putting [(); CONSTANT]: bound on trait causes cycle

[djdisodo]

EDITED(workingjubilee): Inlined source

#![feature(generic_const_exprs)]
#![allow(incomplete_features)]
#![allow(dead_code)]

trait Foo: where [(); Self::BAR]: {
    const BAR: usize;
}

struct Baz<F: Foo> {
    _a: [u8; F::BAR],
    _b: u32,
}

struct Qux;
impl Foo for Qux {
    const BAR: usize = 4;
}

fn main() {
    assert_eq!(std::mem::size_of::<Baz<Qux>>(), 8);
}

---

source

   Compiling playground v0.0.1 (/playground)
error[E0391]: cycle detected when building an abstract representation for `Foo::{constant#0}`
 --> src/main.rs:5:23
  |
5 | trait Foo: where [(); Self::BAR]: {
  |                       ^^^^^^^^^
  |
note: ...which requires building THIR for `Foo::{constant#0}`...
 --> src/main.rs:5:23
  |
5 | trait Foo: where [(); Self::BAR]: {
  |                       ^^^^^^^^^
note: ...which requires type-checking `Foo::{constant#0}`...
 --> src/main.rs:5:23
  |
5 | trait Foo: where [(); Self::BAR]: {
  |                       ^^^^^^^^^
  = note: ...which again requires building an abstract representation for `Foo::{constant#0}`, completing the cycle
note: cycle used when checking that `Foo` is well-formed
 --> src/main.rs:5:1
  |
5 | trait Foo: where [(); Self::BAR]: {
  | ^^^^^^^^^

For more information about this error, try `rustc --explain E0391`.
error: could not compile `playground` due to previous error

[djdisodo]

#104400