-Znext-solver overflow evaluating the requirement

I tried this code:

use std::ops::Add;

impl<S, const LEN: usize> MyTrait for [S; LEN]
where
    N<LEN>: ToNum,
    S: ToNum<Num: Add<Helper<LEN>, Output: Num>>,
{
}

type Helper<const LEN: usize> = <N<LEN> as ToNum>::Num;

trait MyTrait {}

struct N<const N: usize>;

trait ToNum {
    type Num: Num;
}

trait Num: ToNum<Num = Self> {}

I expected to see this happen: On the current trait solver the above code compiles so it would be expected that the new trait solver also does.

Instead, this happened: The new trait solver reports an overflow attempting to evaluate the trait bound.

error[E0275]: overflow evaluating the requirement `[S; LEN]: MyTrait`
 --> <source>:3:39
  |
3 | impl<S, const LEN: usize> MyTrait for [S; LEN]
  |                                       ^^^^^^^^

error[E0275]: overflow evaluating the requirement `<S as ToNum>::Num: Add<<N<LEN> as ToNum>::Num>`
 --> <source>:6:19
  |
6 |     S: ToNum<Num: Add<Helper<LEN>, Output: Num>>,
  |                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

error[E0275]: overflow evaluating the requirement `<<S as ToNum>::Num as Add<<N<LEN> as ToNum>::Num>>::Output: Num`
 --> <source>:6:44
  |
6 |     S: ToNum<Num: Add<Helper<LEN>, Output: Num>>,
  |                                            ^^^

error[E0275]: overflow evaluating the requirement `<N<LEN> as ToNum>::Num: Sized`
 --> <source>:6:19
  |
6 |     S: ToNum<Num: Add<Helper<LEN>, Output: Num>>,
  |                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
note: required by an implicit `Sized` bound in `Add`
 --> /rustc/f4a216d28ee635afce685b4206e713579f66e130/library/core/src/ops/arith.rs:78:1

error[E0275]: overflow evaluating the requirement `<<S as ToNum>::Num as Add<<N<LEN> as ToNum>::Num>>::Output well-formed`
 --> <source>:6:44
  |
6 |     S: ToNum<Num: Add<Helper<LEN>, Output: Num>>,
  |                                            ^^^

I isolated this from a project of mine that has hit this a few times. The core issue seems to be the

trait A: B<A = Self> {}

pattern seen in the code above. However, the failure to evaluate the bound doesn't always trigger when using a trait of that form.

Meta

rustc --version --verbose:

rustc 1.87.0-nightly (f4a216d28 2025-03-02)
binary: rustc
commit-hash: f4a216d28ee635afce685b4206e713579f66e130
commit-date: 2025-03-02
host: x86_64-unknown-linux-gnu
release: 1.87.0-nightly
LLVM version: 20.1.0

Start by compiling the minimal reproducer with the reported nightly toolchain and compare the current and new trait solvers. Read the trait-bound diagnostics and the referenced core/ops/arith.rs Add definition to understand the involved implicit bound. Done means the new solver no longer overflows for this pattern while preserving the expected compilation behavior.