Error messages are odd for delegating traits

[nagisa]

This is a more general version of #22478. (i.e. probably should be considered to supersede it)

trait A {
    fn a(self);
}

trait B {
    fn b(self);
}

impl<T: B> A for T {
    fn a(self) {
        B::b(self)
    }
}

struct C;

fn main() {
    A::a(C);
}

Compiling this will result in

t.rs:18:5: 18:9 error: the trait `B` is not implemented for the type `C` [E0277]
t.rs:18     A::a(C);
            ^~~~
error: aborting due to previous error

but impl<T: B> A for T, T = C and C does not implement B and therefore the impl in question should not even be taken into consideration, even if it is the only implementation of a trait.

[alexcrichton]

triage: I-nominated

nominating for 1.0 polish as this is quite a confusing error message that leaks into many places. For example if you pass something to a function requiring a FnMut and doesn't satisfy it the compiler will complain about a lack of Fn implementation.

[pnkfelix]

we could provide more of a trace in the error message.

anyway this is polish at best, not a 1.0 blocker. P-low. not on 1.0 milestone.