`overflow evaluating the requirement` for Recursively-implemented Traits with Additional Parameters

[zheland]

The current stable trait solver may enter an infinite recursion when recursively-implemented traits with additional implementation parameters used without full type annotations (with type inference).

Compilation success depends on the compilation cache (see fn1 comments).
This issue is resolved in next-generation trait solver (RUSTFLAGS="-Znext-solver").

Code

pub trait Trait<T> {
    type Ty;
    // Error remains for `fn get() -> Result<T, Self::Ty>`.
    // Error remains for `fn get(&self) -> (T, Self::Ty);`.
    fn get() -> (T, Self::Ty);
}

// Error remains for `impl<C, T, A> Trait<&'static T>> for C`
// Error remains for `impl<C, T, A> Trait<Box<T>> for C`
// Error remains for `impl<C, T, A> Trait<Option<T>> for C`
// Error remains for `impl<C, T, E, A> Trait<Result<T, E>> for C`
impl<C, T, A> Trait<(T,)> for C
where
    C: Trait<T, Ty = A>,
{
    // Removing `A` impl argument and using `type Ty = C::Ty;` fixes the error.
    type Ty = A;
    fn get() -> ((T,), Self::Ty) {
        todo!()
    }
}

struct Type;

// Error remains when using other primitive types instead of `u8`.
impl Trait<u8> for Type {
    type Ty = ();
    fn get() -> (u8, Self::Ty) {
        todo!()
    }
}

// Uncommenting the function below fixes the compilation of `fn2`.
// Compilation is fixed only if `fn1` is located before `fn2`.
// Commenting it back doesn't break the compilation of `fn2` back.
// Using `cargo clean` with commented `fn1` will break the compilation back.
// fn fn1() {
//     let res: ((u8,), ()) = Type::get();
// }

// This fails to compile.
fn fn2() {
    let res: ((u8,), ()) = <Type as Trait<(_,)>>::get();
}

// This succeeds to compile.
fn fn3() {
    // Fully-specified trait arguments fixes the issue.
    let res: ((u8,), ()) = <Type as Trait<(u8,)>>::get();
}

Checked with: cargo check
Expected: success
Result:

error[E0275]: overflow evaluating the requirement `Type: Trait<(_,)>`
  --> src/lib.rs:43:28
   |
43 |     let res: ((u8,), ()) = <Type as Trait<(_,)>>::get();
   |                            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = help: consider increasing the recursion limit by adding a `#![recursion_limit = "256"]` attribute to your crate (`test_lib2`)
note: required for `Type` to implement `Trait<((_,),)>`
  --> src/lib.rs:12:15
   |
12 | impl<C, T, A> Trait<(T,)> for C
   |               ^^^^^^^^^^^     ^
13 | where
14 |     C: Trait<T, Ty = A>,
   |                 ------ unsatisfied trait bound introduced here
   = note: 127 redundant requirements hidden
   = note: required for `Type` to implement `Trait<(((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((_,),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),),)>`

Meta

Affected Rust versions:

• 1.31.0 - reproducible.
• Stable (rustc 1.88.0 (6b00bc388 2025-06-23)) - reproducible.
• Beta (rustc 1.89.0-beta.3 (b5e10d8c0 2025-07-02))- reproducible.
• Nightly (rustc 1.90.0-nightly (da58c0513 2025-07-03)) - reproducible.

$ rustc --version --verbose
rustc 1.88.0 (6b00bc388 2025-06-23)
binary: rustc
commit-hash: 6b00bc3880198600130e1cf62b8f8a93494488cc
commit-date: 2025-06-23
host: x86_64-unknown-linux-gnu
release: 1.88.0
LLVM version: 20.1.5

The issue doesn't occur when using RUSTFLAGS="-Znext-solver" cargo +nightly check