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Trait impls conflict even with different associated const bounds #103292

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joshlf opened this issue Oct 20, 2022 · 1 comment
Open

Trait impls conflict even with different associated const bounds #103292

joshlf opened this issue Oct 20, 2022 · 1 comment
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A-trait-system Area: Trait system C-bug Category: This is a bug. F-associated_const_equality `#![feature(associated_const_equality)]` T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. T-types Relevant to the types team, which will review and decide on the PR/issue.

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@joshlf
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joshlf commented Oct 20, 2022

I have the following code:

#![feature(associated_const_equality)]

pub trait Align {
    const ALIGN: usize;
}

impl<T> Align for T {
    const ALIGN: usize = std::mem::size_of::<T>();
}

pub trait AlignedTo<const N: usize> {}

impl<T: Align<ALIGN=1>> AlignedTo<1> for T {}
impl<T: Align<ALIGN=2>> AlignedTo<2> for T {}
impl<T: Align<ALIGN=2>> AlignedTo<1> for T {}
impl<T: Align<ALIGN=4>> AlignedTo<4> for T {}
impl<T: Align<ALIGN=4>> AlignedTo<2> for T {}
impl<T: Align<ALIGN=4>> AlignedTo<1> for T {}

I would expect that this code would be accepted. While there are multiple blanket impls of AlignedTo<1> and AlignedTo<2>, they're bounded by mutually-exclusive bounds - e.g., a type cannot be both Align<ALIGN=1> and Align<ALIGN=2> at the same time. However, I get this error:

error[[E0119]](https://doc.rust-lang.org/nightly/error-index.html#E0119): conflicting implementations of trait `AlignedTo<1>`
  --> src/lib.rs:15:1
   |
13 | impl<T: Align<ALIGN=1>> AlignedTo<1> for T {}
   | ------------------------------------------ first implementation here
14 | impl<T: Align<ALIGN=2>> AlignedTo<2> for T {}
15 | impl<T: Align<ALIGN=2>> AlignedTo<1> for T {}
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation

error[[E0119]](https://doc.rust-lang.org/nightly/error-index.html#E0119): conflicting implementations of trait `AlignedTo<2>`
  --> src/lib.rs:17:1
   |
14 | impl<T: Align<ALIGN=2>> AlignedTo<2> for T {}
   | ------------------------------------------ first implementation here
...
17 | impl<T: Align<ALIGN=4>> AlignedTo<2> for T {}
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation

error[[E0119]](https://doc.rust-lang.org/nightly/error-index.html#E0119): conflicting implementations of trait `AlignedTo<1>`
  --> src/lib.rs:18:1
   |
13 | impl<T: Align<ALIGN=1>> AlignedTo<1> for T {}
   | ------------------------------------------ first implementation here
...
18 | impl<T: Align<ALIGN=4>> AlignedTo<1> for T {}
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation

Possibly related to #20400.

Version

Nightly 2022-10-19 4b8f431

@Rageking8
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@rustbot label +C-bug +T-compiler +F-associated_const_equality

@rustbot rustbot added C-bug Category: This is a bug. F-associated_const_equality `#![feature(associated_const_equality)]` T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. labels Oct 20, 2022
@fmease fmease added A-trait-system Area: Trait system T-types Relevant to the types team, which will review and decide on the PR/issue. labels May 30, 2024
@fmease fmease added A-trait-system Area: Trait system and removed A-trait-system Area: Trait system labels Dec 21, 2024
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Labels
A-trait-system Area: Trait system C-bug Category: This is a bug. F-associated_const_equality `#![feature(associated_const_equality)]` T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. T-types Relevant to the types team, which will review and decide on the PR/issue.
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