Merge pull request microsoft#22323 from Microsoft/checkInferredConstraints

Check inferred constraints for 'infer X' type variables

Author: ahejlsberg

src/compiler/checker.ts

@@ -3966,7 +3966,8 @@ namespace ts {
 
     /* @internal */
     export interface InferenceContext extends TypeMapper {
-        signature: Signature;               // Generic signature for which inferences are made
+        typeParameters: TypeParameter[];    // Type parameters for which inferences are made
+        signature: Signature;               // Generic signature for which inferences are made (if any)
         inferences: InferenceInfo[];        // Inferences made for each type parameter
         flags: InferenceFlags;              // Inference flags
         compareTypes: TypeComparer;         // Type comparer function

src/compiler/types.ts

@@ -17,7 +17,7 @@ tests/cases/conformance/types/conditional/inferTypes1.ts(75,43): error TS2304: C
 tests/cases/conformance/types/conditional/inferTypes1.ts(75,43): error TS4081: Exported type alias 'T62' has or is using private name 'U'.
 tests/cases/conformance/types/conditional/inferTypes1.ts(81,44): error TS2344: Type 'U' does not satisfy the constraint 'string'.
   Type 'number' is not assignable to type 'string'.
-tests/cases/conformance/types/conditional/inferTypes1.ts(134,40): error TS2322: Type 'T' is not assignable to type 'string'.
+tests/cases/conformance/types/conditional/inferTypes1.ts(143,40): error TS2322: Type 'T' is not assignable to type 'string'.
 
 
 ==== tests/cases/conformance/types/conditional/inferTypes1.ts (16 errors) ====
@@ -144,6 +144,15 @@ tests/cases/conformance/types/conditional/inferTypes1.ts(134,40): error TS2322:
     type T77<T> = T extends T76<infer X, infer Y> ? T76<X, Y> : never;
     type T78<T> = T extends T76<infer X, infer X> ? T76<X, X> : never;
 
+    type Foo<T extends string, U extends T> = [T, U];
+    type Bar<T> = T extends Foo<infer X, infer Y> ? Foo<X, Y> : never;
+    
+    type T90 = Bar<[string, string]>;  // [string, string]
+    type T91 = Bar<[string, "a"]>;  // [string, "a"]
+    type T92 = Bar<[string, "a"] & { x: string }>;  // [string, "a"]
+    type T93 = Bar<["a", string]>;  // never
+    type T94 = Bar<[number, number]>;  // never
+    
     // Example from #21496
 
     type JsonifiedObject<T extends object> = { [K in keyof T]: Jsonified<T[K]> };
@@ -206,4 +215,12 @@ tests/cases/conformance/types/conditional/inferTypes1.ts(134,40): error TS2322:
 
     type T80 = MatchingKeys<test, void>;
     type T81 = VoidKeys<test>;
+    
+    // Repro from #22221
+    
+    type MustBeString<T extends string> = T;
+    type EnsureIsString<T> = T extends MustBeString<infer U> ? U : never;
+    
+    type Test1 = EnsureIsString<"hello">;  // "hello"
+    type Test2 = EnsureIsString<42>;  // never
 

tests/baselines/reference/inferTypes1.errors.txt

@@ -88,6 +88,15 @@ type T76<T extends T[], U extends T> = { x: T };
 type T77<T> = T extends T76<infer X, infer Y> ? T76<X, Y> : never;
 type T78<T> = T extends T76<infer X, infer X> ? T76<X, X> : never;
 
+type Foo<T extends string, U extends T> = [T, U];
+type Bar<T> = T extends Foo<infer X, infer Y> ? Foo<X, Y> : never;
+
+type T90 = Bar<[string, string]>;  // [string, string]
+type T91 = Bar<[string, "a"]>;  // [string, "a"]
+type T92 = Bar<[string, "a"] & { x: string }>;  // [string, "a"]
+type T93 = Bar<["a", string]>;  // never
+type T94 = Bar<[number, number]>;  // never
+
 // Example from #21496
 
 type JsonifiedObject<T extends object> = { [K in keyof T]: Jsonified<T[K]> };
@@ -148,6 +157,14 @@ interface test {
 
 type T80 = MatchingKeys<test, void>;
 type T81 = VoidKeys<test>;
+
+// Repro from #22221
+
+type MustBeString<T extends string> = T;
+type EnsureIsString<T> = T extends MustBeString<infer U> ? U : never;
+
+type Test1 = EnsureIsString<"hello">;  // "hello"
+type Test2 = EnsureIsString<42>;  // never
 
 
 //// [inferTypes1.js]