Unique function types are not unique enough

[petrochenkov]

In this example

trait Check: std::marker::MarkerTrait {
    fn check();
}
impl Check for u8 {
    fn check() { println!("u8"); }
}
impl Check for u16 {
    fn check() { println!("u16"); }
}

fn f<T: Check>() {
    T::check();
}



fn main() {
    fn same_type<T>(_: T, _: T) {}
    same_type(f::<u8>, f::<u16>);
    println!("{} {}", f::<u8> as usize, f::<u16> as usize);
}

functions f::<u8> and f::<u16> have the same type fn() {f} tied to the id of the generic function item, but different function pointers. So, in general case a function pointer can't be unambiguosly obtained from function item's type.

cc @nikomatsakis

[petrochenkov]

Related to #19925

[nikomatsakis]

Duplicate of #19925 basically, which is already P-high (but hasn't happened...sadly).