Trait coherency regression in 1.78 #124664
Description
Code
I tried this code:
trait Vendor { type Processor<'a>: VendorProcessor; } trait VendorProcessor { type EventGroup: Group; } trait Group {} trait EventProcessor<T> {} struct MyEventProcessor<T: Vendor> { t: T, } struct MyEvent {} impl Group for MyEvent {} impl<T: Vendor> EventProcessor<MyEvent> for MyEventProcessor<T> {} impl<T, Group> EventProcessor<Group> for MyEventProcessor<T> where T: Vendor, for<'a> T::Processor<'a>: VendorProcessor<EventGroup = Group>, {}I expected a working build.
Instead, this happened:
error[E0119]: conflicting implementations of trait `EventProcessor<MyEvent>` for type `MyEventProcessor<_>` --> minimal-repro/src/lib.rs:23:1 | 21 | impl<T: Vendor> EventProcessor<MyEvent> for MyEventProcessor<T> {} | --------------------------------------------------------------- first implementation here 22 | 23 | / impl<T, Group> EventProcessor<Group> for MyEventProcessor<T> 24 | | where 25 | | T: Vendor, 26 | | for<'a> T::Processor<'a>: VendorProcessor<EventGroup = Group>, | |__________________________________________________________________^ conflicting implementation for `MyEventProcessor<_>` If this was a bugfix for trait coherency, it doesn't seem to have been documented in the changelog. The two implementations could technically be irrefutably non-conflicting though if there is no impl VendorProcessor<EventGroup = MyEvent> on private types.
Version it worked on
It most recently worked on: 1.77.2
Version with regression
rustc --version --verbose:
rustc 1.78.0 (9b00956e5 2024-04-29) binary: rustc commit-hash: 9b00956e56009bab2aa15d7bff10916599e3d6d6 commit-date: 2024-04-29 host: aarch64-apple-darwin release: 1.78.0 LLVM version: 18.1.2 @rustbot modify labels: +regression-from-stable-to-stable -regression-untriaged