Wrong error about the size of a parameter not being known at compiletime

[DutchGhost]

I tried this code:

struct B<A: Sized = [(); {
    let x = [0u8; !0usize];
    1
}]> {
    a: A
}

fn main() {

}

I expected to see this happen: This code should compile, as A is a generic type with the default type being [(); 1]. When I remove the let x = [0u8; !0usize] line, everything compiles fine.

Instead, this happened: I got an error message, error[E0277]

Meta

rustc --version --verbose:

Stable version: 1.47.0
Beta version: 1.48.0-beta.5
1.49.0-nightly (2020-11-05 9d78d1d02761b906038b)

Backtrace

   Compiling playground v0.0.1 (/playground)
error[E0277]: the size for values of type `A` cannot be known at compilation time
  --> src/main.rs:32:19
   |
31 | struct B<A: Sized = [(); {
   |          -
   |          |
   |          this type parameter needs to be `Sized`
   |          required by this bound in `B`
32 |     let x = [0u8; !0usize];
   |                   ^^^^^^^ doesn't have a size known at compile-time

error: aborting due to previous error

For more information about this error, try `rustc --explain E0277`.
error: could not compile `playground`

To learn more, run the command again with --verbose.

[Dylan-DPC]

Current output:

error[[E0277]](https://doc.rust-lang.org/stable/error_codes/E0277.html): the size for values of type `A` cannot be known at compilation time
 --> src/main.rs:2:19
  |
1 |   struct B<A: Sized = [(); {
  |  __________-
2 | |     let x = [0u8; !0usize];
  | |                   ^^^^^^^ doesn't have a size known at compile-time
3 | |     1
4 | | }]> {
  | |__- this type parameter needs to be `std::marker::Sized`
  |
note: required by a bound in `B`
 --> src/main.rs:1:10
  |
1 |   struct B<A: Sized = [(); {
  |  __________^
2 | |     let x = [0u8; !0usize];
3 | |     1
4 | | }]> {
  | |__^ required by this bound in `B`

For more information about this error, try `rustc --explain E0277`.

[fmease]

If you opt out of Sized for the type parameter A, then rustc emits the correct error (I assume it's the correct one):

struct B<
    A: ?Sized = [(); {
                    let x = [0u8; !0usize];
                    1
                }],
> {
    a: A,
}

fn main() {}

error[E0080]: values of the type `[u8; usize::MAX]` are too big for the current architecture
 --> src/main.rs:3:29
  |
3 |                     let x = [0u8; !0usize];
  |                             ^^^^^^^^^^^^^^

[estebank]

Current output:

error[E0080]: evaluation of constant value failed
 --> src/main.rs:2:13
  |
2 |     let x = [0u8; !0usize];
  |             ^^^^^^^^^^^^^^ values of the type `[u8; usize::MAX]` are too big for the target architecture

and

error[E0080]: evaluation of constant value failed
 --> src/main.rs:3:29
  |
3 |                     let x = [0u8; !0usize];
  |                             ^^^^^^^^^^^^^^ values of the type `[u8; usize::MAX]` are too big for the target architecture

[HypheX]

@rustbot claim
Thanks, I'll work on this shortly.