mark first uncovered DefineOpaqueTypes in can_eq

compiler/rustc_infer/src/infer/mod.rs

@@ -917,6 +917,7 @@ impl<'tcx> InferCtxt<'tcx> {
         T: at::ToTrace<'tcx>,
     {
         let origin = &ObligationCause::dummy();
+        // FIXME(DefineOpaqueTypes): no test exercizes using `DefineOpaqueTypes::Yes` below.
         self.probe(|_| self.at(origin, param_env).eq(DefineOpaqueTypes::No, a, b).is_ok())
     }