Merge pull request #29338 from Microsoft/fixConditionalTypeResolution

Fix conditional type resolution

Author: ahejlsberg

src/compiler/checker.ts

@@ -3916,7 +3916,9 @@ namespace ts {
         aliasTypeArguments?: ReadonlyArray<Type>;     // Alias type arguments (if any)
         /* @internal */ aliasTypeArgumentsContainsMarker?: boolean;   // Alias type arguments (if any)
         /* @internal */
-        wildcardInstantiation?: Type;    // Instantiation with type parameters mapped to wildcard type
+        permissiveInstantiation?: Type;  // Instantiation with type parameters mapped to wildcard type
+        /* @internal */
+        restrictiveInstantiation?: Type; // Instantiation with type parameters mapped to unconstrained form
         /* @internal */
         immediateBaseConstraint?: Type;  // Immediate base constraint cache
     }

src/compiler/types.ts

@@ -471,4 +471,12 @@ tests/cases/conformance/types/conditional/conditionalTypes1.ts(288,43): error TS
 
     var a = {o: 1, b: 2, c: [{a: 1, c: '213'}]}
     assign(a, {o: 2, c: {0: {a: 2, c: '213123'}}})
+    
+    // Repros from #23843
+    
+    type Weird1 = (<U extends boolean>(a: U) => never) extends 
+        (<U extends true>(a: U) => never) ? never : never;
+    
+    type Weird2 = (<U extends boolean>(a: U) => U) extends 
+        (<U extends true>(a: U) => infer T) ? T : never;
 

tests/baselines/reference/conditionalTypes1.errors.txt

@@ -351,6 +351,14 @@ declare function assign<T>(o: T, a: RecursivePartial<T>): void;
 
 var a = {o: 1, b: 2, c: [{a: 1, c: '213'}]}
 assign(a, {o: 2, c: {0: {a: 2, c: '213123'}}})
+
+// Repros from #23843
+
+type Weird1 = (<U extends boolean>(a: U) => never) extends 
+    (<U extends true>(a: U) => never) ? never : never;
+
+type Weird2 = (<U extends boolean>(a: U) => U) extends 
+    (<U extends true>(a: U) => infer T) ? T : never;
 
 
 //// [conditionalTypes1.js]
@@ -715,3 +723,5 @@ declare var a: {
         c: string;
     }[];
 };
+declare type Weird1 = (<U extends boolean>(a: U) => never) extends (<U extends true>(a: U) => never) ? never : never;
+declare type Weird2 = (<U extends boolean>(a: U) => U) extends (<U extends true>(a: U) => infer T) ? T : never;

tests/baselines/reference/conditionalTypes1.js

@@ -1371,3 +1371,30 @@ assign(a, {o: 2, c: {0: {a: 2, c: '213123'}}})
 >a : Symbol(a, Decl(conditionalTypes1.ts, 351, 25))
 >c : Symbol(c, Decl(conditionalTypes1.ts, 351, 30))
 
+// Repros from #23843
+
+type Weird1 = (<U extends boolean>(a: U) => never) extends 
+>Weird1 : Symbol(Weird1, Decl(conditionalTypes1.ts, 351, 46))
+>U : Symbol(U, Decl(conditionalTypes1.ts, 355, 16))
+>a : Symbol(a, Decl(conditionalTypes1.ts, 355, 35))
+>U : Symbol(U, Decl(conditionalTypes1.ts, 355, 16))
+
+    (<U extends true>(a: U) => never) ? never : never;
+>U : Symbol(U, Decl(conditionalTypes1.ts, 356, 6))
+>a : Symbol(a, Decl(conditionalTypes1.ts, 356, 22))
+>U : Symbol(U, Decl(conditionalTypes1.ts, 356, 6))
+
+type Weird2 = (<U extends boolean>(a: U) => U) extends 
+>Weird2 : Symbol(Weird2, Decl(conditionalTypes1.ts, 356, 54))
+>U : Symbol(U, Decl(conditionalTypes1.ts, 358, 16))
+>a : Symbol(a, Decl(conditionalTypes1.ts, 358, 35))
+>U : Symbol(U, Decl(conditionalTypes1.ts, 358, 16))
+>U : Symbol(U, Decl(conditionalTypes1.ts, 358, 16))
+
+    (<U extends true>(a: U) => infer T) ? T : never;
+>U : Symbol(U, Decl(conditionalTypes1.ts, 359, 6))
+>a : Symbol(a, Decl(conditionalTypes1.ts, 359, 22))
+>U : Symbol(U, Decl(conditionalTypes1.ts, 359, 6))
+>T : Symbol(T, Decl(conditionalTypes1.ts, 359, 36))
+>T : Symbol(T, Decl(conditionalTypes1.ts, 359, 36))
+

tests/baselines/reference/conditionalTypes1.symbols

@@ -1054,3 +1054,21 @@ assign(a, {o: 2, c: {0: {a: 2, c: '213123'}}})
 >c : string
 >'213123' : "213123"
 
+// Repros from #23843
+
+type Weird1 = (<U extends boolean>(a: U) => never) extends 
+>Weird1 : never
+>a : U
+
+    (<U extends true>(a: U) => never) ? never : never;
+>true : true
+>a : U
+
+type Weird2 = (<U extends boolean>(a: U) => U) extends 
+>Weird2 : boolean
+>a : U
+
+    (<U extends true>(a: U) => infer T) ? T : never;
+>true : true
+>a : U
+

tests/baselines/reference/conditionalTypes1.types

@@ -353,3 +353,11 @@ declare function assign<T>(o: T, a: RecursivePartial<T>): void;
 
 var a = {o: 1, b: 2, c: [{a: 1, c: '213'}]}
 assign(a, {o: 2, c: {0: {a: 2, c: '213123'}}})
+
+// Repros from #23843
+
+type Weird1 = (<U extends boolean>(a: U) => never) extends 
+    (<U extends true>(a: U) => never) ? never : never;
+
+type Weird2 = (<U extends boolean>(a: U) => U) extends 
+    (<U extends true>(a: U) => infer T) ? T : never;