNeverType;
                 }
                 propTypes.push(type);
@@ -15124,9 +15126,24 @@ namespace ts {
                 /*aliasSymbol*/ undefined, /*aliasTypeArguments*/ undefined, origin);
         }
 
+        /**
+         * A union type which is reducible upon instantiation (meaning some members are removed under certain instantiations)
+         * must be kept generic, as that instantiation information needs to flow through the type system. By replacing all
+         * type parameters in the union with a special never type that is treated as a literal in `getReducedType`, we can cause the `getReducedType` logic
+         * to reduce the resulting type if possible (since only intersections with conflicting literal-typed properties are reducible).
+         */
+        function isPossiblyReducibleByInstantiation(type: UnionType): boolean {
+            return some(type.types, t => {
+                const uniqueFilled = getUniqueLiteralFilledInstantiation(t);
+                return getReducedType(uniqueFilled) !== uniqueFilled;
+            });
+        }
+
         function getIndexType(type: Type, stringsOnly = keyofStringsOnly, noIndexSignatures?: boolean): Type {
             type = getReducedType(type);
-            return type.flags & TypeFlags.Union ? getIntersectionType(map((type as UnionType).types, t => getIndexType(t, stringsOnly, noIndexSignatures))) :
+            return type.flags & TypeFlags.Union ? isPossiblyReducibleByInstantiation(type as UnionType)
+                    ? getIndexTypeForGenericType(type as InstantiableType | UnionOrIntersectionType, stringsOnly)
+                    : getIntersectionType(map((type as UnionType).types, t => getIndexType(t, stringsOnly, noIndexSignatures))) :
                 type.flags & TypeFlags.Intersection ? getUnionType(map((type as IntersectionType).types, t => getIndexType(t, stringsOnly, noIndexSignatures))) :
                 type.flags & TypeFlags.InstantiableNonPrimitive || isGenericTupleType(type) || isGenericMappedType(type) && !hasDistributiveNameType(type) ? getIndexTypeForGenericType(type as InstantiableType | UnionOrIntersectionType, stringsOnly) :
                 getObjectFlags(type) & ObjectFlags.Mapped ? getIndexTypeForMappedType(type as MappedType, stringsOnly, noIndexSignatures) :
@@ -17070,6 +17087,11 @@ namespace ts {
             return type; // Nested invocation of `inferTypeForHomomorphicMappedType` or the `source` instantiated into something unmappable
         }
 
+        function getUniqueLiteralFilledInstantiation(type: Type) {
+            return type.flags & (TypeFlags.Primitive | TypeFlags.AnyOrUnknown | TypeFlags.Never) ? type :
+                type.uniqueLiteralFilledInstantiation || (type.uniqueLiteralFilledInstantiation = instantiateType(type, uniqueLiteralMapper));
+        }
+
         function getPermissiveInstantiation(type: Type) {
             return type.flags & (TypeFlags.Primitive | TypeFlags.AnyOrUnknown | TypeFlags.Never) ? type :
                 type.permissiveInstantiation || (type.permissiveInstantiation = instantiateType(type, permissiveMapper));

src/compiler/types.ts

@@ -5244,6 +5244,8 @@ namespace ts {
         /* @internal */
         restrictiveInstantiation?: Type; // Instantiation with type parameters mapped to unconstrained form
         /* @internal */
+        uniqueLiteralFilledInstantiation?: Type;  // Instantiation with type parameters mapped to never type
+        /* @internal */
         immediateBaseConstraint?: Type;  // Immediate base constraint cache
         /* @internal */
         widened?: Type; // Cached widened form of the type

