Fix conditional type type parameter leak

src/compiler/checker.ts

@@ -10406,14 +10406,34 @@ namespace ts {
             const checkTypeInstantiable = maybeTypeOfKind(checkType, TypeFlags.Instantiable | TypeFlags.GenericMappedType);
             let combinedMapper: TypeMapper | undefined;
             if (root.inferTypeParameters) {
-                const context = createInferenceContext(root.inferTypeParameters, /*signature*/ undefined, InferenceFlags.None);
+                const freshParams = map(root.inferTypeParameters, cloneTypeParameter);
+                const freshMapper = createTypeMapper(root.inferTypeParameters, freshParams);
+                const context = createInferenceContext(freshParams, /*signature*/ undefined, InferenceFlags.None);
+                // We have three mappers that need applying:
+                //    * The original `mapper` used to create this conditional
+                //    * The mapper that maps the old root type parameter to the clone (`freshMapper`)
+                //    * The mapper that maps the clone to its inference result (`context.mapper`)
+                // When we're looking at making an inference for a clone, when we get its constraint, it's autmagically be
+                // instantiated with the context, so it doesn't need that one - however the constraint may refer to another _root_, _uncloned_
+                // `infer` type parameter.
+                // Eg, if we have `Foo<T, U extends T>` and `Foo<number, infer B>` - `B` is constrained to `T`, which, in turn, has been instantiated
+                // as `number`
+                // Conversely, if we have `Foo<infer A, infer B>`, `B` is still constrained to `T` and `T` is instantiated as `A`
+                // So we need to:
+                //    * Clone the type parameters so their constraints can be instantiated in the context of `mapper`
+                //    * Set the clones to both map the context and the original params
+                //    * instantiate the extends type with the clones
+                //    * incorporate all of the component mappers into the combined mapper for the members
+                for (const p of freshParams) {
+                    p.mapper = combineTypeMappers(mapper, freshMapper);
+                }
                 if (!checkTypeInstantiable) {
                     // We don't want inferences from constraints as they may cause us to eagerly resolve the
                     // conditional type instead of deferring resolution. Also, we always want strict function
                     // types rules (i.e. proper contravariance) for inferences.
-                    inferTypes(context.inferences, checkType, extendsType, InferencePriority.NoConstraints | InferencePriority.AlwaysStrict);
+                    inferTypes(context.inferences, checkType, instantiateType(extendsType, freshMapper), InferencePriority.NoConstraints | InferencePriority.AlwaysStrict);
                 }
-                combinedMapper = combineTypeMappers(mapper, context.mapper);
+                combinedMapper = combineTypeMappers(combineTypeMappers(freshMapper, context.mapper), mapper);
             }
             // Instantiate the extends type including inferences for 'infer T' type parameters
             const inferredExtendsType = combinedMapper ? instantiateType(root.extendsType, combinedMapper) : extendsType;

tests/baselines/reference/conditionalDoesntLeakUninstantiatedTypeParameter.errors.txt

@@ -0,0 +1,14 @@
+tests/cases/compiler/conditionalDoesntLeakUninstantiatedTypeParameter.ts(7,7): error TS2322: Type '"3"' is not assignable to type 'number'.
+
+
+==== tests/cases/compiler/conditionalDoesntLeakUninstantiatedTypeParameter.ts (1 errors) ====
+    interface Synthetic<A, B extends A> {}
+    type SyntheticDestination<T, U> = U extends Synthetic<T, infer V> ? V : never;
+    type TestSynthetic = // Resolved to T, should be `number` or an inference failure (`unknown`)
+        SyntheticDestination<number, Synthetic<number, number>>;
+
+    const y: TestSynthetic = 3; // Type '3' is not assignable to type 'T'. (shouldn't error)
+    const z: TestSynthetic = '3'; // Type '"3""' is not assignable to type 'T'. (should not mention T)
+          ~
+!!! error TS2322: Type '"3"' is not assignable to type 'number'.
+

tests/baselines/reference/conditionalDoesntLeakUninstantiatedTypeParameter.js

@@ -0,0 +1,13 @@
+//// [conditionalDoesntLeakUninstantiatedTypeParameter.ts]
+interface Synthetic<A, B extends A> {}
+type SyntheticDestination<T, U> = U extends Synthetic<T, infer V> ? V : never;
+type TestSynthetic = // Resolved to T, should be `number` or an inference failure (`unknown`)
+    SyntheticDestination<number, Synthetic<number, number>>;
+
+const y: TestSynthetic = 3; // Type '3' is not assignable to type 'T'. (shouldn't error)
+const z: TestSynthetic = '3'; // Type '"3""' is not assignable to type 'T'. (should not mention T)
+
+
+//// [conditionalDoesntLeakUninstantiatedTypeParameter.js]
+var y = 3; // Type '3' is not assignable to type 'T'. (shouldn't error)
+var z = '3'; // Type '"3""' is not assignable to type 'T'. (should not mention T)

tests/baselines/reference/conditionalDoesntLeakUninstantiatedTypeParameter.symbols

