Specialization can switch to DefineOpaqueTypes::Yes without having an effect.

The reason is that in specialization graph computation we use `DefiningAnchor::Error`, so there's no difference anyway. And in the other use cases, we

* already errored in the specialization_graph computation, or
* already errored in coherence, or
* are comparing opaque types with inference variables already, or
* there are no opaque types involved

Author: oli-obk

compiler/rustc_trait_selection/src/traits/specialize/mod.rs

@@ -246,7 +246,12 @@ fn fulfill_implication<'tcx>(
     // do the impls unify? If not, no specialization.
     let Ok(InferOk { obligations: more_obligations, .. }) = infcx
         .at(&ObligationCause::dummy(), param_env)
-        .eq(DefineOpaqueTypes::No, source_trait, target_trait)
+        // Ok to use `Yes`, as all the generic params are already replaced by inference variables,
+        // which will match the opaque type no matter if it is defining or not.
+        // Any concrete type that would match the opaque would already be handled by coherence rules,
+        // and thus either be ok to match here and already have errored, or it won't match, in which
+        // case there is no issue anyway.
+        .eq(DefineOpaqueTypes::Yes, source_trait, target_trait)
     else {
         debug!("fulfill_implication: {:?} does not unify with {:?}", source_trait, target_trait);
         return Err(());