rust-lang / rust-lang/rust

Overflow evaluating the requirement on `associated_type_defaults` bounded on the field it's being assigned

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A-trait-system C-bug F-associated_type_defaults T-compiler
Dominant language
Rust
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Description

I tried this code:

#![feature(associated_type_defaults)]

trait Trait {
    type A;
}

trait Other<T>
where
    T: Trait<A = Self::A>,
{
    type A = T::A;
}

See playground.

I expected the associated type default to compile since the bound is valid when implementing the trait. In my mind type A = T::A should be compatible with the T: Trait<A = Self::A> constraint;

Instead, this happened:

error[E0275]: overflow evaluating the requirement `<Self as UnsizedVisitor<T>>::A == _`
  --> src/lib.rs:12:14
   |
12 |     type A = T::A;
   |              ^^^^
Meta

rustc --version --verbose:

binary: rustc
commit-hash: 794c12416b2138064af1f2746646973fafd9419d
commit-date: 2025-02-21
host: x86_64-pc-windows-msvc
release: 1.87.0-nightly
LLVM version: 20.1.0

Contributor guide

Open the contributing guide

First steps

  1. Read the whole issue, then the project's contributing guide.
  2. Comment on the issue to say you are picking it up — it saves two people doing the same work.
  3. Fork the repository and make your change on a branch.
  4. Open a pull request that references the issue number.

Research direction

Start by reproducing the overflow from the example in src/lib.rs at line 12 or through the linked Rust Playground, using the reported nightly version. Investigate how rustc evaluates the associated_type_defaults bound and determine what behavior should replace the recursive requirement; done means the example is handled without an overflow and the behavior is covered by an appropriate compiler test.

Written by the indexing model from the issue text.

Assessment

Tech stack
rust
Domain
compilers
Issue type
Bug
Difficulty
5/5
Estimated time
Over a week
Activity status
Stale
Clarity
Mostly clear
Newbie friendliness
35/100

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