Unexpected trait behavior with default_type_params

[carllerche]

This seems more like an error message issue than anything, but I was very confused for a while. Here is a repro:

#![feature(unboxed_closures)]
#![feature(default_type_params)]

pub trait Invoke<A, R = ()> {
    fn invoke(&mut self, args: A) -> R;
}

impl<A, R, F: Send + FnMut(A) -> R> Invoke<A, R> for F {
    fn invoke(&mut self, args: A) -> R {
        self.call_mut((args,))
    }
}

pub struct Foo;


impl Invoke<uint, uint> for Foo {
    fn invoke(&mut self, args: uint) -> uint {
        args
    }
}

fn repro<A, I: Invoke<A>>(_: I) {
    unimplemented!();
}

pub fn main() {
    repro(Foo);
}

output:

repro.rs:28:5: 28:10 error: the trait `core::ops::Fn<(_,), ()>` is not implemented for the type `Foo`
repro.rs:28     repro(Foo);
                ^~~~~
repro.rs:28:5: 28:10 note: the trait `core::ops::Fn` must be implemented because it is required by `repro`
repro.rs:28     repro(Foo);
                ^~~~~
error: aborting due to previous error

[nikomatsakis]

cc me

[steveklabnik]

Triage: updated code:

#![feature(unboxed_closures)]
#![feature(default_type_params)]

pub trait Invoke<A, R = ()> {
    fn invoke(&mut self, args: A) -> R;
}

impl<A, R, F: Send + FnMut(A) -> R> Invoke<A, R> for F {
    fn invoke(&mut self, args: A) -> R {
        self.call_mut((args,))
    }
}

pub struct Foo;


impl Invoke<usize, usize> for Foo {
    fn invoke(&mut self, args: usize) -> usize {
        args
    }
}

fn repro<A, I: Invoke<A>>(_: I) {
    unimplemented!();
}

pub fn main() {
    repro(Foo);
}

same error.

[steveklabnik]

Triage: updated code

#![feature(fn_traits)]
#![feature(default_type_params)]

pub trait Invoke<A, R = ()> {
    fn invoke(&mut self, args: A) -> R;
}

impl<A, R, F: Send + FnMut(A) -> R> Invoke<A, R> for F {
    fn invoke(&mut self, args: A) -> R {
        self.call_mut((args,))
    }
}

pub struct Foo;


impl Invoke<usize, usize> for Foo {
    fn invoke(&mut self, args: usize) -> usize {
        args
    }
}

fn repro<A, I: Invoke<A>>(_: I) {
    unimplemented!();
}

pub fn main() {
    repro(Foo);
}

updated error:

error[E0277]: expected a `std::ops::FnMut<(_,)>` closure, found `Foo`
  --> src/main.rs:28:5
   |
28 |     repro(Foo);
   |     ^^^^^ expected an `FnMut<(_,)>` closure, found `Foo`
   |
   = help: the trait `std::ops::FnMut<(_,)>` is not implemented for `Foo`
   = note: required because of the requirements on the impl of `Invoke<_>` for `Foo`
note: required by `repro`
  --> src/main.rs:23:1
   |
23 | fn repro<A, I: Invoke<A>>(_: I) {
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^