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Merged
jakebailey merged 12 commits into from
Dec 3, 2025
Merged

Filter return type inferences by constraint applicability #58910

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b9936e3
Filter return type inferences by constraint applicability
Andarist Jun 18, 2024
ffb2da0
Merge remote-tracking branch 'origin/main' into fix/filter-out-return…
Andarist Jun 18, 2024
74e6799
Merge remote-tracking branch 'origin/main' into fix/filter-out-return…
Andarist Jul 13, 2024
0ecedb6
fixed a silly mistake
Andarist Jul 13, 2024
b7418c7
add extra test case
Andarist Jul 13, 2024
1c744ca
add an extra test case
Andarist Jul 13, 2024
24f5964
dont forget about the fallback type
Andarist Jul 13, 2024
b0563e8
Merge remote-tracking branch 'origin/main' into fix/filter-out-return…
Andarist Sep 9, 2024
97c164d
add extra test case
Andarist Sep 9, 2024
c8e938c
Merge remote-tracking branch 'origin/main' into fix/filter-out-return…
Andarist Jun 22, 2025
6db731b
Merge remote-tracking branch 'origin/main' into fix/filter-out-return…
Andarist Dec 3, 2025
35e0002
match ts-go changes
Andarist Dec 3, 2025
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23 changes: 20 additions & 3 deletions src/compiler/checker.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -26852,14 +26852,31 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
const constraint = getConstraintOfTypeParameter(inference.typeParameter);
if (constraint) {
const instantiatedConstraint = instantiateType(constraint, context.nonFixingMapper);
if (!inferredType || !context.compareTypes(inferredType, getTypeWithThisArgument(instantiatedConstraint, inferredType))) {
// If the fallback type satisfies the constraint, we pick it. Otherwise, we pick the constraint.
inference.inferredType = fallbackType && context.compareTypes(fallbackType, getTypeWithThisArgument(instantiatedConstraint, fallbackType)) ? fallbackType : instantiatedConstraint;
if (!inferredType) {
inference.inferredType = instantiatedConstraint;
}
else {
if (inference.priority! & InferencePriority.ReturnType) {
inference.inferredType = filterContextualInferredType(inferredType, instantiatedConstraint) ?? (fallbackType && filterContextualInferredType(fallbackType, instantiatedConstraint)) ?? instantiatedConstraint;
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@weswigham weswigham Jun 18, 2024

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We only need to bother with this filtering if the constraint check for the whole union fails, right? (Since if it passes, no filtering will occur, as every union member must also pass.) This can probably be inside the comparison branch below?

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@Andarist Andarist Jun 19, 2024

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That’s right but it felt cleaner to have this in a separate branch. Shouldnt filterType already optimize for array/type allocations?

}
else if (!context.compareTypes(inferredType, getTypeWithThisArgument(instantiatedConstraint, inferredType))) {
// If the fallback type satisfies the constraint, we pick it. Otherwise, we pick the constraint.
inference.inferredType = fallbackType && context.compareTypes(fallbackType, getTypeWithThisArgument(instantiatedConstraint, fallbackType)) ? fallbackType : instantiatedConstraint;
}
}
}
}

return inference.inferredType;

function filterContextualInferredType(inferredType: Type, constraint: Type) {
if (inferredType.flags & TypeFlags.Never) {
return inferredType;
}
const constraintWithThisArgument = getTypeWithThisArgument(constraint, inferredType);
const applicableByConstraint = filterType(inferredType, t => !!context.compareTypes(t, constraintWithThisArgument));
return !(applicableByConstraint.flags & TypeFlags.Never) ? inferredType : undefined;
}
}

function getDefaultTypeArgumentType(isInJavaScriptFile: boolean): Type {
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53 changes: 53 additions & 0 deletions tests/baselines/reference/inferenceContextualReturnTypeUnion1.symbols
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@@ -0,0 +1,53 @@
//// [tests/cases/compiler/inferenceContextualReturnTypeUnion1.ts] ////

