Don't add implicit number constraint to mapped type's type parameter when it has an as clause

Author: gabritto

src/compiler/checker.ts

@@ -16243,7 +16243,7 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
             }
             // Given a homomorphic mapped type { [K in keyof T]: XXX }, where T is constrained to an array or tuple type, in the
             // template type XXX, K has an added constraint of number | `${number}`.
-            else if (type.flags & TypeFlags.TypeParameter && parent.kind === SyntaxKind.MappedType && node === (parent as MappedTypeNode).type) {
+            else if (type.flags & TypeFlags.TypeParameter && parent.kind === SyntaxKind.MappedType && !(parent as MappedTypeNode).nameType && node === (parent as MappedTypeNode).type) {
                 const mappedType = getTypeFromTypeNode(parent as TypeNode) as MappedType;
                 if (getTypeParameterFromMappedType(mappedType) === getActualTypeVariable(type)) {
                     const typeParameter = getHomomorphicTypeVariable(mappedType);

tests/baselines/reference/mappedTypeWithNameClauseAppliedToArrayType.symbols

@@ -0,0 +1,28 @@
+//// [tests/cases/compiler/mappedTypeWithNameClauseAppliedToArrayType.ts] ////
+
+=== mappedTypeWithNameClauseAppliedToArrayType.ts ===
+type Mappy<T extends unknown[]> = { [K in keyof T as K]: T[K] };
+>Mappy : Symbol(Mappy, Decl(mappedTypeWithNameClauseAppliedToArrayType.ts, 0, 0))
+>T : Symbol(T, Decl(mappedTypeWithNameClauseAppliedToArrayType.ts, 0, 11))
+>K : Symbol(K, Decl(mappedTypeWithNameClauseAppliedToArrayType.ts, 0, 37))
+>T : Symbol(T, Decl(mappedTypeWithNameClauseAppliedToArrayType.ts, 0, 11))
+>K : Symbol(K, Decl(mappedTypeWithNameClauseAppliedToArrayType.ts, 0, 37))
+>T : Symbol(T, Decl(mappedTypeWithNameClauseAppliedToArrayType.ts, 0, 11))
+>K : Symbol(K, Decl(mappedTypeWithNameClauseAppliedToArrayType.ts, 0, 37))
+
+type NotArray = Mappy<number[]>;
+>NotArray : Symbol(NotArray, Decl(mappedTypeWithNameClauseAppliedToArrayType.ts, 0, 64))
+>Mappy : Symbol(Mappy, Decl(mappedTypeWithNameClauseAppliedToArrayType.ts, 0, 0))
+
+declare function doArrayStuff(x: unknown[]): void;
+>doArrayStuff : Symbol(doArrayStuff, Decl(mappedTypeWithNameClauseAppliedToArrayType.ts, 1, 32))
+>x : Symbol(x, Decl(mappedTypeWithNameClauseAppliedToArrayType.ts, 3, 30))
+
+declare const x: NotArray;
+>x : Symbol(x, Decl(mappedTypeWithNameClauseAppliedToArrayType.ts, 4, 13))
+>NotArray : Symbol(NotArray, Decl(mappedTypeWithNameClauseAppliedToArrayType.ts, 0, 64))
+
+doArrayStuff(x);
+>doArrayStuff : Symbol(doArrayStuff, Decl(mappedTypeWithNameClauseAppliedToArrayType.ts, 1, 32))
+>x : Symbol(x, Decl(mappedTypeWithNameClauseAppliedToArrayType.ts, 4, 13))
+

tests/baselines/reference/mappedTypeWithNameClauseAppliedToArrayType.types

@@ -0,0 +1,21 @@
+//// [tests/cases/compiler/mappedTypeWithNameClauseAppliedToArrayType.ts] ////
+
+=== mappedTypeWithNameClauseAppliedToArrayType.ts ===
+type Mappy<T extends unknown[]> = { [K in keyof T as K]: T[K] };
+>Mappy : Mappy<T>
+
+type NotArray = Mappy<number[]>;
+>NotArray : Mappy<number[]>
+
+declare function doArrayStuff(x: unknown[]): void;
+>doArrayStuff : (x: unknown[]) => void
+>x : unknown[]
+
+declare const x: NotArray;
+>x : Mappy<number[]>
+
+doArrayStuff(x);
+>doArrayStuff(x) : void
+>doArrayStuff : (x: unknown[]) => void
+>x : Mappy<number[]>
+

tests/cases/compiler/mappedTypeWithNameClauseAppliedToArrayType.ts

@@ -0,0 +1,9 @@
+// @strict: true
+// @noEmit: true
+
+type Mappy<T extends unknown[]> = { [K in keyof T as K]: T[K] };
+type NotArray = Mappy<number[]>;
+
+declare function doArrayStuff(x: unknown[]): void;
+declare const x: NotArray;
+doArrayStuff(x);