rust-lang / rust-lang/rust

cannot specialize const trait impl with non-const

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#147,130 2 comments 0 reactions 0 assignees View on GitHub

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C-bug F-const_trait_impl F-min_specialization T-compiler
Dominant language
Rust
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Description

#![feature(const_trait_impl, min_specialization)]

#[allow(dead_code)]
const trait Trait {
    fn item();
}

impl<T> const Trait for T {
    default fn item() {}
}

// works when adding const here too
impl Trait for () {
    fn item() {}
}

fn main() {}
error[E0119]: conflicting implementations of trait `Trait` for type `()`
  --> conflict.rs:14:1
   |
 9 | impl<T> const Trait for T {
   | ------------------------- first implementation here
...
14 | impl Trait for () {
   | ^^^^^^^^^^^^^^^^^ conflicting implementation for `()`

error: aborting due to 1 previous error

For more information about this error, try `rustc --explain E0119`.
rustc 1.92.0-nightly (4082d6a3f 2025-09-27)

i would like to expect this to work it may however be intentional behaviour

Contributor guide

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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 compiling the provided minimal reproduction with rustc 1.92.0-nightly and compare the const and non-const implementations. Trace the compiler's overlap check for const trait implementations; done means determining whether the conflict is intentional or correcting the behavior to allow the expected specialization.

Written by the indexing model from the issue text.

Assessment

Tech stack
rust
Domain
compilers
Issue type
Bug
Difficulty
4/5
Estimated time
3-5 days
Activity status
Stale
Clarity
Mostly clear
Newbie friendliness
35/100

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