<Foo as Trait>::AssocType triggers non-Copy union check even when it is copy

[TheEdward162]

Hello, I ran into what I think is a bug with unions and type normalization.

trait Trait {
    type Assoc: Copy;
}

struct Foo;
impl Trait for Foo {
    type Assoc = u8;
}

union Bar {
    // The `Copy` bound on `Trait::Assoc` shouldn't even be required as this isn't even generic, the type is known to be `u8`
    foo: <Foo as Trait>::Assoc
}

// Ideally this is what Bar should normalize to
union Baz {
    foo: u8
}

Playground:
https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=b9c65cbec84f266463da8192cf7c2d21

I expected to see this happen: This should compile: <Foo as Trait>::Assoc is both bounded to be Copy and should normalize directly to u8 since it uses a concrete type.

Instead, this happened:

   Compiling playground v0.0.1 (/playground)
error[E0658]: unions with non-`Copy` fields other than `ManuallyDrop<T>` are unstable
  --> src/lib.rs:12:5
   |
12 |     foo: <Foo as Trait>::Assoc
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = note: see issue #55149 <https://github.com/rust-lang/rust/issues/55149> for more information

For more information about this error, try `rustc --explain E0658`.
error: could not compile `playground` due to previous error

Wrapping the type in ManuallyDrop does indeed work, and is a possible workaround for simple types, but I believe this should still work.

Meta

Stable 1.56.1 playground, happens on nightly 1.58.0-nightly (2021-11-13 b416e38) playground as well

Maybe this is connected to #81199? If I remove impl Trait for Foo it ICEs on stable and errors on nightly (i.e. fixed on nightly).

[Dylan-DPC]

Yeah this is fixed and no longer errors on stable