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Allow extends type to be supertype of constraint types when narrowing conditional return type #61117

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Allow extends type to be supertype of constraint types when narrowing conditional return type #61117

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a668b6b
allow extends type to be supertype of constraint types
gabritto Feb 4, 2025
c140449
update comment
gabritto Feb 5, 2025
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4 changes: 2 additions & 2 deletions src/compiler/checker.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -46082,7 +46082,7 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
// (0) The conditional type is distributive;
// (1) The conditional type has no `infer` type parameters;
// (2) The conditional type's check type is a narrowable type parameter (i.e. a type parameter with a union constraint);
// (3) The extends type `A` is a type or a union of types belonging to the union constraint of the type parameter;
// (3) The extends type `A` is a type or a union of types that are supertypes of the union constraint of the type parameter;
// (4) `TrueBranch<T>` and `FalseBranch<T>` must be valid, recursively.
// In particular, the false-most branch of the conditional type must be `never`.
function isNarrowableConditionalTypeWorker(type: ConditionalType): boolean {
Expand Down Expand Up @@ -46111,7 +46111,7 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
!everyType(type.extendsType, extendsType =>
some(
(constraintType as UnionType).types,
constraintType => isTypeIdenticalTo(constraintType, extendsType),
constraintType => isTypeAssignableTo(constraintType, extendsType),
))
) {
return false;
Expand Down
37 changes: 15 additions & 22 deletions tests/baselines/reference/dependentReturnType1.errors.txt
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Expand Up @@ -26,25 +26,23 @@ dependentReturnType1.ts(275,9): error TS2322: Type '1' is not assignable to type
dependentReturnType1.ts(278,9): error TS2322: Type '2' is not assignable to type 'HelperCond<{ x: U; y: V; }, { x: string; y: true; }, 1, { x: number; y: false; }, 2>'.
dependentReturnType1.ts(280,5): error TS2322: Type '0' is not assignable to type 'HelperCond<{ x: U; y: V; }, { x: string; y: true; }, 1, { x: number; y: false; }, 2>'.
dependentReturnType1.ts(302,9): error TS2322: Type 'string' is not assignable to type 'string[]'.
dependentReturnType1.ts(311,9): error TS2322: Type 'undefined' is not assignable to type 'T extends {} ? void : T extends undefined ? number : never'.
dependentReturnType1.ts(313,5): error TS2322: Type 'number' is not assignable to type 'T extends {} ? void : T extends undefined ? number : never'.
dependentReturnType1.ts(334,9): error TS2322: Type '1' is not assignable to type '4'.
dependentReturnType1.ts(367,13): error TS2322: Type 'number' is not assignable to type 'T extends 1 ? number : T extends 2 ? string : never'.
dependentReturnType1.ts(369,9): error TS2322: Type 'string' is not assignable to type 'T extends 1 ? number : T extends 2 ? string : never'.
dependentReturnType1.ts(392,9): error TS2322: Type '2' is not assignable to type 'T extends true ? 1 : T extends false ? 2 : never'.
dependentReturnType1.ts(402,13): error TS2322: Type 'number' is not assignable to type 'string'.
dependentReturnType1.ts(412,9): error TS2322: Type 'true' is not assignable to type 'T extends [infer R] ? R : T extends number ? boolean : never'.
dependentReturnType1.ts(414,5): error TS2322: Type '""' is not assignable to type 'T extends [infer R] ? R : T extends number ? boolean : never'.
dependentReturnType1.ts(439,15): error TS2322: Type '1' is not assignable to type 'T extends true ? 1 : T extends false ? 2 : never'.
dependentReturnType1.ts(441,11): error TS2322: Type '2' is not assignable to type 'T extends true ? 1 : T extends false ? 2 : never'.
dependentReturnType1.ts(470,13): error TS2322: Type 'R' is not assignable to type 'ConditionalReturnType<T, R, EOp>'.
dependentReturnType1.ts(472,13): error TS2322: Type 'R' is not assignable to type 'ConditionalReturnType<T, R, EOp>'.
dependentReturnType1.ts(474,13): error TS2322: Type 'T' is not assignable to type 'ConditionalReturnType<T, R, EOp>'.
dependentReturnType1.ts(488,9): error TS2322: Type 'R' is not assignable to type 'ConditionalReturnType<T, R, EOp>'.
dependentReturnType1.ts(514,5): error TS2322: Type '1' is not assignable to type 'never'.
dependentReturnType1.ts(333,9): error TS2322: Type '1' is not assignable to type '4'.
dependentReturnType1.ts(366,13): error TS2322: Type 'number' is not assignable to type 'T extends 1 ? number : T extends 2 ? string : never'.
dependentReturnType1.ts(368,9): error TS2322: Type 'string' is not assignable to type 'T extends 1 ? number : T extends 2 ? string : never'.
dependentReturnType1.ts(391,9): error TS2322: Type '2' is not assignable to type 'T extends true ? 1 : T extends false ? 2 : never'.
dependentReturnType1.ts(401,13): error TS2322: Type 'number' is not assignable to type 'string'.
dependentReturnType1.ts(411,9): error TS2322: Type 'true' is not assignable to type 'T extends [infer R] ? R : T extends number ? boolean : never'.
dependentReturnType1.ts(413,5): error TS2322: Type '""' is not assignable to type 'T extends [infer R] ? R : T extends number ? boolean : never'.
dependentReturnType1.ts(438,15): error TS2322: Type '1' is not assignable to type 'T extends true ? 1 : T extends false ? 2 : never'.
dependentReturnType1.ts(440,11): error TS2322: Type '2' is not assignable to type 'T extends true ? 1 : T extends false ? 2 : never'.
dependentReturnType1.ts(469,13): error TS2322: Type 'R' is not assignable to type 'ConditionalReturnType<T, R, EOp>'.
dependentReturnType1.ts(471,13): error TS2322: Type 'R' is not assignable to type 'ConditionalReturnType<T, R, EOp>'.
dependentReturnType1.ts(473,13): error TS2322: Type 'T' is not assignable to type 'ConditionalReturnType<T, R, EOp>'.
dependentReturnType1.ts(487,9): error TS2322: Type 'R' is not assignable to type 'ConditionalReturnType<T, R, EOp>'.
dependentReturnType1.ts(513,5): error TS2322: Type '1' is not assignable to type 'never'.


==== dependentReturnType1.ts (39 errors) ====
==== dependentReturnType1.ts (37 errors) ====
interface A {
1: number;
2: string;
Expand Down Expand Up @@ -401,18 +399,13 @@ dependentReturnType1.ts(514,5): error TS2322: Type '1' is not assignable to type
}
}

// No narrowing because conditional's extends type is different from type parameter constraint types
function voidRet<T extends { a: string } | undefined>(
x: T
): T extends {} ? void : T extends undefined ? number : never {
if (x) {
return;
~~~~~~
!!! error TS2322: Type 'undefined' is not assignable to type 'T extends {} ? void : T extends undefined ? number : never'.
}
return 1;
~~~~~~
!!! error TS2322: Type 'number' is not assignable to type 'T extends {} ? void : T extends undefined ? number : never'.
}

// Multiple type parameters at once
Expand Down
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