tests/baselines/reference/mappedTypeNotMistakenlyHomomorphic.errors.txt

@@ -0,0 +1,46 @@
+tests/cases/compiler/mappedTypeNotMistakenlyHomomorphic.ts(33,1): error TS2741: Property 'a' is missing in type 'Gen2<ABC.B>' but required in type 'Gen2<ABC.A>'.
+tests/cases/compiler/mappedTypeNotMistakenlyHomomorphic.ts(34,1): error TS2741: Property 'b' is missing in type 'Gen2<ABC.A>' but required in type 'Gen2<ABC.B>'.
+
+
+==== tests/cases/compiler/mappedTypeNotMistakenlyHomomorphic.ts (2 errors) ====
+    enum ABC { A, B }
+
+    type Gen<T extends ABC> = { v: T; } & (
+      {
+        v: ABC.A,
+        a: string,
+      } | {
+        v: ABC.B,
+        b: string,
+      }
+    )
+
+    // Quick info: ???
+    //
+    // type Gen2<T extends ABC> = {
+    //    v: string;
+    // }
+    //
+    type Gen2<T extends ABC> = {
+      [Property in keyof Gen<T>]: string;
+    };
+
+    // 'a' and 'b' properties required !?!?
+    const gen2TypeA: Gen2<ABC.A> = { v:  "I am A", a: "" };
+    const gen2TypeB: Gen2<ABC.B> = { v:  "I am B", b: "" };
+
+    // 'v' ???
+    type K = keyof Gen2<ABC.A>;
+
+    // :(
+    declare let a: Gen2<ABC.A>;
+    declare let b: Gen2<ABC.B>;
+    a = b;
+    ~
+!!! error TS2741: Property 'a' is missing in type 'Gen2<ABC.B>' but required in type 'Gen2<ABC.A>'.
+!!! related TS2728 tests/cases/compiler/mappedTypeNotMistakenlyHomomorphic.ts:6:5: 'a' is declared here.
+    b = a;
+    ~
+!!! error TS2741: Property 'b' is missing in type 'Gen2<ABC.A>' but required in type 'Gen2<ABC.B>'.
+!!! related TS2728 tests/cases/compiler/mappedTypeNotMistakenlyHomomorphic.ts:9:5: 'b' is declared here.
+

tests/baselines/reference/mappedTypeNotMistakenlyHomomorphic.js

@@ -0,0 +1,48 @@
+//// [mappedTypeNotMistakenlyHomomorphic.ts]
+enum ABC { A, B }
+
+type Gen<T extends ABC> = { v: T; } & (
+  {
+    v: ABC.A,
+    a: string,
+  } | {
+    v: ABC.B,
+    b: string,
+  }
+)
+
+// Quick info: ???
+//
+// type Gen2<T extends ABC> = {
+//    v: string;
+// }
+//
+type Gen2<T extends ABC> = {
+  [Property in keyof Gen<T>]: string;
+};
+
+// 'a' and 'b' properties required !?!?
+const gen2TypeA: Gen2<ABC.A> = { v:  "I am A", a: "" };
+const gen2TypeB: Gen2<ABC.B> = { v:  "I am B", b: "" };
+
+// 'v' ???
+type K = keyof Gen2<ABC.A>;
+
+// :(
+declare let a: Gen2<ABC.A>;
+declare let b: Gen2<ABC.B>;
+a = b;
+b = a;
+
+
+//// [mappedTypeNotMistakenlyHomomorphic.js]
+var ABC;
+(function (ABC) {
+    ABC[ABC["A"] = 0] = "A";
+    ABC[ABC["B"] = 1] = "B";
+})(ABC || (ABC = {}));
+// 'a' and 'b' properties required !?!?
+var gen2TypeA = { v: "I am A", a: "" };
+var gen2TypeB = { v: "I am B", b: "" };
+a = b;
+b = a;

