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Allow parameters to be contextually-typed based on contextual rest instantiated using return mapper #62341

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Allow parameters to be contextually-typed based on contextual rest instantiated using return mapper #62341

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f3691a3
Allow parameters to be contextually-typed based on contextual rest in…
Andarist Aug 27, 2025
37499a8
fix filename typo
Andarist Aug 27, 2025
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4 changes: 3 additions & 1 deletion src/compiler/checker.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -39547,7 +39547,9 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
inferFromAnnotatedParametersAndReturn(signature, contextualSignature, inferenceContext!);
const restType = getEffectiveRestType(contextualSignature);
if (restType && restType.flags & TypeFlags.TypeParameter) {
instantiatedContextualSignature = instantiateSignature(contextualSignature, inferenceContext!.nonFixingMapper);
const info = find(inferenceContext!.inferences, info => info.typeParameter === restType);
const mapper = combineTypeMappers(info && !hasInferenceCandidates(info) ? inferenceContext!.returnMapper : undefined, inferenceContext!.nonFixingMapper);
instantiatedContextualSignature = instantiateSignature(contextualSignature, mapper);
}
}
instantiatedContextualSignature ||= inferenceContext ?
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42 changes: 42 additions & 0 deletions ...baselines/reference/contextuallyTypeParametersUsingInstantiantedRestReturnMapper1.symbols
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@@ -0,0 +1,42 @@
//// [tests/cases/compiler/contextuallyTypeParametersUsingInstantiantedRestReturnMapper1.ts] ////

=== contextuallyTypeParametersUsingInstantiantedRestReturnMapper1.ts ===
// https://github.com/microsoft/TypeScript/issues/62336

declare function call<T>(fn: (a: string, b: T) => unknown): (b: T) => unknown;
>call : Symbol(call, Decl(contextuallyTypeParametersUsingInstantiantedRestReturnMapper1.ts, 0, 0))
>T : Symbol(T, Decl(contextuallyTypeParametersUsingInstantiantedRestReturnMapper1.ts, 2, 22))
>fn : Symbol(fn, Decl(contextuallyTypeParametersUsingInstantiantedRestReturnMapper1.ts, 2, 25))
>a : Symbol(a, Decl(contextuallyTypeParametersUsingInstantiantedRestReturnMapper1.ts, 2, 30))
>b : Symbol(b, Decl(contextuallyTypeParametersUsingInstantiantedRestReturnMapper1.ts, 2, 40))
>T : Symbol(T, Decl(contextuallyTypeParametersUsingInstantiantedRestReturnMapper1.ts, 2, 22))
>b : Symbol(b, Decl(contextuallyTypeParametersUsingInstantiantedRestReturnMapper1.ts, 2, 61))
>T : Symbol(T, Decl(contextuallyTypeParametersUsingInstantiantedRestReturnMapper1.ts, 2, 22))

declare function fn<Args extends any[]>(
>fn : Symbol(fn, Decl(contextuallyTypeParametersUsingInstantiantedRestReturnMapper1.ts, 2, 78))
>Args : Symbol(Args, Decl(contextuallyTypeParametersUsingInstantiantedRestReturnMapper1.ts, 4, 20))

fn: (...args: Args) => unknown,
>fn : Symbol(fn, Decl(contextuallyTypeParametersUsingInstantiantedRestReturnMapper1.ts, 4, 40))
>args : Symbol(args, Decl(contextuallyTypeParametersUsingInstantiantedRestReturnMapper1.ts, 5, 7))
>Args : Symbol(Args, Decl(contextuallyTypeParametersUsingInstantiantedRestReturnMapper1.ts, 4, 20))

