Conditional Types: Type parameters with unknown constraint are not the most restrictive type when relating from any.

[jack-williams]

TypeScript Version: master

A conditional type immediately resolves to the true branch when the condition is satisfied with type parameters instantiated at the most restrictive type---a type parameter with unknown constraint.

A type parameter with unknown constraint is not the most restrictive type in all cases: any may be assigned to any type parameter, but any may not be assigned to never.

Code `

type Contrived<T> = [any] extends [T] ? true : false; // true
type ContrivedConcrete = [any] extends [never] ? true : false; // false

Related Issues: These PR's #29338 #29437

To be honest, this issue is incredibly niche, and almost exclusively a result of any (that I currently know of). The behaviour is also consistent with how things work in other cases, such as:

function foo<T>(x: any): T {
    return x;
}
const a = foo<never>(3);

While the conditional behaviour is consistent with other cases where a type parameter can later be instantiated to never, I'm not sure anyone would ever want that behaviour in a conditional. Regardless of whether this is by design, a limitation, or a bug---I think it would be good to have this documented somewhere in the tracker.

[jack-williams]

FWIW: I'm not actually sure what the right thing should be here; I can see arguments for both approaches. I guess all I would like to get out of the issue is some explanation I can link to if someone ever stumbles upon this.

[jack-williams]

Closing for house-keeping---seems unactionable right now.