Do not assemble candidates for auto traits of opaque types in their defining scope

1 files changed, 6 insertions, 1 deletions

diff --git a/compiler/rustc_trait_selection/src/traits/select/candidate_assembly.rs b/compiler/rustc_trait_selection/src/traits/select/candidate_assembly.rs
index 4c3d833b0f9..06b79ea63ca 100644
--- a/compiler/rustc_trait_selection/src/traits/select/candidate_assembly.rs
+++ b/compiler/rustc_trait_selection/src/traits/select/candidate_assembly.rs
@@ -772,7 +772,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                     );
                 }
 
-                ty::Alias(ty::Opaque, _) => {
+                ty::Alias(ty::Opaque, alias) => {
                     if candidates.vec.iter().any(|c| matches!(c, ProjectionCandidate(_))) {
                         // We do not generate an auto impl candidate for `impl Trait`s which already
                         // reference our auto trait.
@@ -787,6 +787,11 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                         // We do not emit auto trait candidates for opaque types in coherence.
                         // Doing so can result in weird dependency cycles.
                         candidates.ambiguous = true;
+                    } else if self.infcx.can_define_opaque_ty(alias.def_id) {
+                        // We do not emit auto trait candidates for opaque types in their defining scope, as
+                        // we need to know the hidden type first, which we can't reliably know within the defining
+                        // scope.
+                        candidates.ambiguous = true;
                     } else {
                         candidates.vec.push(AutoImplCandidate)
                     }