weird error message for trait associated constant in array length

[Monadic-Cat]

I tried this code:

trait Unsigned {
    const REIFY: usize;
}

struct A<T: Unsigned> {
    buf: [u8; <T as Unsigned>::REIFY]
}

I expected to see an error about associated constants not being allowed in array lengths or something.

Instead, this is the error given by rustc.

error[E0277]: the trait bound `T: Unsigned` is not satisfied
 --> src/lib.rs:6:15
  |
2 |     const REIFY: usize;
  |     ------------------- required by `Unsigned::REIFY`
...
6 |     buf: [u8; <T as Unsigned>::REIFY]
  |               ^^^^^^^^^^^^^^^^^^^^^^ the trait `Unsigned` is not implemented for `T`
  |
help: consider further restricting this bound
  |
5 | struct A<T: Unsigned + Unsigned> {

(There is an additional "T is never used" error, but that one makes sense, since there are no valid uses of T in the struct definition.)

Meta

rustc --version --verbose:

rustc 1.48.0 (7eac88abb 2020-11-16)
binary: rustc
commit-hash: 7eac88abb2e57e752f3302f02be5f3ce3d7adfb4
commit-date: 2020-11-16
host: x86_64-unknown-linux-gnu
release: 1.48.0
LLVM version: 11.0

I tested this in Stable, Beta, and Nightly, on the Rust Playground.

[jonas-schievink]

Duplicate of #43408