diff --git a/flang/include/flang/Lower/Todo.h b/flang/include/flang/Lower/Todo.h
new file mode 100644
--- /dev/null
+++ b/flang/include/flang/Lower/Todo.h
@@ -0,0 +1,49 @@
+//===-- Lower/Todo.h --------------------------------------------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+//
+// Coding style: https://mlir.llvm.org/getting_started/DeveloperGuide/
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef FORTRAN_LOWER_TODO_H
+#define FORTRAN_LOWER_TODO_H
+
+#include "llvm/Support/ErrorHandling.h"
+#include "llvm/Support/raw_ostream.h"
+#include <cstdlib>
+
+// This is throw-away code used to mark areas of the code that have not yet been
+// developed.
+
+#undef TODO
+
+#ifdef NDEBUG
+
+// In a release build, just give a message and exit.
+#define TODO(ToDoMsg)                                                          \
+  do {                                                                         \
+    llvm::errs() << __FILE__ << ':' << __LINE__ << ": not yet implemented "    \
+                 << ToDoMsg << '\n';                                           \
+    std::exit(1);                                                              \
+  } while (false)
+
+#else
+
+#undef TODOQUOTE
+#define TODOQUOTE(X) #X
+
+// In a developer build, print a message and give a backtrace.
+#define TODO(ToDoMsg)                                                          \
+  do {                                                                         \
+    llvm::report_fatal_error(                                                  \
+        __FILE__ ":" TODOQUOTE(__LINE__) ": not yet implemented " ToDoMsg);    \
+  } while (false)
+
+#endif
+
+#endif // FORTRAN_LOWER_TODO_H