): (...args: Args) => unknown;
>args : Symbol(args, Decl(contextuallyTypeParametersUsingInstantiantedRestReturnMapper1.ts, 6, 4))
>Args : Symbol(Args, Decl(contextuallyTypeParametersUsingInstantiantedRestReturnMapper1.ts, 4, 20))

call(
>call : Symbol(call, Decl(contextuallyTypeParametersUsingInstantiantedRestReturnMapper1.ts, 0, 0))

fn(function (a, b: number) {
>fn : Symbol(fn, Decl(contextuallyTypeParametersUsingInstantiantedRestReturnMapper1.ts, 2, 78))
>a : Symbol(a, Decl(contextuallyTypeParametersUsingInstantiantedRestReturnMapper1.ts, 9, 15))
>b : Symbol(b, Decl(contextuallyTypeParametersUsingInstantiantedRestReturnMapper1.ts, 9, 17))

a; // string
>a : Symbol(a, Decl(contextuallyTypeParametersUsingInstantiantedRestReturnMapper1.ts, 9, 15))

}),
);

56 changes: 56 additions & 0 deletions ...s/baselines/reference/contextuallyTypeParametersUsingInstantiantedRestReturnMapper1.types
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@@ -0,0 +1,56 @@
//// [tests/cases/compiler/contextuallyTypeParametersUsingInstantiantedRestReturnMapper1.ts] ////

=== contextuallyTypeParametersUsingInstantiantedRestReturnMapper1.ts ===
// https://github.com/microsoft/TypeScript/issues/62336

declare function call<T>(fn: (a: string, b: T) => unknown): (b: T) => unknown;
>call : <T>(fn: (a: string, b: T) => unknown) => (b: T) => unknown
> : ^ ^^ ^^ ^^^^^
>fn : (a: string, b: T) => unknown
> : ^ ^^ ^^ ^^ ^^^^^
>a : string
> : ^^^^^^
>b : T
> : ^
>b : T
> : ^

declare function fn<Args extends any[]>(
>fn : <Args extends any[]>(fn: (...args: Args) => unknown) => (...args: Args) => unknown
> : ^ ^^^^^^^^^ ^^ ^^ ^^^^^

fn: (...args: Args) => unknown,
>fn : (...args: Args) => unknown
> : ^^^^ ^^ ^^^^^
>args : Args
> : ^^^^

): (...args: Args) => unknown;
>args : Args
> : ^^^^

call(
>call( fn(function (a, b: number) { a; // string }),) : (b: number) => unknown
> : ^ ^^^^^^^^^^^^^
>call : <T>(fn: (a: string, b: T) => unknown) => (b: T) => unknown
> : ^ ^^ ^^ ^^^^^

fn(function (a, b: number) {
>fn(function (a, b: number) { a; // string }) : (a: string, b: number) => unknown
> : ^^^^^^^^^^^^^^^^^^^^^^^^^^
>fn : <Args extends any[]>(fn: (...args: Args) => unknown) => (...args: Args) => unknown
> : ^ ^^^^^^^^^ ^^ ^^ ^^^^^
>function (a, b: number) { a; // string } : (a: string, b: number) => void
> : ^ ^^^^^^^^^^ ^^ ^^^^^^^^^
>a : string
> : ^^^^^^
>b : number
> : ^^^^^^

a; // string
>a : string
> : ^^^^^^

}),
);

16 changes: 16 additions & 0 deletions tests/cases/compiler/contextuallyTypeParametersUsingInstantiantedRestReturnMapper1.ts
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@@ -0,0 +1,16 @@
// @strict: true
// @noEmit: true

// https://github.com/microsoft/TypeScript/issues/62336

declare function call<T>(fn: (a: string, b: T) => unknown): (b: T) => unknown;

declare function fn<Args extends any[]>(
fn: (...args: Args) => unknown,
): (...args: Args) => unknown;

call(
fn(function (a, b: number) {
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In the presented case, there are no regular inference candidates for this rest because the return type candidate that could be gathered is only partially inferable ([a: string, b: silentNeverType]). That means it's treated as non-inferrable candidate and thus it's ignored.

But the return mapper manages to capture a [a: string, b: unknown] inference for the same Args.

So the proposal here is to utilize that mapping in the absence of regular candidates as that allows the inferrable part of the original partially candidate to be used when contextually typing parameters. Of course, that inferrable part is coming now from a completely different place (return mapper instead of that original partially inferrable candidate) - but conceptually they both used the information from the contextual signature so their "root origin" is the same.

a; // string
}),
);
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