=== inferenceContextualReturnTypeUnion1.ts ===
// https://github.com/microsoft/TypeScript/issues/50719

declare function useCallback1<T extends Function>(fn: T): T;
>useCallback1 : Symbol(useCallback1, Decl(inferenceContextualReturnTypeUnion1.ts, 0, 0))
>T : Symbol(T, Decl(inferenceContextualReturnTypeUnion1.ts, 2, 30))
>Function : Symbol(Function, Decl(lib.es5.d.ts, --, --), Decl(lib.es5.d.ts, --, --))
>fn : Symbol(fn, Decl(inferenceContextualReturnTypeUnion1.ts, 2, 50))
>T : Symbol(T, Decl(inferenceContextualReturnTypeUnion1.ts, 2, 30))
>T : Symbol(T, Decl(inferenceContextualReturnTypeUnion1.ts, 2, 30))

declare function ex2(callback?: (x: number) => void): void;
>ex2 : Symbol(ex2, Decl(inferenceContextualReturnTypeUnion1.ts, 2, 60))
>callback : Symbol(callback, Decl(inferenceContextualReturnTypeUnion1.ts, 4, 21))
>x : Symbol(x, Decl(inferenceContextualReturnTypeUnion1.ts, 4, 33))

ex2(useCallback1((x) => {}));
>ex2 : Symbol(ex2, Decl(inferenceContextualReturnTypeUnion1.ts, 2, 60))
>useCallback1 : Symbol(useCallback1, Decl(inferenceContextualReturnTypeUnion1.ts, 0, 0))
>x : Symbol(x, Decl(inferenceContextualReturnTypeUnion1.ts, 5, 18))

declare function ex3(callback: ((x: number) => void) | 5): void;
>ex3 : Symbol(ex3, Decl(inferenceContextualReturnTypeUnion1.ts, 5, 29))
>callback : Symbol(callback, Decl(inferenceContextualReturnTypeUnion1.ts, 7, 21))
>x : Symbol(x, Decl(inferenceContextualReturnTypeUnion1.ts, 7, 33))

ex3(useCallback1((x) => {}));
>ex3 : Symbol(ex3, Decl(inferenceContextualReturnTypeUnion1.ts, 5, 29))
>useCallback1 : Symbol(useCallback1, Decl(inferenceContextualReturnTypeUnion1.ts, 0, 0))
>x : Symbol(x, Decl(inferenceContextualReturnTypeUnion1.ts, 8, 18))

// https://github.com/microsoft/TypeScript/issues/41461

declare function useCallback2<T extends (...args: any[]) => any>(
>useCallback2 : Symbol(useCallback2, Decl(inferenceContextualReturnTypeUnion1.ts, 8, 29))
>T : Symbol(T, Decl(inferenceContextualReturnTypeUnion1.ts, 12, 30))
>args : Symbol(args, Decl(inferenceContextualReturnTypeUnion1.ts, 12, 41))

callback: T,
>callback : Symbol(callback, Decl(inferenceContextualReturnTypeUnion1.ts, 12, 65))
>T : Symbol(T, Decl(inferenceContextualReturnTypeUnion1.ts, 12, 30))

): T;
>T : Symbol(T, Decl(inferenceContextualReturnTypeUnion1.ts, 12, 30))

const test: ((x: string) => void) | undefined = useCallback2((x) => {});
>test : Symbol(test, Decl(inferenceContextualReturnTypeUnion1.ts, 15, 5))
>x : Symbol(x, Decl(inferenceContextualReturnTypeUnion1.ts, 15, 14))
>useCallback2 : Symbol(useCallback2, Decl(inferenceContextualReturnTypeUnion1.ts, 8, 29))
>x : Symbol(x, Decl(inferenceContextualReturnTypeUnion1.ts, 15, 62))

82 changes: 82 additions & 0 deletions tests/baselines/reference/inferenceContextualReturnTypeUnion1.types
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Original file line number Diff line number Diff line change
@@ -0,0 +1,82 @@
//// [tests/cases/compiler/inferenceContextualReturnTypeUnion1.ts] ////

