Revert "Don't use global caches if opaques can be defined"

This reverts commit 61b5e11.

Author: lqd

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

@@ -1487,7 +1487,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             return false;
         }
 
-        // Avoid using the global cache during coherence and just rely
+        // Avoid using the master 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"
@@ -1498,12 +1498,6 @@ 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
     }