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Checked type of conditional type isn't matched for constraint in true clause unless the type parameter is constrained #31700
Description
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.
Activity
RyanCavanaugh commentedon May 31, 2019
Regressed between 3.3 and 3.4
[-]Type infer broken in 3.5[/-][+]Checked type of conditional type isn't matched for constraint in true clause unless the type parameter is constrained[/+]weswigham commentedon Jul 25, 2019
Technically this is because we erase
anyin a substitution (or rather, we interpret it asunknownso as to not delete information). So aT extends any ? InstanceType<ReturnType<T>> : neveris essentiallyInstanceType<ReturnType<T & unknown>>which is justInstanceType<ReturnType<T>>which is ofc an error. The others all formulate in the same way. Essentially,anyas a conditional check does not disable type checking, likeanyas 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 replacesanyat 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 anybe any different than aT extends unknown???)weswigham commentedon Jul 25, 2019
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 commentedon Jul 28, 2019
This issue has been marked 'Working as Intended' and has seen no recent activity. It has been automatically closed for house-keeping purposes.