Poor inference for argument types in generic arrow function

[Nathan-Fenner]

function onlyIdentity(id: <T>(x: T) => T) {
}

onlyIdentity((x) => x); // okay, as expected

onlyIdentity(<S>(x): S => x); // x gets inferred type "any"

It would be nice if the argument got the correct inferred type (S) from it's context as an argument to onlyIdentity.

In my use case, the return type is small and needs to be narrowed, so I want to specify it, but the argument types are very complex, so I'd prefer that they were inferred.

[RyanCavanaugh]

Functions with type arguments aren't subject to contextual typing; this doesn't seem to be a problem in general and would complicate type inference quite a bit to try to solve that.