functions with destructuring parameters are not inferred as type predicate correctly

[Zzzen]

🔎 Search Terms

infer type predicate, destructuring

🕗 Version & Regression Information

• This changed between versions 5.5
• This changed in commit or PR _______
• This is the behavior in every version I tried, and I reviewed the FAQ for entries about _________
• I was unable to test this on prior versions because _______

⏯ Playground Link

https://www.typescriptlang.org/play/?#code/C4TwDgpgBAGlC8UDeVSQFxQOQEEsG4oBDTAZ2ACcBLAOwHMoBfKAH2VXAkywCECoARmUq0GjfACgANhGBQAHqUwwA2gF0EUdZIkB6XVFIALAPYBXKQBMoNE3IgUKJihIDGJmuSgAzKlOAOEJYAjJgoaFzYeIQkhiL0TOqaihJQUAB0vv4OABQ54ZxMAJQIAHwckAjwiLhYRTrunnJZARRBAExhFZG1McLUCYxJiClpmX6tOfJlCukRVTV49RJAA

💻 Code

type X = { type: 'A'; a: string } | { type: 'B'; b: string };
let xs: X[] = [];

// should not error
const filtered1: { type: 'A'; a: string }[] = xs
  .filter(({ type }) => type === 'A');

const filtered2: { type: 'A'; a: string }[] = xs
  .filter(x => x.type === 'A');

🙁 Actual behavior

error on filtered1

🙂 Expected behavior

not error on filtered1

Additional information about the issue

No response

[Andarist]

That's (partly) because there is no identifier to use there:

const filtered1: { type: 'A'; a: string }[] = xs
  .filter(({ type }): ??? is { type: "A"; a: string; } => type === 'A');

One could be generated though but it would slightly go against the recent "reuse as much as possible from the source file" goal. It would require replacing the whole binding pattern with an auto-generated identifier

[miguel-leon]

But there is an identifier there: type.
It would be beautiful for the predicate to be:

const filtered1: { type: 'A'; a: string }[] = xs
  .filter(({ type }): type is "A" => type === 'A');

[Andarist]

Sure, but then you'd have to make this a valid type predicate because it errors today with:

A type predicate cannot reference element 'type' in a binding pattern.(1230)

Not impossible but it asks new questions about the design of how this should work.