Fix completions of optional properties in generic positions

[andrewbranch]

Builds on #32100

Fixes #30507
Fixes #28470

Notable changes from #32100:

• Doesn’t cache an alternative type during inference (@weswigham’s change request on the original PR)
• Rather than getting the contextual type from the base signature alone, this intersects the types from the base signature and the resolved signature, so that in a case like

type U = { x: boolean };
declare function f<T extends Partial<U>>(x: T, y: T)

f({ a: true }, { /**/ });

you get completions for both x and a. Thanks to @DanielRosenwasser for that suggestion.

[timsuchanek]

Awesome! Would be great to see this merged!

[sandersn]

One test idea plus some nits.

[AviVahl]

@andrewbranch this also fixes #28470

[andrewbranch]

Ugh, committing PR review suggestions from GitHub UI doesn’t work in checker.ts 😑

[DanielRosenwasser]

🎉

[timsuchanek]

Very excited to see this merged 🙏
One question: How can I try it out in VSCode?
Unfortunately I'm not able to use this properly yet, even though setting "typescript.tsdk" in my settings.json to node_modules/typescript/lib .

This example should now work in VSCode and show world as a completion:

interface MyOptions {
  hello?: boolean
  world?: boolean
}
declare function bar<T extends MyOptions>(options?: Partial<T>): void

bar({
  hello: true,
  /* cursor */
})

@andrewbranch can you confirm that this example is now working for you?

[timsuchanek]

I added a small PR to add exactly that test case, as it's important for us at Prisma that this one works.

#34559

[andrewbranch]

@timsuchanek yep, that example works in VS Code for me:

setting "typescript.tsdk" in my settings.json to node_modules/typescript/lib.

If you’re building the source from master, you want to set the path to built/local. The one in the repo’s node_modules is just whatever the nightly was whenever you last did a clean npm install 😄

[timsuchanek]

Awesome. I didn't know about the built/local. That also works for me.
Thanks a lot!