Don't use global caches if opaques can be defined

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

@@ -1498,7 +1498,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             return false;
         }
 
-        // Avoid using the master cache during coherence and just rely
+        // Avoid using the global cache during coherence and just rely
         // on the local cache. This effectively disables caching
         // during coherence. It is really just a simplification to
         // avoid us having to fear that coherence results "pollute"
@@ -1509,6 +1509,12 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             return false;
         }
 
+        // Avoid using the global cache when we're defining opaque types
+        // as their hidden type may impact the result of candidate selection.
+        if !self.infcx.defining_opaque_types().is_empty() {
+            return false;
+        }
+
         // Otherwise, we can use the global cache.
         true
     }