Incorrect type inference from parameter contraint in conditional types

[s-ve]

TypeScript Version:
2.8.0-dev.201802213

Search Terms:
conditional, inference, constraint

Code

type MustBeString<T extends string> = T;
type EnsureIsString<T> = T extends MustBeString<infer U> ? U : never;

type Test1 = EnsureIsString<'hello'>; //'hello'
type Test2 = EnsureIsString<42>; //42

Expected behavior:
Test2 has type never

Actual behavior:
Test2 has type 42, even though typescript correctly infers that U must be a string in the conditional true branch.

Playground Link:
Related issues:
#21937 #21631

[jcalz]

Related to #21937? (Please note that the issue template does ask you for search terms and related issues)

[s-ve]

@jcalz I just updated my issue to match the template

[mhegazy]

The constraint failure does not invalidate the inference really. do you have a use case where this is needed?

[s-ve]

@mhegazy

What motivated me to report this is the fact that this construct can make Typescript ignore type constraints :

type A<T extends string> = {
  [TKey in T]: string
};

type B<T extends string> = T;
type C<T> = T extends B<infer U> ? A<U> : never;
type D = C<boolean>; // Evaluated as A<boolean>, which shouldn't be valid

it also seems inconsistent to me since this code works as expected :

type B<T extends string> = {a: T};
type C<T> = T extends B<infer U> ? A<U> : never;
type D = C<{a: boolean}>; // never, as expected

I don't know if it can be a problem in real world code though... I can't think of any use-case where an "Identity" type such as B would be useful in a conditional type...

EDIT
Please ignore the second part of my comment, the result is exactly the same (A<boolean>, not never). This had nothing to do with the fact that B is a "identity" type. Sorry about that.