diff --git a/flang/include/flang/Frontend/FrontendActions.h b/flang/include/flang/Frontend/FrontendActions.h
--- a/flang/include/flang/Frontend/FrontendActions.h
+++ b/flang/include/flang/Frontend/FrontendActions.h
@@ -128,7 +128,7 @@
 };
 
 class PluginParseTreeAction : public PrescanAndSemaAction {
-  void ExecuteAction() override;
+  void ExecuteAction() override = 0;
 };
 
 } // namespace Fortran::frontend