Generic type parameter deduction fails when used with associated constant defaults

[mehcode]

Compiling playground v0.0.1 (file:///playground)
error[E0283]: type annotations required: cannot resolve `_: Foo`
  --> src/main.rs:12:14
   |
12 |   take_debug(Foo::BAR);
   |              ^^^^^^^^
   |
   = note: required by `Foo::BAR`

error: aborting due to previous error

use std::fmt;

trait Foo {
  const BAR: u8 = 32;
}

fn take_debug<F: fmt::Debug>(f: F) {
    println!("{:?}", f);
}

fn main() {
  // error
  take_debug(Foo::BAR);
}

https://play.rust-lang.org/?gist=b8610b4b6fa35a3c913434e43f18de1c&version=nightly
https://play.rust-lang.org/?gist=fb75d70964d26c4ce1f75ae98895edec&version=nightly

---

$ rustc --version
rustc 1.22.0-nightly (eba374fb2 2017-09-11)

[bluss]

I think this is expected-- a specific implementation of Foo is still needed to resolve the value of the constant. Indeed, is there even a constant to resolve in a program with no implementations of the trait. The role of the default is a default for each implementation.

[Mark-Simulacrum]

Agreed that this is an expected problem, but the error message can be improved I think, perhaps something like the below.

error[EXXXX]: cannot take value of associated constant without concrete impl
  --> test.rs:13:14
   |
13 |   take_debug(Foo::BAR);
   |              ^^^^^^^^

error: aborting due to previous error

[estebank]

Current output:

error[E0283]: type annotations needed
  --> src/main.rs:12:14
   |
4  |   const BAR: u8 = 32;
   |   ------------------- required by `Foo::BAR`
...
12 |   take_debug(Foo::BAR);
   |              ^^^^^^^^ cannot infer type
   |
   = note: cannot resolve `_: Foo`

error[E0283]: type annotations needed
  --> src/main.rs:20:12
   |
2  |   const BAR: BAR = BAR;
   |   --------------------- required by `Foo::BAR`
...
20 |   take_foo(Foo::BAR);
   |            ^^^^^^^^ cannot infer type
   |
   = note: cannot resolve `_: Foo`

The second one should suggest take_foo(<BAR as Foo>::BAR); or enumerate all of the possible implementors of Foo. The first one should mention that no implementor of Foo has been found. Both cases should mention that associated types can only be used from specific impls, not directly from a trait.

[estebank]

Current output:

error[E0283]: type annotations needed
  --> file13.rs:12:16
   |
4  |     const BAR: u8 = 32;
   |     ------------------- required by `Foo::BAR`
...
12 |     take_debug(Foo::BAR);
   |                ^^^^^^^^
   |                |
   |                cannot infer type
   |                help: use the fully qualified path to an implementation: `<Type as Foo>::BAR`
   |
   = note: cannot satisfy `_: Foo`
   = note: associated constants cannot be accessed directly on a `trait`, they can only be accessed through a specific `impl`