Type parameter not usable as type argument in identical type; union in constraint becomes intersection

🔎 Search Terms

indexed access, constraints, generic, unions, ts2344

🕗 Version & Regression Information

• This changed between versions 3.3 and 3.5 (likely #30769)

⏯ Playground Link

Playground link

💻 Code

interface Foo<T extends { x: string }, U extends (T | { x: "a" })['x']> {
    z: Foo<T, U> // error!
    //        ~ Type 'U' does not satisfy the constraint 'T["x"] & "a"'. 🙃
}

🙁 Actual behavior

The U type parameter is rejected as a type argument with a TS2344 error about how it doesn't satisfy the constraint. The constraint seems to have shifted from a union to an intersection, even though it comes from an identical place.

🙂 Expected behavior

The U type parameter should be accepted as a type argument.

Additional information about the issue

Distilled from SO question.

I expect this is a consequence of #30769 as per https://github.com/microsoft/TypeScript/issues/31731#issuecomment-498465358, but it is at least somewhat surprising that this should happen when the types involved are identical. One could rewrite the constraint to U extends T["x"] | "a", of course, but this is distilled from the above SO question which is presumably distilled from some use case. What's happening here, exactly, and is it intended, a design limitation, or a bona fide bug?

Comienza con el TypeScript Playground enlazado y el ejemplo mínimo de una restricción genérica del issue. Investiga por qué la restricción de acceso indexado se trata como una intersección en lugar de la unión esperada, teniendo en cuenta la regresión reportada entre las versiones 3.3 y 3.5 y el issue relacionado #30769. Se considera terminado cuando se haya determinado si esto es intencionado o se haya corregido el comportamiento para que U se acepte sin TS2344.