Infer on tuples broken on nightly

[mikearnaldi]

Bug Report

The latest nightly release breaks non distributive infer

🕗 Version & Regression Information

Bug introduced in the latest nighty release

💻 Code

type _R<T> = [T] extends [{ _R: (_: infer R) => void }] ? R : never
type _E<T> = [T] extends [{ _E: () => infer E }] ? E : never
type _A<T> = [T] extends [{ _A: () => infer A }] ? A : never

interface Sync<R, E, A> {
  _R: (_: R) => void
  _E: () => E
  _A: () => A
}

type R = _R<Sync<number, string, void>>
type E = _E<Sync<number, string, void>>
type A = _A<Sync<number, string, void>>

🙁 Actual behavior

R = never
E = never
A = never

https://www.typescriptlang.org/play?ts=4.2.0-dev.20210114#code/C4TwDgpgBA+gSgHgCoD4oF4oG0kF0oQAewEAdgCYDO2A3rHAFxQAUMTAlqQGYQBOUcAJQY0ANwD27clAC++APwCoTUhFF8AUKEiwAosjSYc+IiQrUsdGLqbNh6NJx79dshVFcq1m7dBgBBAwxsPAJiMipaWH9be0duPih-NyhFZK91Xg0NThJeLgBDAGNoAGUQUiKEOAAaDzr-NBoNKHpbNgE4qAkpFr1YkQ8+gIGHJI0ZbN8lTHgEcsqEUgBXAFsAIz46ymBeTgBzOp7yFBQtcGhXWf0FqpWNragdvdJD7skTs+nk2cDbpbWm14212ByOH1OGiAA

🙂 Expected behavior

R = number
E = string
A = void

https://www.typescriptlang.org/play?ts=4.2.0-beta#code/C4TwDgpgBA+gSgHgCoD4oF4oG0kF0oQAewEAdgCYDO2A3rHAFxQAUMTAlqQGYQBOUcAJQY0ANwD27clAC++APwCoTUhFF8AUKEiwAosjSYc+IiQrUsdGLqbNh6NJx79dshVFcq1m7dBgBBAwxsPAJiMipaWH9be0duPih-NyhFZK91Xg0NThJeLgBDAGNoAGUQUiKEOAAaDzr-NBoNKHpbNgE4qAkpFr1YkQ8+gIGHJI0ZbN8lTHgEcsqEUgBXAFsAIz46ymBeTgBzOp7yFBQtcGhXWf0FqpWNragdvdJD7skTs+nk2cDbpbWm14212ByOH1OGiAA

[r-cyr]

I can reproduce it, last working build was 4.2.0-dev.20210112.

[mikearnaldi]

It seems related to tuples, rewriting as:

type _R<T> = {t: T} extends {t: { _R: (_: infer R) => void }} ? R : never
type _E<T> = {t: T} extends {t: { _E: () => infer E }} ? E : never
type _A<T> = {t: T} extends {t: { _A: () => infer A }} ? A : never

interface Sync<R, E, A> {
  _R: (_: R) => void
  _E: () => E
  _A: () => A
}

type R = _R<Sync<number, string, void>>
type E = _E<Sync<number, string, void>>
type A = _A<Sync<number, string, void>>

Works on nightly too

[mikearnaldi]

Well seems definitely related to tuples:

type X = [0] extends [infer A] ? A : never

X here is never instead of 0

[ahejlsberg]

@weswigham Might this be caused by #42248?

[weswigham]

Hm, must be? I'll have to see why it caused this, since unwrapping the tuples for the "is generic" checks doesn't seem like it should affect inference of infer variables. And add some coverage into our test suite since this is pretty common.

[jcalz]

Is this related or unrelated?

type Hmm = [never] extends [infer T, ...any[]] ?
    [T, [never] extends [T] ? true : false]
    : never

// TS 4.1.5
// type Hmm = [never, true] ✔

// TS 4.2.2
// type Hmm = [never, false] ❌
// So T is inferred as never, but [never] extends [T] is false?