Can no longer write deep predicates in conditional types

[geoffreytools]

Bug Report

🔎 Search Terms

conditional types, generics, deep, predicate

🕗 Version & Regression Information

This changed between versions 4.8.4 and 4.9.4

⏯ Playground Link

Playground link with the code bellow

💻 Code

type Foo<A,B> = [A, B] extends unknown[][] ? Bar<[A, B]> : 'else'
//                                               ~~~~~~
// Type '[A, B]' does not satisfy the constraint 'unknown[][]'.

type Bar<T extends unknown[][]> = T

This is not limited to tuples

type Foo<A> = Set<A> extends Set<unknown[]> ? Bar<Set<A>> : 'else'
//                                                ~~~~~
// Type 'Set<A>' does not satisfy the constraint 'Set<unknown[]>'.

type Bar<T extends Set<unknown[]>> = T

🙁 Actual behavior

There is a type error that literally contradicts the predicate of the conditional

🙂 Expected behavior

It should compile with no error

[jakebailey]

Both of these compile in nightly (soon to be beta) as of #51621, so this looks to be a duplicate of #51614.

(Flip your playground to nightly to see.)

[geoffreytools]

It's great that it is fixed but it must have been in a different PR, or maybe it was an unintentional side-effect, because these 2 issues are not related to me: the outcome of the conditional cannot be known here.

I feel that a test case should be added for this scenario if it hasn't been already.

[jakebailey]

That PR definitely fixed it; I bisected to confirm before replying.

[jakebailey]

I've also checked and it was #50397 which regressed this.

[jakebailey]

Sent #52333 with a test.