Checked type of conditional type isn't matched for constraint in true clause unless the type parameter is constrained

[sirian]

TypeScript Version: 3.5.1

Code

type Ctor = new () => Date;

 // ERROR, WTF?
type Z1<T> = T extends () => Ctor ? InstanceType<ReturnType<T>> : never; 

// But here NO ERROR
type Z2<T extends any> = T extends () => Ctor ? InstanceType<ReturnType<T>> : never;


 // ERROR
type Z3<T> = T extends () => any ? InstanceType<ReturnType<T>> : never;

// NO ERROR again
type Z4<T extends () => any> = InstanceType<ReturnType<T>>;

Actual behaviour
type Z1<T> = T extends () => Ctor ? InstanceType<ReturnType<T>> : never; errors

Expected behaviour
type Z1<T> = T extends () => Ctor ? InstanceType<ReturnType<T>> : never; should be valid

More examples.

Playground Link:

[RyanCavanaugh]

Regressed between 3.3 and 3.4

[weswigham]

Technically this is because we erase any in a substitution (or rather, we interpret it as unknown so as to not delete information). So a T extends any ? InstanceType<ReturnType<T>> : never is essentially InstanceType<ReturnType<T & unknown>> which is just InstanceType<ReturnType<T>> which is ofc an error. The others all formulate in the same way. Essentially, any as a conditional check does not disable type checking, like any as a constraint (accidentally!) does. #29571 would go a lot of the way to making the constraint cases behave like the conditionals (though it's still admittedly incomplete! it only replaces any at the top level of a constraint and not throughout the constraint type!)

So this is technically a regression, but it's an intended consequence of an intentional change, and what we have been looking at is making the constraint case behave in the same way, too (as why would a T extends any be any different than a T extends unknown???)

[weswigham]

Ryan Cavanaugh (@RyanCavanaugh) if you want, we can discuss this, but after actually looking at it, everything seems in order except for our constraint behavior which we already acknowledge as wrong.

[typescript-bot]

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