Merge "C++17 compatibility: unreachable `return`."
diff --git a/runtime/gc/accounting/mod_union_table_test.cc b/runtime/gc/accounting/mod_union_table_test.cc
index b39628b..e66a174 100644
--- a/runtime/gc/accounting/mod_union_table_test.cc
+++ b/runtime/gc/accounting/mod_union_table_test.cc
@@ -162,9 +162,9 @@
     }
     default: {
       UNIMPLEMENTED(FATAL) << "Invalid type " << type;
+      UNREACHABLE();
     }
   }
-  return nullptr;
 }
 
 TEST_F(ModUnionTableTest, TestCardCache) {