Fix type mapper combination

src/compiler/checker.ts

@@ -21840,6 +21840,20 @@ namespace ts {
             return t;
         }
 
+        function mapToInferredTypeIncludingReturnTypeInferences(context: InferenceContext, returnContext: InferenceContext, t: Type): Type {
+            const inferences = context.inferences;
+            for (let i = 0; i < inferences.length; i++) {
+                const inference = inferences[i];
+                if (t === inference.typeParameter) {
+                    if (inference.inferredType || hasInferenceCandidates(inference)) {
+                        return getInferredType(context, i);
+                    }
+                    return getMappedType(t, returnContext.mapper);
+                }
+            }
+            return t;
+        }
+
         function clearCachedInferences(inferences: InferenceInfo[]) {
             for (const inference of inferences) {
                 if (!inference.isFixed) {
@@ -27055,12 +27069,10 @@ namespace ts {
                 const inferenceContext = getInferenceContext(node);
                 // If no inferences have been made, nothing is gained from instantiating as type parameters
                 // would just be replaced with their defaults similar to the apparent type.
-                if (inferenceContext && contextFlags! & ContextFlags.Signature && (inferenceContext.returnMapper || some(inferenceContext.inferences, hasInferenceCandidates))) {
+                if (inferenceContext && contextFlags! & ContextFlags.Signature && (inferenceContext.combinedReturnMapper || some(inferenceContext.inferences, hasInferenceCandidates))) {
                     // For contextual signatures we incorporate all inferences made so far, e.g. from return
                     // types as well as arguments to the left in a function call.
-                    return instantiateInstantiableTypes(contextualType, inferenceContext.returnMapper
-                        ? combineTypeMappers(inferenceContext.nonFixingMapper, inferenceContext.returnMapper)
-                        : inferenceContext.nonFixingMapper);
+                    return instantiateInstantiableTypes(contextualType, inferenceContext.combinedReturnMapper || inferenceContext.nonFixingMapper);
                 }
                 if (inferenceContext?.returnMapper) {
                     // For other purposes (e.g. determining whether to produce literal types) we only
@@ -29950,7 +29962,11 @@ namespace ts {
                         const returnContext = createInferenceContext(signature.typeParameters!, signature, context.flags);
                         const returnSourceType = instantiateType(contextualType, outerContext && outerContext.returnMapper);
                         inferTypes(returnContext.inferences, returnSourceType, inferenceTargetType);
-                        context.returnMapper = some(returnContext.inferences, hasInferenceCandidates) ? getMapperFromContext(cloneInferredPartOfContext(returnContext)) : undefined;
+                        if (some(returnContext.inferences, hasInferenceCandidates)) {
+                            const clonedReturnContext = cloneInferredPartOfContext(returnContext)!;
+                            context.returnMapper = getMapperFromContext(clonedReturnContext);
+                            context.combinedReturnMapper = makeFunctionTypeMapper(t => mapToInferredTypeIncludingReturnTypeInferences(context, clonedReturnContext, t));
+                        }
                     }
                 }
             }

src/compiler/types.ts

@@ -5922,6 +5922,7 @@ namespace ts {
         mapper: TypeMapper;                           // Mapper that fixes inferences
         nonFixingMapper: TypeMapper;                  // Mapper that doesn't fix inferences
         returnMapper?: TypeMapper;                    // Type mapper for inferences from return types (if any)
+        combinedReturnMapper?: TypeMapper;            // Non-fixing mapper combined with return mapper for contextual signature instantiation
         inferredTypeParameters?: readonly TypeParameter[]; // Inferred type parameters for function result
         intraExpressionInferenceSites?: IntraExpressionInferenceSite[];
     }

tests/baselines/reference/objectBindingPatternContextuallyTypesArgument.js

@@ -0,0 +1,7 @@
+//// [objectBindingPatternContextuallyTypesArgument.ts]
+declare function id<T>(x: T): T;
+const { f = (x: string) => x.length } = id({ f: x => x.charAt });
+
+
+//// [objectBindingPatternContextuallyTypesArgument.js]
+var _a = id({ f: function (x) { return x.charAt; } }).f, f = _a === void 0 ? function (x) { return x.length; } : _a;

tests/baselines/reference/objectBindingPatternContextuallyTypesArgument.symbols

@@ -0,0 +1,21 @@
+=== tests/cases/compiler/objectBindingPatternContextuallyTypesArgument.ts ===
+declare function id<T>(x: T): T;
+>id : Symbol(id, Decl(objectBindingPatternContextuallyTypesArgument.ts, 0, 0))
+>T : Symbol(T, Decl(objectBindingPatternContextuallyTypesArgument.ts, 0, 20))
+>x : Symbol(x, Decl(objectBindingPatternContextuallyTypesArgument.ts, 0, 23))
+>T : Symbol(T, Decl(objectBindingPatternContextuallyTypesArgument.ts, 0, 20))
+>T : Symbol(T, Decl(objectBindingPatternContextuallyTypesArgument.ts, 0, 20))
+
+const { f = (x: string) => x.length } = id({ f: x => x.charAt });
+>f : Symbol(f, Decl(objectBindingPatternContextuallyTypesArgument.ts, 1, 7))
+>x : Symbol(x, Decl(objectBindingPatternContextuallyTypesArgument.ts, 1, 13))
+>x.length : Symbol(String.length, Decl(lib.es5.d.ts, --, --))
+>x : Symbol(x, Decl(objectBindingPatternContextuallyTypesArgument.ts, 1, 13))
+>length : Symbol(String.length, Decl(lib.es5.d.ts, --, --))
+>id : Symbol(id, Decl(objectBindingPatternContextuallyTypesArgument.ts, 0, 0))
+>f : Symbol(f, Decl(objectBindingPatternContextuallyTypesArgument.ts, 1, 44))
+>x : Symbol(x, Decl(objectBindingPatternContextuallyTypesArgument.ts, 1, 47))
+>x.charAt : Symbol(String.charAt, Decl(lib.es5.d.ts, --, --))
+>x : Symbol(x, Decl(objectBindingPatternContextuallyTypesArgument.ts, 1, 47))
+>charAt : Symbol(String.charAt, Decl(lib.es5.d.ts, --, --))
+

tests/baselines/reference/objectBindingPatternContextuallyTypesArgument.types

@@ -0,0 +1,22 @@
+=== tests/cases/compiler/objectBindingPatternContextuallyTypesArgument.ts ===
+declare function id<T>(x: T): T;
+>id : <T>(x: T) => T
+>x : T
+
+const { f = (x: string) => x.length } = id({ f: x => x.charAt });
+>f : ((x: string) => number) | ((x: string) => (pos: number) => string)
+>(x: string) => x.length : (x: string) => number
+>x : string
+>x.length : number
+>x : string
+>length : number
+>id({ f: x => x.charAt }) : { f: (x: string) => (pos: number) => string; }
+>id : <T>(x: T) => T
+>{ f: x => x.charAt } : { f: (x: string) => (pos: number) => string; }
+>f : (x: string) => (pos: number) => string
+>x => x.charAt : (x: string) => (pos: number) => string
+>x : string
+>x.charAt : (pos: number) => string
+>x : string
+>charAt : (pos: number) => string
+

tests/cases/compiler/objectBindingPatternContextuallyTypesArgument.ts

@@ -0,0 +1,2 @@
+declare function id<T>(x: T): T;
+const { f = (x: string) => x.length } = id({ f: x => x.charAt });