@@ -0,0 +1,32 @@
+=== tests/cases/compiler/conditionalDoesntLeakUninstantiatedTypeParameter.ts ===
+interface Synthetic<A, B extends A> {}
+>Synthetic : Symbol(Synthetic, Decl(conditionalDoesntLeakUninstantiatedTypeParameter.ts, 0, 0))
+>A : Symbol(A, Decl(conditionalDoesntLeakUninstantiatedTypeParameter.ts, 0, 20))
+>B : Symbol(B, Decl(conditionalDoesntLeakUninstantiatedTypeParameter.ts, 0, 22))
+>A : Symbol(A, Decl(conditionalDoesntLeakUninstantiatedTypeParameter.ts, 0, 20))
+
+type SyntheticDestination<T, U> = U extends Synthetic<T, infer V> ? V : never;
+>SyntheticDestination : Symbol(SyntheticDestination, Decl(conditionalDoesntLeakUninstantiatedTypeParameter.ts, 0, 38))
+>T : Symbol(T, Decl(conditionalDoesntLeakUninstantiatedTypeParameter.ts, 1, 26))
+>U : Symbol(U, Decl(conditionalDoesntLeakUninstantiatedTypeParameter.ts, 1, 28))
+>U : Symbol(U, Decl(conditionalDoesntLeakUninstantiatedTypeParameter.ts, 1, 28))
+>Synthetic : Symbol(Synthetic, Decl(conditionalDoesntLeakUninstantiatedTypeParameter.ts, 0, 0))
+>T : Symbol(T, Decl(conditionalDoesntLeakUninstantiatedTypeParameter.ts, 1, 26))
+>V : Symbol(V, Decl(conditionalDoesntLeakUninstantiatedTypeParameter.ts, 1, 62))
+>V : Symbol(V, Decl(conditionalDoesntLeakUninstantiatedTypeParameter.ts, 1, 62))
+
+type TestSynthetic = // Resolved to T, should be `number` or an inference failure (`unknown`)
+>TestSynthetic : Symbol(TestSynthetic, Decl(conditionalDoesntLeakUninstantiatedTypeParameter.ts, 1, 78))
+
+    SyntheticDestination<number, Synthetic<number, number>>;
+>SyntheticDestination : Symbol(SyntheticDestination, Decl(conditionalDoesntLeakUninstantiatedTypeParameter.ts, 0, 38))
+>Synthetic : Symbol(Synthetic, Decl(conditionalDoesntLeakUninstantiatedTypeParameter.ts, 0, 0))
+
+const y: TestSynthetic = 3; // Type '3' is not assignable to type 'T'. (shouldn't error)
+>y : Symbol(y, Decl(conditionalDoesntLeakUninstantiatedTypeParameter.ts, 5, 5))
+>TestSynthetic : Symbol(TestSynthetic, Decl(conditionalDoesntLeakUninstantiatedTypeParameter.ts, 1, 78))
+
+const z: TestSynthetic = '3'; // Type '"3""' is not assignable to type 'T'. (should not mention T)
+>z : Symbol(z, Decl(conditionalDoesntLeakUninstantiatedTypeParameter.ts, 6, 5))
+>TestSynthetic : Symbol(TestSynthetic, Decl(conditionalDoesntLeakUninstantiatedTypeParameter.ts, 1, 78))
+

tests/baselines/reference/conditionalDoesntLeakUninstantiatedTypeParameter.types

@@ -0,0 +1,18 @@
+=== tests/cases/compiler/conditionalDoesntLeakUninstantiatedTypeParameter.ts ===
+interface Synthetic<A, B extends A> {}
+type SyntheticDestination<T, U> = U extends Synthetic<T, infer V> ? V : never;
+>SyntheticDestination : SyntheticDestination<T, U>
+
+type TestSynthetic = // Resolved to T, should be `number` or an inference failure (`unknown`)
+>TestSynthetic : number
+
+    SyntheticDestination<number, Synthetic<number, number>>;
+
+const y: TestSynthetic = 3; // Type '3' is not assignable to type 'T'. (shouldn't error)
+>y : number
+>3 : 3
+
+const z: TestSynthetic = '3'; // Type '"3""' is not assignable to type 'T'. (should not mention T)
+>z : number
+>'3' : "3"
+

tests/cases/compiler/conditionalDoesntLeakUninstantiatedTypeParameter.ts

@@ -0,0 +1,7 @@
+interface Synthetic<A, B extends A> {}
+type SyntheticDestination<T, U> = U extends Synthetic<T, infer V> ? V : never;
+type TestSynthetic = // Resolved to T, should be `number` or an inference failure (`unknown`)
+    SyntheticDestination<number, Synthetic<number, number>>;
+
+const y: TestSynthetic = 3; // Type '3' is not assignable to type 'T'. (shouldn't error)
+const z: TestSynthetic = '3'; // Type '"3""' is not assignable to type 'T'. (should not mention T)