When relating a deferred index type over a mapped type on the source side ...

src/compiler/checker.ts

@@ -22142,6 +22142,18 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
             return result;
         }
 
+        function getApparentMappedTypeKeys(nameType: Type, targetType: MappedType) {
+            const modifiersType = getApparentType(getModifiersTypeFromMappedType(targetType));
+            const mappedKeys: Type[] = [];
+            forEachMappedTypePropertyKeyTypeAndIndexSignatureKeyType(
+                modifiersType,
+                TypeFlags.StringOrNumberLiteralOrUnique,
+                /*stringsOnly*/ false,
+                t => void mappedKeys.push(instantiateType(nameType, appendTypeMapping(targetType.mapper, getTypeParameterFromMappedType(targetType), t))),
+            );
+            return getUnionType(mappedKeys);
+        }
+
         function structuredTypeRelatedToWorker(source: Type, target: Type, reportErrors: boolean, intersectionState: IntersectionState, saveErrorInfo: ReturnType<typeof captureErrorCalculationState>): Ternary {
             let result: Ternary;
             let originalErrorInfo: DiagnosticMessageChain | undefined;
@@ -22305,16 +22317,9 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
                         if (nameType && isMappedTypeWithKeyofConstraintDeclaration(targetType)) {
                             // we need to get the apparent mappings and union them with the generic mappings, since some properties may be
                             // missing from the `constraintType` which will otherwise be mapped in the object
-                            const modifiersType = getApparentType(getModifiersTypeFromMappedType(targetType));
-                            const mappedKeys: Type[] = [];
-                            forEachMappedTypePropertyKeyTypeAndIndexSignatureKeyType(
-                                modifiersType,
-                                TypeFlags.StringOrNumberLiteralOrUnique,
-                                /*stringsOnly*/ false,
-                                t => void mappedKeys.push(instantiateType(nameType, appendTypeMapping(targetType.mapper, getTypeParameterFromMappedType(targetType), t))),
-                            );
+                            const mappedKeys = getApparentMappedTypeKeys(nameType, targetType);
                             // We still need to include the non-apparent (and thus still generic) keys in the target side of the comparison (in case they're in the source side)
-                            targetKeys = getUnionType([...mappedKeys, nameType]);
+                            targetKeys = getUnionType([mappedKeys, nameType]);
                         }
                         else {
                             targetKeys = nameType || constraintType;
@@ -22507,9 +22512,23 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
                 }
             }
             else if (sourceFlags & TypeFlags.Index) {
-                if (result = isRelatedTo(keyofConstraintType, target, RecursionFlags.Source, reportErrors)) {
+                const isDeferredMappedIndex = shouldDeferIndexType((source as IndexType).type, (source as IndexType).indexFlags) && getObjectFlags((source as IndexType).type) & ObjectFlags.Mapped;
+                if (result = isRelatedTo(keyofConstraintType, target, RecursionFlags.Source, reportErrors && !isDeferredMappedIndex)) {
                     return result;
                 }
+                if (isDeferredMappedIndex) {
+                    const mappedType = (source as IndexType).type as MappedType;
+                    const nameType = getNameTypeFromMappedType(mappedType);
+                    // Unlike on the target side, on the source side we do *not* include the generic part of the `nameType`, since that comes from a
+                    // (potentially anonymous) mapped type local type parameter, so that'd never assign outside the mapped type body, but we still want to
+                    // allow assignments of index types of identical (or similar enough) mapped types.
+                    // eg, `keyof {[X in keyof A]: Obj[X]}` should be assignable to `keyof {[Y in keyof A]: Tup[Y]}` because both map over the same set of keys (`keyof A`).
+                    // Without this source-side breakdown, a `keyof {[X in keyof A]: Obj[X]}` style type won't be assignable to anything except itself, which is much too strict.
+                    const sourceMappedKeys = nameType && isMappedTypeWithKeyofConstraintDeclaration(mappedType) ? getApparentMappedTypeKeys(nameType, mappedType) : (nameType || getConstraintTypeFromMappedType(mappedType));
+                    if (result = isRelatedTo(sourceMappedKeys, target, RecursionFlags.Source, reportErrors)) {
+                        return result;
+                    }
+                }
             }
             else if (sourceFlags & TypeFlags.TemplateLiteral && !(targetFlags & TypeFlags.Object)) {
                 if (!(targetFlags & TypeFlags.TemplateLiteral)) {

tests/baselines/reference/declarationEmitNestedAnonymousMappedType.js

@@ -0,0 +1,26 @@
+//// [tests/cases/compiler/declarationEmitNestedAnonymousMappedType.ts] ////
+
+//// [declarationEmitNestedAnonymousMappedType.ts]
+export function enumFromStrings<const Members extends readonly string[]>() {
+    type Part1 = {
+        [key in keyof Members as Members[key] extends string
+        ? Members[key]
+        : never]: Members[key];
+    };
+    type Part2 = { [Property in keyof Part1]: Part1[Property] };
+    return Object.create(null) as Part2;
+}
+
+
+//// [declarationEmitNestedAnonymousMappedType.js]
+"use strict";
+Object.defineProperty(exports, "__esModule", { value: true });
+exports.enumFromStrings = void 0;
+function enumFromStrings() {
+    return Object.create(null);
+}
+exports.enumFromStrings = enumFromStrings;
+
+
+//// [declarationEmitNestedAnonymousMappedType.d.ts]
+export declare function enumFromStrings<const Members extends readonly string[]>(): { [Property in keyof { [key in keyof Members as Members[key] extends string ? Members[key] : never]: Members[key]; }]: { [key in keyof Members as Members[key] extends string ? Members[key] : never]: Members[key]; }[Property]; };

