Why can't this conditional with generic keyof be eagerly resolved?

Question

function broken<T>(value: T) {
  type Test = keyof T extends keyof T ? 1 : 0
}

Why can't Test in the example above be eagerly resolved to 1? There's something I don't understand about the compiler that should answer this question for me.

Interestingly enough, there is a workaround:

function workaround<T>(value: T) {
  type Test = [keyof T] extends [keyof T] ? 1 : 0
}

Playground here

Answer 1

The answer to the question probably involves a bug fix introduced in TypeScript 3.3. The description of the new way of evaluating conditional types is:

For a conditional type T extends U ? X : Y, the algorithm we use to determine whether to defer resolution of a conditional type is:

• we resolve to Y when T is not assignable to U considering all type parameters referenced in T and U related (i.e. T is definitely not assignable to U),
• otherwise we resolve to X when T is assignable to U considering all type parameters referenced in T and U unrelated (i.e. T is definitely assignable to U),
• otherwise we defer resolution.

So it looks like keyof T extends keyof T ? 1 : 0 is deferred, because keyof T1 is not assignable to keyof T2 when T1 and T2 are unrelated types. This deferral isn't exactly wrong, but it does seem like a limitation.

Indeed if I try your code in TS 3.2.1 it is eagerly resolved, so this was a change introduced for TS3.3.

Curiously my reading of that bug fix's algorithm description looks like it should apply to your workaround also. I'm not sure why [keyof T] extends [keyof T] would change things, since [keyof T1] isn't definitely assignable to [keyof T2], if T1 and T2 are unrelated. So some mystery still remains here. Haven't been able to crack it yet, so I'll probably just give up now. Oh well!

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It also looks like someone reported this exact issue where X extends X ? 1 : 0 is not eagerly resolved (and they also noticed that wrapping in a one-tuple "fixes" it), and the issue was tagged as a bug, so maybe it will be addressed?

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Okay, hope that helps; good luck!