in conditional type, type parameter T is inferred as X, but [X] extends [T] is false

[jcalz]

Bug Report

🔎 Search Terms

conditional inference, tuple, never

🕗 Version & Regression Information

• This changed between versions 4.2.0-dev.20210112 and 4.2.0-dev.20210113, is present in 4.2.2, and continues to be present in nightly ( 4.3.0-dev.20210306 ). This is probably caused by Better detect when typical nondistributive conditionals need to be deferred...  #42248, possibly related to Infer on tuples broken on nightly #42331, but was not fixed by Add missing unwrap call for the inferredExtendsType #42409.

⏯ Playground Link

Playground link

💻 Code

type Hmm = [0] extends [infer T, any?] ?
    [T, [0] extends [T] ? true : false]
    : never

🙁 Actual behavior

Hmm is evaluated as [0, false]. Somehow T is inferred as 0, but [0] doesn't extend [T]?

🙂 Expected behavior

Hmm should be evaluated as [0, true] as it does in TS 4.1.5 and below.

This behavior was brought up in a Stack Overflow question asking why it happens; not sure if there's any actual use case for this.