Trait coercion issue

[edef1c]

https://gist.github.com/edef1c/f968f1840b3ed6f007e0

 /tmp/f968f1840b3ed6f007e0 ⮀ ⭠ master ⮀ rustc fail.rs 
fail.rs:17:11: 17:24 error: mismatched types: expected `Box serialize::serialize::Encodable<serialize::json::Encoder<'a>,std::io::IoError>:Send` but found `~Output` (expected trait serialize::serialize::Encodable but found box)
fail.rs:17   tx.send(box Output(0));
                     ^~~~~~~~~~~~~
error: aborting due to previous error
 ✘ ⮀ /tmp/f968f1840b3ed6f007e0 ⮀ ⭠ master ⮀ rustc works.rs 
 /tmp/f968f1840b3ed6f007e0 ⮀ ⭠ master± ⮀ 

[luqmana]

Here is a smaller test case:

fn main() {
    send::<Box<Foo>>(box Output(0));               // 1  free function
    Test::<Box<Foo>>::foo(box Output(0));          // 2  static function
    Test::<Box<Foo>>.send(box Output(0));          // 3  method
}

fn send<T>(_: T) {}

struct Test<T>;
impl<T> Test<T> {
    fn foo(_: T) {}
    fn send(&self, _: T) {}
}

trait Foo {}
struct Output(int);
impl Foo for Output {}

All three should work but 3 fails to coerce. It seems to be because we have unresolved types and thus we can't add an adjustment.

[luqmana]

Closed by #14628.