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Can no longer write deep predicates in conditional types #52271

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#52333
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Can no longer write deep predicates in conditional types#52271
#52333
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jakebailey
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BugA bug in TypeScript

Description

@geoffreytools
Geoffrey (geoffreytools)
opened on Jan 17, 2023

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

Activity

  1. jakebailey commented on Jan 20, 2023

    @jakebailey
    Jake Bailey (jakebailey)
    on Jan 20, 2023
    Member

    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.)

  2. geoffreytools commented on Jan 20, 2023

    @geoffreytools
    Geoffrey (geoffreytools)
    on Jan 20, 2023
    Author

    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.

  3. jakebailey commented on Jan 20, 2023

    @jakebailey
    Jake Bailey (jakebailey)
    on Jan 20, 2023
    Member

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

  4. jakebailey commented on Jan 20, 2023

    @jakebailey
    Jake Bailey (jakebailey)
    on Jan 20, 2023
    Member

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

  5. Jake Bailey (jakebailey)
    added 2 commits that reference this issue on Jan 20, 2023

    Add test case for microsoft#52271

    ae40d80

    Add test case for microsoft#52271

    cbf4be5

  6. jakebailey commented on Jan 20, 2023

    @jakebailey
    Jake Bailey (jakebailey)
    on Jan 20, 2023
    Member

    Sent #52333 with a test.

  7. Ryan Cavanaugh (RyanCavanaugh)
    assigned
    Jake Bailey (jakebailey)
    on Jan 23, 2023
  8. Ryan Cavanaugh (RyanCavanaugh)
    added
    BugA bug in TypeScript
    on Jan 23, 2023
  9. Jake Bailey (jakebailey)
    closed this as completedin #52333on Jan 23, 2023
  10. Jake Bailey (jakebailey)
    added a commit that references this issue on Jan 23, 2023

    Add test case for #52271 (#52333)

    87c808f
  11. microsoft
    locked as resolved and limited conversation to collaborators on Oct 22, 2025
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