[generic_const_exprs] False positive E0391 cycle when `const fn` `where` clause references trait with const param

Summary

A false positive E0391 cycle error occurs when a struct has a const generic parameter of array type, and its where clause calls a const fn that itself has a where clause referencing a const expression.

Code

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

pub struct Trait<const B: bool>;

pub struct Y<const V: [(); 3]>
where
    Trait<{ foo(&V) }>:, {}

const fn foo<const N: usize>(x: &[(); N]) -> bool
where
    Trait<{ N == 3 }>:,
{
    true
}

fn main() {}

Error

error[E0391]: cycle detected when building an abstract representation for `Y::{constant#1}`
 --> src\main.rs:8:11
  |
8 |     Trait<{ foo(&V) }>:, {}
  |           ^^^^^^^^^^^
  |
note: ...which requires building THIR for `Y::{constant#1}`...
 --> src\main.rs:8:11
  |
8 |     Trait<{ foo(&V) }>:, {}
  |           ^^^^^^^^^^^
note: ...which requires type-checking `Y::{constant#1}`...
 --> src\main.rs:8:11
  |
8 |     Trait<{ foo(&V) }>:, {}
  |           ^^^^^^^^^^^
  = note: ...which again requires building an abstract representation for `Y::{constant#1}`, completing the cycle
note: cycle used when checking that `Y` is well-formed
 --> src\main.rs:6:1
  |
6 | pub struct Y<const V: [(); 3]>
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Notes

There is no actual semantic cycle here — this appears to be a false positive. The interaction between a const generic of array type ([(); N]) and a nested where clause const expression in the called const fn seems to confuse the abstract representation builder.

Version

rustc 1.96.0-nightly (80381278a 2026-03-01)

Related Issues

• #92961 — False positive cycle detection with generic_const_exprs
• #106994 — generic_const_exprs does not work properly with const/type param defaults
• #134978 — Cycle error with const expression in function where clause

Reproduce the false-positive E0391 using the provided src/main.rs example, then trace the reported cycle through abstract representation building, THIR construction, and type-checking for Y::{constant#1}. Compare the behavior with related issues #92961, #106994, and #134978; done means this valid example no longer reports a cycle and the regression is covered by a test.