tests/baselines/reference/mappedTypeNotMistakenlyHomomorphic.symbols

@@ -0,0 +1,95 @@
+=== tests/cases/compiler/mappedTypeNotMistakenlyHomomorphic.ts ===
+enum ABC { A, B }
+>ABC : Symbol(ABC, Decl(mappedTypeNotMistakenlyHomomorphic.ts, 0, 0))
+>A : Symbol(ABC.A, Decl(mappedTypeNotMistakenlyHomomorphic.ts, 0, 10))
+>B : Symbol(ABC.B, Decl(mappedTypeNotMistakenlyHomomorphic.ts, 0, 13))
+
+type Gen<T extends ABC> = { v: T; } & (
+>Gen : Symbol(Gen, Decl(mappedTypeNotMistakenlyHomomorphic.ts, 0, 17))
+>T : Symbol(T, Decl(mappedTypeNotMistakenlyHomomorphic.ts, 2, 9))
+>ABC : Symbol(ABC, Decl(mappedTypeNotMistakenlyHomomorphic.ts, 0, 0))
+>v : Symbol(v, Decl(mappedTypeNotMistakenlyHomomorphic.ts, 2, 27))
+>T : Symbol(T, Decl(mappedTypeNotMistakenlyHomomorphic.ts, 2, 9))
+  {
+    v: ABC.A,
+>v : Symbol(v, Decl(mappedTypeNotMistakenlyHomomorphic.ts, 3, 3))
+>ABC : Symbol(ABC, Decl(mappedTypeNotMistakenlyHomomorphic.ts, 0, 0))
+>A : Symbol(ABC.A, Decl(mappedTypeNotMistakenlyHomomorphic.ts, 0, 10))
+
+    a: string,
+>a : Symbol(a, Decl(mappedTypeNotMistakenlyHomomorphic.ts, 4, 13))
+
+  } | {
+    v: ABC.B,
+>v : Symbol(v, Decl(mappedTypeNotMistakenlyHomomorphic.ts, 6, 7))
+>ABC : Symbol(ABC, Decl(mappedTypeNotMistakenlyHomomorphic.ts, 0, 0))
+>B : Symbol(ABC.B, Decl(mappedTypeNotMistakenlyHomomorphic.ts, 0, 13))
+
+    b: string,
+>b : Symbol(b, Decl(mappedTypeNotMistakenlyHomomorphic.ts, 7, 13))
+  }
+)
+
+// Quick info: ???
+//
+// type Gen2<T extends ABC> = {
+//    v: string;
+// }
+//
+type Gen2<T extends ABC> = {
+>Gen2 : Symbol(Gen2, Decl(mappedTypeNotMistakenlyHomomorphic.ts, 10, 1))
+>T : Symbol(T, Decl(mappedTypeNotMistakenlyHomomorphic.ts, 18, 10))
+>ABC : Symbol(ABC, Decl(mappedTypeNotMistakenlyHomomorphic.ts, 0, 0))
+
+  [Property in keyof Gen<T>]: string;
+>Property : Symbol(Property, Decl(mappedTypeNotMistakenlyHomomorphic.ts, 19, 3))
+>Gen : Symbol(Gen, Decl(mappedTypeNotMistakenlyHomomorphic.ts, 0, 17))
+>T : Symbol(T, Decl(mappedTypeNotMistakenlyHomomorphic.ts, 18, 10))
+
+};
+
+// 'a' and 'b' properties required !?!?
+const gen2TypeA: Gen2<ABC.A> = { v:  "I am A", a: "" };
+>gen2TypeA : Symbol(gen2TypeA, Decl(mappedTypeNotMistakenlyHomomorphic.ts, 23, 5))
+>Gen2 : Symbol(Gen2, Decl(mappedTypeNotMistakenlyHomomorphic.ts, 10, 1))
+>ABC : Symbol(ABC, Decl(mappedTypeNotMistakenlyHomomorphic.ts, 0, 0))
+>A : Symbol(ABC.A, Decl(mappedTypeNotMistakenlyHomomorphic.ts, 0, 10))
+>v : Symbol(v, Decl(mappedTypeNotMistakenlyHomomorphic.ts, 23, 32))
+>a : Symbol(a, Decl(mappedTypeNotMistakenlyHomomorphic.ts, 23, 46))
+
+const gen2TypeB: Gen2<ABC.B> = { v:  "I am B", b: "" };
+>gen2TypeB : Symbol(gen2TypeB, Decl(mappedTypeNotMistakenlyHomomorphic.ts, 24, 5))
+>Gen2 : Symbol(Gen2, Decl(mappedTypeNotMistakenlyHomomorphic.ts, 10, 1))
+>ABC : Symbol(ABC, Decl(mappedTypeNotMistakenlyHomomorphic.ts, 0, 0))
+>B : Symbol(ABC.B, Decl(mappedTypeNotMistakenlyHomomorphic.ts, 0, 13))
+>v : Symbol(v, Decl(mappedTypeNotMistakenlyHomomorphic.ts, 24, 32))
+>b : Symbol(b, Decl(mappedTypeNotMistakenlyHomomorphic.ts, 24, 46))
+
+// 'v' ???
+type K = keyof Gen2<ABC.A>;
+>K : Symbol(K, Decl(mappedTypeNotMistakenlyHomomorphic.ts, 24, 55))
+>Gen2 : Symbol(Gen2, Decl(mappedTypeNotMistakenlyHomomorphic.ts, 10, 1))
+>ABC : Symbol(ABC, Decl(mappedTypeNotMistakenlyHomomorphic.ts, 0, 0))
+>A : Symbol(ABC.A, Decl(mappedTypeNotMistakenlyHomomorphic.ts, 0, 10))
+
+// :(
+declare let a: Gen2<ABC.A>;
+>a : Symbol(a, Decl(mappedTypeNotMistakenlyHomomorphic.ts, 30, 11))
+>Gen2 : Symbol(Gen2, Decl(mappedTypeNotMistakenlyHomomorphic.ts, 10, 1))
+>ABC : Symbol(ABC, Decl(mappedTypeNotMistakenlyHomomorphic.ts, 0, 0))
+>A : Symbol(ABC.A, Decl(mappedTypeNotMistakenlyHomomorphic.ts, 0, 10))
+
+declare let b: Gen2<ABC.B>;
+>b : Symbol(b, Decl(mappedTypeNotMistakenlyHomomorphic.ts, 31, 11))
+>Gen2 : Symbol(Gen2, Decl(mappedTypeNotMistakenlyHomomorphic.ts, 10, 1))
+>ABC : Symbol(ABC, Decl(mappedTypeNotMistakenlyHomomorphic.ts, 0, 0))
+>B : Symbol(ABC.B, Decl(mappedTypeNotMistakenlyHomomorphic.ts, 0, 13))
+
+a = b;
+>a : Symbol(a, Decl(mappedTypeNotMistakenlyHomomorphic.ts, 30, 11))
+>b : Symbol(b, Decl(mappedTypeNotMistakenlyHomomorphic.ts, 31, 11))
+
+b = a;
+>b : Symbol(b, Decl(mappedTypeNotMistakenlyHomomorphic.ts, 31, 11))
+>a : Symbol(a, Decl(mappedTypeNotMistakenlyHomomorphic.ts, 30, 11))
+

