[generic_const_exprs + adt_const_params] False positive E0391 cycle when const param type itself has a GCE where clause

Summary

A false positive E0391 cycle error occurs when combining adt_const_params and generic_const_exprs: if a struct used as a const generic parameter type itself carries a GCE where clause, using that const parameter inside another GCE where clause triggers a spurious cycle — even when the called function is a trivially simple generic const fn with no where clause of its own.

Code

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

use std::marker::ConstParamTy;

struct T<const B: bool>;

#[derive(PartialEq, Eq, ConstParamTy)]
struct X<const N: usize>
where
    T<{ N == 4 }>:, {}

struct Y<const N: X<4>>
where
    T<{ foo(&N) }>:, {}

const fn foo<T>(_: &T) -> bool {
    true
}

fn main() {}

Error

error[E0391]: cycle detected when building an abstract representation for `Y::{constant#1}`
  --> src\main.rs:16:7
   |
16 |     T<{ foo(&N) }>:, {}
   |       ^^^^^^^^^^^
   |
note: ...which requires building THIR for `Y::{constant#1}`...
  --> src\main.rs:16:7
   |
16 |     T<{ foo(&N) }>:, {}
   |       ^^^^^^^^^^^
note: ...which requires type-checking `Y::{constant#1}`...
  --> src\main.rs:16:7
   |
16 |     T<{ foo(&N) }>:, {}
   |       ^^^^^^^^^^^
   = 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:14:1
   |
14 | struct Y<const N: X<4>>
   | ^^^^^^^^^^^^^^^^^^^^^^^

Notes

There is no actual semantic cycle here. foo is a trivial generic function — it takes &T and returns true, with no where clause and no const generic constraints.

The distinguishing characteristic compared to similar issues is that the source of the "complexity" is entirely in the type of the const parameter (X<4>), not in the called function.

Version

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

Related Issues

• #92961 — False positive cycle detection with generic_const_exprs (similar symptom, different trigger: associated const self-reference in impl)
• #134978 — Cycle error with GCE where clause in function (similar symptom, different trigger: associated const access, no adt_const_params)
• #112248 — Type inference fails with adt_const_params + generic_const_exprs
• #90455 — Spurious deref errors combining adt_const_params + generic_const_exprs

Reproduce the example from src/main.rs with the reported rustc nightly version, then compare the cycle trace involving Y::{constant#1}, THIR construction, and type-checking. Trace why the GCE where clause on the const parameter type X<4> is treated as a cycle, and consider the issue resolved when this example compiles without the false-positive E0391 error.