Check inferred constraints for 'infer X' type variables

[ahejlsberg]

With this PR we properly check inferred constrains for infer X type variables in conditional types. Specifically, during type inference in a conditional type, when a candidate type for an infer X type variable doesn't satisfy the constraint inferred for that type variable (see #21709), the candidate is replaced with the constraint. That will subsequently cause the conditional check to be false. For example:

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

type T1 = EnsureIsString<"hello">;  // "hello"
type T2 = EnsureIsString<42>;  // never

Previously, T2 would be the type 42 which doesn't satisfy the constraint string inferred from MustBeString.

Fixes #22221.

[zpdDG4gta8XKpMCd]

@ahejlsberg is there a way to express Not?

type Not<T, U> = ... // T can be anything but U, if T is U then error
// use case:
function notSupposedToTakeAsyncCallbacks<X>(callback: () => Not<X, Promise<any>>) {
}

[mhegazy]

there is not really a way to throw an error. so the best that is possible today is to return a type that will cause a down-stream error, something like never would be good.
So you can define Not as:

type Not<T, U> = T extends U ? never : T;

type t1 = Not<{ a: number }, Promise<any>>; // { a: number } 

type t2 = Not<Promise<number>, Promise<any>>; // never

[zpdDG4gta8XKpMCd]

another silly questions please

#22375 (comment)