tests/baselines/reference/mappedTypeNotMistakenlyHomomorphic.types

@@ -0,0 +1,82 @@
+=== tests/cases/compiler/mappedTypeNotMistakenlyHomomorphic.ts ===
+enum ABC { A, B }
+>ABC : ABC
+>A : ABC.A
+>B : ABC.B
+
+type Gen<T extends ABC> = { v: T; } & (
+>Gen : Gen<T>
+>v : T
+  {
+    v: ABC.A,
+>v : ABC.A
+>ABC : any
+
+    a: string,
+>a : string
+
+  } | {
+    v: ABC.B,
+>v : ABC.B
+>ABC : any
+
+    b: string,
+>b : string
+  }
+)
+
+// Quick info: ???
+//
+// type Gen2<T extends ABC> = {
+//    v: string;
+// }
+//
+type Gen2<T extends ABC> = {
+>Gen2 : Gen2<T>
+
+  [Property in keyof Gen<T>]: string;
+};
+
+// 'a' and 'b' properties required !?!?
+const gen2TypeA: Gen2<ABC.A> = { v:  "I am A", a: "" };
+>gen2TypeA : Gen2<ABC.A>
+>ABC : any
+>{ v:  "I am A", a: "" } : { v: string; a: string; }
+>v : string
+>"I am A" : "I am A"
+>a : string
+>"" : ""
+
+const gen2TypeB: Gen2<ABC.B> = { v:  "I am B", b: "" };
+>gen2TypeB : Gen2<ABC.B>
+>ABC : any
+>{ v:  "I am B", b: "" } : { v: string; b: string; }
+>v : string
+>"I am B" : "I am B"
+>b : string
+>"" : ""
+
+// 'v' ???
+type K = keyof Gen2<ABC.A>;
+>K : "v" | "a"
+>ABC : any
+
+// :(
+declare let a: Gen2<ABC.A>;
+>a : Gen2<ABC.A>
+>ABC : any
+
+declare let b: Gen2<ABC.B>;
+>b : Gen2<ABC.B>
+>ABC : any
+
+a = b;
+>a = b : Gen2<ABC.B>
+>a : Gen2<ABC.A>
+>b : Gen2<ABC.B>
+
+b = a;
+>b = a : Gen2<ABC.A>
+>b : Gen2<ABC.B>
+>a : Gen2<ABC.A>
+

tests/cases/compiler/mappedTypeNotMistakenlyHomomorphic.ts

@@ -0,0 +1,34 @@
+enum ABC { A, B }
+
+type Gen<T extends ABC> = { v: T; } & (
+  {
+    v: ABC.A,
+    a: string,
+  } | {
+    v: ABC.B,
+    b: string,
+  }
+)
+
+// Quick info: ???
+//
+// type Gen2<T extends ABC> = {
+//    v: string;
+// }
+//
+type Gen2<T extends ABC> = {
+  [Property in keyof Gen<T>]: string;
+};
+
+// 'a' and 'b' properties required !?!?
+const gen2TypeA: Gen2<ABC.A> = { v:  "I am A", a: "" };
+const gen2TypeB: Gen2<ABC.B> = { v:  "I am B", b: "" };
+
+// 'v' ???
+type K = keyof Gen2<ABC.A>;
+
+// :(
+declare let a: Gen2<ABC.A>;
+declare let b: Gen2<ABC.B>;
+a = b;
+b = a;