Fixed a ghost error popping up after completions request caching a wrong contextual function type

[Andarist]

fixes #58351

[Andarist]

This becomes more and more complicated and error-prone. I'm definitely not a fan of this. It builds on top of the existing solution... so at least it shouldn't be significantly worse now. It's just that perhaps there is a completely better way to do it than the existing mechanism. I think that alternatives could be explored separately.

[Andarist]

I thought about using infra introduced in #57421 but that PR is still in a draft. The problem space is also a little bit different - in this PR here we don't need to revert some recent work. In here, we need to temporarily evict some preexisting caches and restore them - we don't need to speculate per se. I assume that both are interested in the same lists of caches though.

[Andarist]

The key here is to reset NodeCheckFlags.ContextChecked so contextuallyCheckFunctionExpressionOrObjectLiteralMethod can correctly resolve (and cache) signature.resolvedReturnType multiple times (for requests with inference blocked and for requests without inference blocked). Otherwise, it has to be resolved later on... but without proper contextual information.

So I could just remove that bit and re-add it later if needed. I looked through all NodeCheckFlags and it might also be needed to do the same for NodeCheckFlags.TypeChecked. Although, I'm not 100% sure if that's really the case.

I just tried with a complete flags reset here and it does seem to work... but 🤷‍♂️ I'd probably be in favor of resetting solely what's really-really needed.

[Andarist]

@jakebailey could u build a playground for this one?

[jakebailey]

@typescript-bot pack this

[typescript-bot]

Starting jobs; this comment will be updated as builds start and complete.

Command	Status	Results
pack this	✅ Started	✅ Results

[typescript-bot]

Hey @jakebailey, I've packed this into an installable tgz. You can install it for testing by referencing it in your package.json like so:

{
    "devDependencies": {
        "typescript": "https://typescript.visualstudio.com/cf7ac146-d525-443c-b23c-0d58337efebc/_apis/build/builds/165443/artifacts?artifactName=tgz&fileId=19A54E0F368D999E48289F2485543C4D299B83DBD5ECD3179955337BE892D2E802&fileName=/typescript-5.9.0-insiders.20250623.tgz"
    }
}

and then running npm install.

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There is also a playground for this build and an npm module you can use via "typescript": "npm:@typescript-deploys/pr-build@5.9.0-pr-58378-3".;