Improve contextual types inferred from return types

src/compiler/checker.ts

@@ -26727,7 +26727,9 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
                     }
                     if (!inference.isFixed) {
                         const candidate = propagationType || source;
-                        if (candidate === blockedStringType) {
+                        // ReturnMapper inferences are used only for contextual types. We exclude `any` and `unknown` types
+                        // that might otherwise swallow other useful inferences.
+                        if (candidate === blockedStringType || candidate.flags & TypeFlags.AnyOrUnknown && priority & InferencePriority.ReturnMapper) {
                             return;
                         }
                         if (inference.priority === undefined || priority < inference.priority) {
@@ -35698,7 +35700,7 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
                     // type parameters for which the inferences are being made.
                     const returnContext = createInferenceContext(signature.typeParameters!, signature, context.flags);
                     const returnSourceType = instantiateType(contextualType, outerContext && createOuterReturnMapper(outerContext));
-                    inferTypes(returnContext.inferences, returnSourceType, inferenceTargetType);
+                    inferTypes(returnContext.inferences, returnSourceType, inferenceTargetType, InferencePriority.ReturnMapper);
                     context.returnMapper = some(returnContext.inferences, hasInferenceCandidates) ? getMapperFromContext(cloneInferredPartOfContext(returnContext)) : undefined;
                 }
             }

src/compiler/types.ts

@@ -7103,9 +7103,10 @@ export const enum InferencePriority {
     LiteralKeyof                 = 1 << 8,  // Inference made from a string literal to a keyof T
     NoConstraints                = 1 << 9,  // Don't infer from constraints of instantiable types
     AlwaysStrict                 = 1 << 10, // Always use strict rules for contravariant inferences
-    MaxValue                     = 1 << 11, // Seed for inference priority tracking
+    ReturnMapper                 = 1 << 11, // Inferring for return mapper
+    MaxValue                     = 1 << 12, // Seed for inference priority tracking
 
-    PriorityImpliesCombination = ReturnType | MappedTypeConstraint | LiteralKeyof, // These priorities imply that the resulting type should be a combination of all candidates
+    PriorityImpliesCombination = ReturnType | ReturnMapper | MappedTypeConstraint | LiteralKeyof, // These priorities imply that the resulting type should be a combination of all candidates
     Circularity = -1,  // Inference circularity (value less than all other priorities)
 }
 
@@ -7129,6 +7130,7 @@ export const enum InferenceFlags {
     NoDefault       = 1 << 0,  // Infer silentNeverType for no inferences (otherwise anyType or unknownType)
     AnyDefault      = 1 << 1,  // Infer anyType (in JS files) for no inferences (otherwise unknownType)
     SkippedGenericFunction = 1 << 2, // A generic function was skipped during inference
+    NoUnknownInference = 1 << 3,
 }
 
 /**

tests/baselines/reference/api/typescript.d.ts

@@ -6894,7 +6894,8 @@ declare namespace ts {
         LiteralKeyof = 256,
         NoConstraints = 512,
         AlwaysStrict = 1024,
-        MaxValue = 2048,
+        ReturnMapper = 2048,
+        MaxValue = 4096,
         PriorityImpliesCombination = 416,
         Circularity = -1,
     }