Fixed mapped types not being considered as homomorphic with substitution constraints

Author: Andarist

src/compiler/checker.ts

@@ -20860,7 +20860,7 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
     }
 
     function getHomomorphicTypeVariable(type: MappedType) {
-        const constraintType = getConstraintTypeFromMappedType(type);
+        const constraintType = getActualTypeVariable(getConstraintTypeFromMappedType(type));
         if (constraintType.flags & TypeFlags.Index) {
             const typeVariable = getActualTypeVariable((constraintType as IndexType).type);
             if (typeVariable.flags & TypeFlags.TypeParameter) {

tests/baselines/reference/mappedTypeNotMistakenlyHomomorphic2.symbols

@@ -0,0 +1,39 @@
+//// [tests/cases/compiler/mappedTypeNotMistakenlyHomomorphic2.ts] ////
+
+=== mappedTypeNotMistakenlyHomomorphic2.ts ===
+// https://github.com/microsoft/TypeScript/issues/63132
+
+type KeyMap<T> = keyof T extends PropertyKey ? { [K in keyof T]: K } : never;
+>KeyMap : Symbol(KeyMap, Decl(mappedTypeNotMistakenlyHomomorphic2.ts, 0, 0))
+>T : Symbol(T, Decl(mappedTypeNotMistakenlyHomomorphic2.ts, 2, 12))
+>T : Symbol(T, Decl(mappedTypeNotMistakenlyHomomorphic2.ts, 2, 12))
+>PropertyKey : Symbol(PropertyKey, Decl(lib.es5.d.ts, --, --))
+>K : Symbol(K, Decl(mappedTypeNotMistakenlyHomomorphic2.ts, 2, 50))
+>T : Symbol(T, Decl(mappedTypeNotMistakenlyHomomorphic2.ts, 2, 12))
+>K : Symbol(K, Decl(mappedTypeNotMistakenlyHomomorphic2.ts, 2, 50))
+
+type X = KeyMap<number>;
+>X : Symbol(X, Decl(mappedTypeNotMistakenlyHomomorphic2.ts, 2, 77))
+>KeyMap : Symbol(KeyMap, Decl(mappedTypeNotMistakenlyHomomorphic2.ts, 0, 0))
+
+type U = { a?: number; b: string } | { b: string; readonly c: boolean };
+>U : Symbol(U, Decl(mappedTypeNotMistakenlyHomomorphic2.ts, 3, 24))
+>a : Symbol(a, Decl(mappedTypeNotMistakenlyHomomorphic2.ts, 5, 10))
+>b : Symbol(b, Decl(mappedTypeNotMistakenlyHomomorphic2.ts, 5, 22))
+>b : Symbol(b, Decl(mappedTypeNotMistakenlyHomomorphic2.ts, 5, 38))
+>c : Symbol(c, Decl(mappedTypeNotMistakenlyHomomorphic2.ts, 5, 49))
+
+type MapStringOnly<T> = keyof T extends string ? { [K in keyof T]: [T[K]] } : never;
+>MapStringOnly : Symbol(MapStringOnly, Decl(mappedTypeNotMistakenlyHomomorphic2.ts, 5, 72))
+>T : Symbol(T, Decl(mappedTypeNotMistakenlyHomomorphic2.ts, 6, 19))
+>T : Symbol(T, Decl(mappedTypeNotMistakenlyHomomorphic2.ts, 6, 19))
+>K : Symbol(K, Decl(mappedTypeNotMistakenlyHomomorphic2.ts, 6, 52))
+>T : Symbol(T, Decl(mappedTypeNotMistakenlyHomomorphic2.ts, 6, 19))
+>T : Symbol(T, Decl(mappedTypeNotMistakenlyHomomorphic2.ts, 6, 19))
+>K : Symbol(K, Decl(mappedTypeNotMistakenlyHomomorphic2.ts, 6, 52))
+
+type Y = MapStringOnly<U>;
+>Y : Symbol(Y, Decl(mappedTypeNotMistakenlyHomomorphic2.ts, 6, 84))
+>MapStringOnly : Symbol(MapStringOnly, Decl(mappedTypeNotMistakenlyHomomorphic2.ts, 5, 72))
+>U : Symbol(U, Decl(mappedTypeNotMistakenlyHomomorphic2.ts, 3, 24))
+

tests/baselines/reference/mappedTypeNotMistakenlyHomomorphic2.types

@@ -0,0 +1,33 @@
+//// [tests/cases/compiler/mappedTypeNotMistakenlyHomomorphic2.ts] ////
+
+=== mappedTypeNotMistakenlyHomomorphic2.ts ===
+// https://github.com/microsoft/TypeScript/issues/63132
+
+type KeyMap<T> = keyof T extends PropertyKey ? { [K in keyof T]: K } : never;
+>KeyMap : KeyMap<T>
+>       : ^^^^^^^^^
+
+type X = KeyMap<number>;
+>X : number
+>  : ^^^^^^
+
+type U = { a?: number; b: string } | { b: string; readonly c: boolean };
+>U : U
+>  : ^
+>a : number | undefined
+>  : ^^^^^^^^^^^^^^^^^^
+>b : string
+>  : ^^^^^^
+>b : string
+>  : ^^^^^^
+>c : boolean
+>  : ^^^^^^^
+
+type MapStringOnly<T> = keyof T extends string ? { [K in keyof T]: [T[K]] } : never;
+>MapStringOnly : MapStringOnly<T>
+>              : ^^^^^^^^^^^^^^^^
+
+type Y = MapStringOnly<U>;
+>Y : { a?: [number | undefined] | undefined; b: [string]; } | { b: [string]; readonly c: [boolean]; }
+>  : ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+

tests/cases/compiler/mappedTypeNotMistakenlyHomomorphic2.ts

@@ -0,0 +1,11 @@
+// @strict: true
+// @noEmit: true
+
+// https://github.com/microsoft/TypeScript/issues/63132
+
+type KeyMap<T> = keyof T extends PropertyKey ? { [K in keyof T]: K } : never;
+type X = KeyMap<number>;
+
+type U = { a?: number; b: string } | { b: string; readonly c: boolean };
+type MapStringOnly<T> = keyof T extends string ? { [K in keyof T]: [T[K]] } : never;
+type Y = MapStringOnly<U>;