=== inferenceContextualReturnTypeUnion1.ts ===
// https://github.com/microsoft/TypeScript/issues/50719

declare function useCallback1<T extends Function>(fn: T): T;
>useCallback1 : <T extends Function>(fn: T) => T
> : ^ ^^^^^^^^^ ^^ ^^ ^^^^^
>fn : T
> : ^

declare function ex2(callback?: (x: number) => void): void;
>ex2 : (callback?: (x: number) => void) => void
> : ^ ^^^ ^^^^^
>callback : ((x: number) => void) | undefined
> : ^^ ^^ ^^^^^ ^^^^^^^^^^^^^
>x : number
> : ^^^^^^

ex2(useCallback1((x) => {}));
>ex2(useCallback1((x) => {})) : void
> : ^^^^
>ex2 : (callback?: (x: number) => void) => void
> : ^ ^^^ ^^^^^
>useCallback1((x) => {}) : (x: number) => void
> : ^ ^^^^^^^^^^^^^^^^^
>useCallback1 : <T extends Function>(fn: T) => T
> : ^ ^^^^^^^^^ ^^ ^^ ^^^^^
>(x) => {} : (x: number) => void
> : ^ ^^^^^^^^^^^^^^^^^
>x : number
> : ^^^^^^

declare function ex3(callback: ((x: number) => void) | 5): void;
>ex3 : (callback: ((x: number) => void) | 5) => void
> : ^ ^^ ^^^^^
>callback : ((x: number) => void) | 5
> : ^^ ^^ ^^^^^ ^^^^^
>x : number
> : ^^^^^^

ex3(useCallback1((x) => {}));
>ex3(useCallback1((x) => {})) : void
> : ^^^^
>ex3 : (callback: ((x: number) => void) | 5) => void
> : ^ ^^ ^^^^^
>useCallback1((x) => {}) : (x: number) => void
> : ^ ^^^^^^^^^^^^^^^^^
>useCallback1 : <T extends Function>(fn: T) => T
> : ^ ^^^^^^^^^ ^^ ^^ ^^^^^
>(x) => {} : (x: number) => void
> : ^ ^^^^^^^^^^^^^^^^^
>x : number
> : ^^^^^^

// https://github.com/microsoft/TypeScript/issues/41461

declare function useCallback2<T extends (...args: any[]) => any>(
>useCallback2 : <T extends (...args: any[]) => any>(callback: T) => T
> : ^ ^^^^^^^^^ ^^ ^^ ^^^^^
>args : any[]
> : ^^^^^

callback: T,
>callback : T
> : ^

): T;
const test: ((x: string) => void) | undefined = useCallback2((x) => {});
>test : ((x: string) => void) | undefined
> : ^^ ^^ ^^^^^ ^^^^^^^^^^^^^
>x : string
> : ^^^^^^
>useCallback2((x) => {}) : (x: string) => void
> : ^ ^^^^^^^^^^^^^^^^^
>useCallback2 : <T extends (...args: any[]) => any>(callback: T) => T
> : ^ ^^^^^^^^^ ^^ ^^ ^^^^^
>(x) => {} : (x: string) => void
> : ^ ^^^^^^^^^^^^^^^^^
>x : string
> : ^^^^^^

19 changes: 19 additions & 0 deletions tests/cases/compiler/inferenceContextualReturnTypeUnion1.ts
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Original file line number Diff line number Diff line change
@@ -0,0 +1,19 @@
// @strict: true
// @noEmit: true

// https://github.com/microsoft/TypeScript/issues/50719

declare function useCallback1<T extends Function>(fn: T): T;

declare function ex2(callback?: (x: number) => void): void;
ex2(useCallback1((x) => {}));

declare function ex3(callback: ((x: number) => void) | 5): void;
ex3(useCallback1((x) => {}));

// https://github.com/microsoft/TypeScript/issues/41461

declare function useCallback2<T extends (...args: any[]) => any>(
callback: T,
): T;
const test: ((x: string) => void) | undefined = useCallback2((x) => {});