regression: error[E0119]: conflicting implementations of trait AsStr for type String

[cuviper]

• https://crater-reports.s3.amazonaws.com/beta-1.84.0-4-retry2/beta-2024-12-08/reg/temporary_enum_delegate_0_3_0-0.3.0/log.txt

[INFO] [stdout] error[E0119]: conflicting implementations of trait `AsStr` for type `String`
[INFO] [stdout]  --> examples/basic.rs:3:1
[INFO] [stdout]   |
[INFO] [stdout] 3 | #[delegate]
[INFO] [stdout]   | ^^^^^^^^^^^ conflicting implementation for `String`
[INFO] [stdout] ...
[INFO] [stdout] 8 | impl AsStr for String {
[INFO] [stdout]   | --------------------- first implementation here
[INFO] [stdout]   |
[INFO] [stdout]   = note: upstream crates may add a new impl of trait `temporary_enum_delegate_0_3_0::__macros::Convert` for type `std::string::String` in future versions
[INFO] [stdout]   = note: upstream crates may add a new impl of trait `temporary_enum_delegate_0_3_0::__macros::Bound<259758991428806249286467916654556560740, 268186603860731246084428674422145461480, 3, 1>` for type `std::string::String` in future versions
[INFO] [stdout]   = note: this error originates in the attribute macro `delegate` (in Nightly builds, run with -Z macro-backtrace for more info)
[INFO] [stdout] 
[INFO] [stdout] 
[INFO] [stdout] For more information about this error, try `rustc --explain E0119`.
[INFO] [stdout] 
[INFO] [stderr] error: could not compile `temporary_enum_delegate_0_3_0` (example "basic") due to 1 previous error

Version it worked on

It most recently worked on: 1.83.0

Version with regression

Using rustc 1.84.0-beta.4 in crater #134138.

@rustbot modify labels: +regression-from-stable-to-beta -regression-untriaged

[compiler-errors]

@lcnr: do you remember if this is known breakage from #130654? This seems coherence related 🤔

[lcnr]

looking at the expansion, this feels like it's #114061

trait AsStr {}
impl<D> AsStr for D
where
    D: upstream::Convert + upstream::Bound<{ AsStr.0 }, { AsStr.1 }, { AsStr.2 }, { AsStr.3 }>,
    <D as upstream::Convert>::Owned: AsStr__DelegateOwned,
    for<'d> <D as upstream::Convert>::Ref<'d>: AsStr__DelegateRef<'d>,
    for<'d> <D as upstream::Convert>::RefMut<'d>: AsStr__DelegateRefMut<'d>,
{
}

impl AsStr for std::string::String {}

std::string::String: upstream::Convert fails the is_knowable check and should do so. This means that all these trait bounds have an unknown type as the self-type and have to be ambiguous. After-all, upstream could implement String: Convert with arbitrary associated types.