Trait coercion may ask for a static bound in overloaded call

[luqmana]

Similar to #11612 but seems this seems to only happen in an overloaded call?

#![feature(overloaded_calls)]

use std::ops::Fn;

trait Response {}
trait Request {}
trait Ingot<R, S> {
    fn enter(&mut self, _: &mut R, _: &mut S, a: &mut Alloy) -> Status;
}

type IoResult<T> = Result<T, ()>;
struct SendFile<'a>;
struct HelloWorld;
struct Alloy;
enum Status {
    Continue
}

impl Alloy {
    fn find<T>(&self) -> Option<T> {
        None
    }
}

impl<'a, 'b> Fn<(&'b mut Response,), IoResult<()>> for SendFile<'a> {
    /// Serve from specified path.
    fn call(&self, (_res,): (&'b mut Response,)) -> IoResult<()> {
        // Check that path exists (path has already checked for permissions)
        // Serve file
        Ok(())
    }
}

impl<Rq: Request, Rs: Response> Ingot<Rq, Rs> for HelloWorld {
    fn enter(&mut self, _req: &mut Rq, res: &mut Rs, alloy: &mut Alloy) -> Status {
        let mut send_file = alloy.find::<SendFile>().unwrap();
        send_file(res);
        Continue
    }
}

fn main() {}

a.rs:37:19: 37:22 error: value may contain references; add `'static` bound to `Rs`
a.rs:37         send_file(res);
                          ^~~
error: aborting due to previous error

Adding an explicit cast works fine.

Reported originally by zzmp on IRC:

[alexcrichton]

cc #14901

[zzmp]

Originally offending code was in this branch. It was resolved by coercing res using res as &mut Response.
The resolution is here.

The master branch will likely change, but I'll leave those two there if you think it will be helpful (though it is by no means a small, isolated example).

Hope this helps. Thanks for your help!