Conditional type does not work when generic

[ehmicky]

🔎 Search Terms

intersection, empty, generic

🕗 Version & Regression Information

This is the behavior in every version I tried (from 3.3.3 to 5.5.0-beta), and I reviewed the FAQ for entries about intersections and conditional types.

⏯ Playground Link

https://www.typescriptlang.org/play/?ts=5.5.0-beta#code/C4TwDgpgBAggznCAnY84EsDmA7AhgIwBsIAeAVQBooAVKCAD2AmwBM4oyA+KAXhoFgAUENCRYJWnwCu2ANbYA9gHds3PgG8wSBWABcUAAx1GzNjSgB+KMCRTo+gGa5CiAL4BuEeGgwATL1gEZFQELDwiUhgqGBIDTk53KAB6JKgAMVx0FyEvMQAhCQCZeWVVAM1tPUNjJlZ2WisbOyhHZzcoADIodQ9c6Dz-PjRgtDCCYhI8qgK4hOTUgGUpAGNliAg2HMFRaABhQuk5RRU1aoZaswbrW3soJxcIT23vKF3BwMQUUZxx0l2qfazRIpKBLVbrNhAA

💻 Code

type AssertAssignable<U, T extends U> = T

type A<T = unknown> = {prop: 0 extends T ? true : false};
type A2 = AssertAssignable<A, A<0>>; // Fails

type B<T = unknown> = {prop: 0 extends T ? true : false} & {};
type B2 = AssertAssignable<B, B<0>>; // Succeeds

type C<T = unknown> = 0 extends T ? true : false;
type C2 = AssertAssignable<C, C<0>>; // Succeeds

🙁 Actual behavior

Type A2 fails with:

Type 'A<0>' does not satisfy the constraint 'A<unknown>'.
  Type 'unknown' is not assignable to type '0'.

🙂 Expected behavior

A<0> should be assignable to A. I.e. A2 should not fail.

Additional information about the issue

B2 does not fail, i.e. adding & {} seems to strangely work around this problem.

C2 does not fail, i.e. this problem seems to only occur inside object properties.

[RyanCavanaugh]

Duplicate #44945

It's looking increasingly likely the only good fix here is going to be introducing an unreliable (name???) companion to the in and out variance markers and let users specify that

[typescript-bot]

This issue has been marked as "Duplicate" and has seen no recent activity. It has been automatically closed for house-keeping purposes.