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

[Andarist]

fixes #62336

[Copilot]

Pull Request Overview

This PR fixes an issue where parameters were not being contextually typed when using an instantiated rest return mapper. The change allows the TypeScript compiler to properly infer parameter types in complex generic scenarios involving rest parameters.

Key Changes

• Updates type inference logic in the checker to use return mapper when appropriate for rest type parameters
• Adds a test case demonstrating the fixed contextual typing behavior

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
src/compiler/checker.ts	Modifies type inference logic to combine return mapper with non-fixing mapper for rest type parameters
tests/cases/compiler/contextuallyTypeParametersUsingInstantiantedRestReturnMapper1.ts	Adds test case for the contextual typing fix
tests/baselines/reference/contextuallyTypeParametersUsingInstantiantedRestReturnMapper1.types	Baseline file showing expected type inference results
tests/baselines/reference/contextuallyTypeParametersUsingInstantiantedRestReturnMapper1.symbols	Baseline file showing expected symbol resolution

[Andarist]

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.

[typescript-bot]

With 6.0 out as the final release vehicle for this codebase, we're closing all PRs that don't fit the merge criteria for post-6.0 patches. If you think this was a mistake and this PR fits the post-6.0 patch criteria, please post to the 6.0 iteration issue with details (specifically, which PR and which patch criteria it satisfies).

Next steps for PRs:

• For crash bugfixes or language service improvements, PRs are currently accepted at the typescript-go repo
• Changes to type system behavior should wait until after 7.0, at which point mainline TypeScript development will resume in this repository with the Go codebase
• Library file updates (lib.d.ts etc) continue to live in this repo or the DOM Generator repo as appropriate