Old generic bounds don't fulfill where clause bounds in trait impls

[sfackler]

trait T {
    fn foo<U>() -> U where U: Clone;
}

struct Foo;

impl T for Foo {
    fn foo<U: Clone>() -> U { panic!() }
}

fn main() {}

test.rs:8:5: 8:41 error: in method `foo`, type parameter 0 requires bound `core::clone::Clone`, which is not required by the corresponding type parameter in the trait declaration [E0052]
test.rs:8     fn foo<U: Clone>() -> U { panic!() }
              ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
error: aborting due to previous error

[sfackler]

Dup of #20107