[ConstraintSystem] Fix getEffectiveOverloadType handling of mutating methods

Author: xedin

lib/Sema/TypeOfReference.cpp

@@ -1833,11 +1833,15 @@ Type ConstraintSystem::getEffectiveOverloadType(ConstraintLocator *locator,
           type, var, useDC, GetClosureType{*this},
           ClosureIsolatedByPreconcurrency{*this});
     } else if (isa<AbstractFunctionDecl>(decl) || isa<EnumElementDecl>(decl)) {
-      if (decl->isInstanceMember() &&
-          (!overload.getBaseType() ||
-           (!overload.getBaseType()->getAnyNominal() &&
-            !overload.getBaseType()->is<ExistentialType>())))
-        return Type();
+      if (decl->isInstanceMember()) {
+        auto baseTy = overload.getBaseType();
+        if (!baseTy)
+          return Type();
+
+        baseTy = baseTy->getRValueType();
+        if (!baseTy->getAnyNominal() && !baseTy->is<ExistentialType>())
+          return Type();
+      }
 
       // Cope with 'Self' returns.
       if (!decl->getDeclContext()->getSelfProtocolDecl()) {