Fix conditional type type parameter leak

src/compiler/checker.ts

@@ -15817,6 +15817,11 @@ namespace ts {
             return type;
         }
 
+        function maybeCloneTypeParameter(p: TypeParameter) {
+            const constraint = getConstraintOfTypeParameter(p);
+            return constraint && (isGenericObjectType(constraint) || isGenericIndexType(constraint)) ? cloneTypeParameter(p) : p;
+        }
+
         function isTypicalNondistributiveConditional(root: ConditionalRoot) {
             return !root.isDistributive && isSingletonTupleType(root.node.checkType) && isSingletonTupleType(root.node.extendsType);
         }
@@ -15858,17 +15863,49 @@ namespace ts {
                 }
                 let combinedMapper: TypeMapper | undefined;
                 if (root.inferTypeParameters) {
-                    const context = createInferenceContext(root.inferTypeParameters, /*signature*/ undefined, InferenceFlags.None);
-                    if (!checkTypeInstantiable) {
+                    // When we're looking at making an inference for an infer type, when we get its constraint, it'll automagically be
+                    // instantiated with the context, so it doesn't need the mapper for the inference contex - however the constraint
+                    // may refer to another _root_, _uncloned_ `infer` type parameter [1], or to something mapped by `mapper` [2].
+                    // [1] 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`
+                    // [2] Eg, if we have `Foo<T, U extends T>` and `Foo<Q, infer B>` where `Q` is mapped by `mapper` into `number` - `B` is constrained to `T`
+                    // which is in turn instantiated as `Q`, which is in turn instantiated as `number`.
+                    // So we need to:
+                    //    * Clone the type parameters so their constraints can be instantiated in the context of `mapper` (otherwise theyd only get inference context information)
+                    //    * Set the clones to both map the conditional's enclosing `mapper` and the original params
+                    //    * instantiate the extends type with the clones
+                    //    * incorporate all of the component mappers into the combined mapper for the true and false members
+                    // This means 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`)
+                    const freshParams = sameMap(root.inferTypeParameters, maybeCloneTypeParameter);
+                    const freshMapper = freshParams !== root.inferTypeParameters ? createTypeMapper(root.inferTypeParameters, freshParams) : undefined;
+                    const context = createInferenceContext(freshParams, /*signature*/ undefined, InferenceFlags.None);
+                    if (freshMapper) {
+                        const freshCombinedMapper = combineTypeMappers(mapper, freshMapper);
+                        for (const p of freshParams) {
+                            if (root.inferTypeParameters.indexOf(p) === -1) {
+                                p.mapper = freshCombinedMapper;
+                            }
+                        }
+                    }
+                    // We skip inference of the possible `infer` types unles the `extendsType` _is_ an infer type
+                    // if it was, it's trivial to say that extendsType = checkType, however such a pattern is used to
+                    // "reset" the type being build up during constraint calculation and avoid making an apparently "infinite" constraint
+                    // so in those cases we refain from performing inference and retain the uninfered type parameter
+                    if (!checkTypeInstantiable || !some(root.inferTypeParameters, t => t === extendsType)) {
                         // 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);
                     }
+                    const innerMapper = combineTypeMappers(freshMapper, context.mapper);
                     // It's possible for 'infer T' type paramteters to be given uninstantiated constraints when the
                     // those type parameters are used in type references (see getInferredTypeParameterConstraint). For
                     // that reason we need context.mapper to be first in the combined mapper. See #42636 for examples.
-                    combinedMapper = mapper ? combineTypeMappers(context.mapper, mapper) : context.mapper;
+                    combinedMapper = mapper ? combineTypeMappers(innerMapper, mapper) : innerMapper;
                 }
                 // Instantiate the extends type including inferences for 'infer T' type parameters
                 const inferredExtendsType = combinedMapper ? instantiateType(unwrapNondistributiveConditionalTuple(root, root.extendsType), combinedMapper) : extendsType;

tests/baselines/reference/conditionalDoesntLeakUninstantiatedTypeParameter.errors.txt

@@ -0,0 +1,14 @@
+tests/cases/compiler/conditionalDoesntLeakUninstantiatedTypeParameter.ts(7,7): error TS2322: Type 'string' 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 'string' 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)