Wrong branch on conditional type with ReturnType of T condition

I think I found a bug.

Last non-broken TS version for this is 3.1.6 on typescript playground. All later versions are broken.

Which gets me thinking: is this maybe intended behavior because it is “broken” for so many versions?

But i dont know why it should NOT be a bug. Either way if it’s indeed not a bug, i need some serious brain rewiring from you guys please, because cant wrap my head around this behavior…

Let’s get started

TypeScript Version: Nightly, all the way down to and including 3.3.3, tested with all these versions on playground

Search Terms: conditional types return type

Code

type StringOrNumber<T extends () => any> = [ReturnType<T>] extends [string]
  ? string
  : number;

type Test = StringOrNumber<() => number>; // oh noes, "string"

Expected behavior: I expect number. Because [number] just cannot extend [string]

The TS handbook talks about deferred evaluation of conditional types. I seems it is not working here!! Evaluation of “ReturnType<T>” should be deferred until the type is actually used in a concrete way!

Actual behavior: The conditional type evaluates to string no matter what function type is passed in

type StringOrNumber2<T extends () => number> = [ReturnType<T>] extends [string]
  ? string
  : number;

type Test2 = StringOrNumber2<() => number>; // oh noes, "string"

Expected behavior: I expect number, even the generic constaint now demands it!

Actual behavior: Again, always get “string”

type StringOrNumber3<T extends () => number> = [number] extends [string]
  ? string
  : number;

type Test3 = StringOrNumber3<() => number>; // correct, number

This is just for show, I expect number and get number, only difference is I now hardcode “number” instead of “ReturnType<T>”

Playground Link: https://www.typescriptlang.org/play/?ts=Nightly#code/FAkFwTwBwUwAgMpgE4EsB2BzA8sgcgK4C2ARjMgDwAqcMAHmDOgCYDOcAFAJRwC8AfHACG6CIN5wA2gCUYYAsnRVoMavwC6tBkzZTWKDJnXA4cAPxx9aLCbgAuOOmJlkAbnAq4VGPr6IDWLiEpOQU3HyCTiHI-K5wAPTxcAD2ABaOyT4ANHAARFaGuaDAkLD+1jj4zuQATNRajCzs4QKO1TF+MnIKSipqmvSNupIFWMamFqOYtg5RLu4lnt76NX5IFUHtdS2R7bEJSWkZ2XlTRaCl8OuGm9EAzPWDOs08rXPk4lLvyAPaTXoBIy2SaAmZtaILS5eHxgO5rQG3FwPHbglz7RJwADGyWQyBgmLAOW+xRJQA

Related Issues: no, searched the issues

[RyanCavanaugh]

Normally this is where I say “Ah, here’s the subtle thing you missed”, but… yeah that seems totally broken and I don’t understand how it’s been that way for so long without anyone else noticing?

[ahejlsberg]

@jcalz Decided to put up a separate PR for that issue: #41622.