tests/baselines/reference/declarationEmitNestedAnonymousMappedType.symbols

@@ -0,0 +1,39 @@
+//// [tests/cases/compiler/declarationEmitNestedAnonymousMappedType.ts] ////
+
+=== declarationEmitNestedAnonymousMappedType.ts ===
+export function enumFromStrings<const Members extends readonly string[]>() {
+>enumFromStrings : Symbol(enumFromStrings, Decl(declarationEmitNestedAnonymousMappedType.ts, 0, 0))
+>Members : Symbol(Members, Decl(declarationEmitNestedAnonymousMappedType.ts, 0, 32))
+
+    type Part1 = {
+>Part1 : Symbol(Part1, Decl(declarationEmitNestedAnonymousMappedType.ts, 0, 76))
+
+        [key in keyof Members as Members[key] extends string
+>key : Symbol(key, Decl(declarationEmitNestedAnonymousMappedType.ts, 2, 9))
+>Members : Symbol(Members, Decl(declarationEmitNestedAnonymousMappedType.ts, 0, 32))
+>Members : Symbol(Members, Decl(declarationEmitNestedAnonymousMappedType.ts, 0, 32))
+>key : Symbol(key, Decl(declarationEmitNestedAnonymousMappedType.ts, 2, 9))
+
+        ? Members[key]
+>Members : Symbol(Members, Decl(declarationEmitNestedAnonymousMappedType.ts, 0, 32))
+>key : Symbol(key, Decl(declarationEmitNestedAnonymousMappedType.ts, 2, 9))
+
+        : never]: Members[key];
+>Members : Symbol(Members, Decl(declarationEmitNestedAnonymousMappedType.ts, 0, 32))
+>key : Symbol(key, Decl(declarationEmitNestedAnonymousMappedType.ts, 2, 9))
+
+    };
+    type Part2 = { [Property in keyof Part1]: Part1[Property] };
+>Part2 : Symbol(Part2, Decl(declarationEmitNestedAnonymousMappedType.ts, 5, 6))
+>Property : Symbol(Property, Decl(declarationEmitNestedAnonymousMappedType.ts, 6, 20))
+>Part1 : Symbol(Part1, Decl(declarationEmitNestedAnonymousMappedType.ts, 0, 76))
+>Part1 : Symbol(Part1, Decl(declarationEmitNestedAnonymousMappedType.ts, 0, 76))
+>Property : Symbol(Property, Decl(declarationEmitNestedAnonymousMappedType.ts, 6, 20))
+
+    return Object.create(null) as Part2;
+>Object.create : Symbol(ObjectConstructor.create, Decl(lib.es5.d.ts, --, --), Decl(lib.es5.d.ts, --, --))
+>Object : Symbol(Object, Decl(lib.es5.d.ts, --, --), Decl(lib.es5.d.ts, --, --))
+>create : Symbol(ObjectConstructor.create, Decl(lib.es5.d.ts, --, --), Decl(lib.es5.d.ts, --, --))
+>Part2 : Symbol(Part2, Decl(declarationEmitNestedAnonymousMappedType.ts, 5, 6))
+}
+

tests/baselines/reference/declarationEmitNestedAnonymousMappedType.types

@@ -0,0 +1,24 @@
+//// [tests/cases/compiler/declarationEmitNestedAnonymousMappedType.ts] ////
+
+=== declarationEmitNestedAnonymousMappedType.ts ===
+export function enumFromStrings<const Members extends readonly string[]>() {
+>enumFromStrings : <const Members extends readonly string[]>() => { [Property in keyof { [key in keyof Members as Members[key] extends string ? Members[key] : never]: Members[key]; }]: { [key in keyof Members as Members[key] extends string ? Members[key] : never]: Members[key]; }[Property]; }
+
+    type Part1 = {
+>Part1 : { [key in keyof Members as Members[key] extends string ? Members[key] : never]: Members[key]; }
+
+        [key in keyof Members as Members[key] extends string
+        ? Members[key]
+        : never]: Members[key];
+    };
+    type Part2 = { [Property in keyof Part1]: Part1[Property] };
+>Part2 : { [Property in keyof { [key in keyof Members as Members[key] extends string ? Members[key] : never]: Members[key]; }]: { [key in keyof Members as Members[key] extends string ? Members[key] : never]: Members[key]; }[Property]; }
+
+    return Object.create(null) as Part2;
+>Object.create(null) as Part2 : { [Property in keyof { [key in keyof Members as Members[key] extends string ? Members[key] : never]: Members[key]; }]: { [key in keyof Members as Members[key] extends string ? Members[key] : never]: Members[key]; }[Property]; }
+>Object.create(null) : any
+>Object.create : { (o: object): any; (o: object, properties: PropertyDescriptorMap & ThisType<any>): any; }
+>Object : ObjectConstructor
+>create : { (o: object): any; (o: object, properties: PropertyDescriptorMap & ThisType<any>): any; }
+}
+

tests/cases/compiler/declarationEmitNestedAnonymousMappedType.ts

@@ -0,0 +1,10 @@
+// @declaration: true
+export function enumFromStrings<const Members extends readonly string[]>() {
+    type Part1 = {
+        [key in keyof Members as Members[key] extends string
+        ? Members[key]
+        : never]: Members[key];
+    };
+    type Part2 = { [Property in keyof Part1]: Part1[Property] };
+    return Object.create(null) as Part2;
+}