wrong error with associated constant; 'consider adding a ' which already exists

[penguin42]

The following disgusting code gives the error:

error[E0277]: the trait bound `A: std::marker::Sized` is not satisfied
  --> assocconst.rs:40:25
   |
40 |     type Res = [isize; <None0B<A> as ResU>::u];
   |                         ^^^^^^^^^ `A` does not have a constant size known at compile-time
   |
   = help: the trait `std::marker::Sized` is not implemented for `A`
   = help: consider adding a `where A: std::marker::Sized` bound
   = note: required by `None0B`

error[E0277]: the trait bound `A: Res` is not satisfied
  --> assocconst.rs:40:25
   |
40 |     type Res = [isize; <None0B<A> as ResU>::u];
   |                         ^^^^^^^^^ the trait `Res` is not implemented for `A`
   |
   = help: consider adding a `where A: Res` bound
   = note: required by `None0B`

error: aborting due to 2 previous errors

looking at the error on line 40, it's complaining about the lack of Res on A,
yet that is already present at line 37

(I'm sure this code deserves some errors, but I don't think that one is right!)

use std::marker::PhantomData;

trait Res {
      const i: isize;
}

trait ResU {
      const u: usize;
}

trait Zero : Res {
      const i: isize = 0;
}

struct Add<A: Res, B: Res> (PhantomData<A>,PhantomData<B>);
struct Minus1 (PhantomData<isize>);
impl Res for Minus1 {
    const i: isize = -1;
}

impl<A:Res, B: Res> Res for Add<A,B>
{
  const i: isize =  A::Res + B::Res; 
}

struct None0B<A: Res> (PhantomData<A>);
impl<A:Res> ResU for None0B<A>
{
    const u: usize = A::Res != 0;
}

trait ResArrT {
    type Res;
}

impl<A> ResArrT for A
  where A: Res,
        None0B<A> : ResU,
{
    type Res = [isize; <None0B<A> as ResU>::u];   // !!!! Line 40
}


fn main() {
    type Z = <Zero as ResArrT> :: Res;
}

Meta

rustc 1.20.0 (f3d6973 2017-08-27)
binary: rustc
commit-hash: f3d6973
commit-date: 2017-08-27
host: x86_64-unknown-linux-gnu
release: 1.20.0
LLVM version: 4.0

[kennytm]

Hi could you please format the report to make it easier to read? Thanks. You could surround ``` around each code snippet like this:

Explanation

```
some
code
here
```

More explanation

```
more
code
here
```

[penguin42]

Hi Kennytm - is that better?

[kennytm]

@penguin42 Much better, thanks ❤️.

[kennytm]

I think this is a duplicate of #42863. The bad error message is tracked at #44168.

[shepmaster]

Closing as duplicate.