I'm having an issue I can't wrap my head around.

I have a struct B for which I implement AsRef<String>. I now want to implement another AsRef<T>, with T being the associated type of a const-generic trait, implemented for some other A.

See the condensed example below. You can assume the modules a and c are identical (well, a is a crate, and b a local module, but otherwise they are "the same"), and ::T isn't a String.

(Full source to reproduce)

// b/src/lib.rs

impl AsRef<String> for B { ... }

// Enable one of the lines below; a::A fails, c::A works.
impl AsRef<<a::A as Numbered<0>>::T> for B { ... }
impl AsRef<<c::A as Numbered<0>>::T> for B { ... }

The issue is, if the 'same struct and trait impl' resides in the current crate this works, if it resides in another crate, this fails with:

error[E0119]: conflicting implementations of trait `AsRef<String>` for type `B`
  --> b\src\lib.rs:19:1
   |
15 | impl AsRef<String> for B { fn as_ref(&self) -> &String { unimplemented!() }}
   | ------------------------ first implementation here
...
19 | impl AsRef<<a::A as Numbered<0>>::T> for B { fn as_ref( & self ) -> &<a::A as Numbered<0>>::T { unimplemented!() }}
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation for `B`

This feels like a compiler bug, or am I missing something?

[Edit, just realized the <0> const generic has nothing to do with that, even without it this has issues]

[Edit 2, this might actually be #51445]

1 post - 1 participant

Read full topic