diff --git a/flang/include/flang/Parser/message.h b/flang/include/flang/Parser/message.h
--- a/flang/include/flang/Parser/message.h
+++ b/flang/include/flang/Parser/message.h
@@ -32,13 +32,15 @@
 // Use "..."_err_en_US, "..."_warn_en_US, "..."_port_en_US, and "..."_en_US
 // string literals to define the static text and fatality of a message.
 //
-// Error: fatal error that prevents code and module file generation
-// Warning: likely problem,
-// Portability: nonstandard or obsolete features
-// Because: for AttachTo(), explanatory attachment in support of another message
-// Context (internal): attachment from SetContext()
-// None: everything else, common for attachments with source locations
-enum class Severity { Error, Warning, Portability, Because, Context, None };
+enum class Severity {
+  Error, // fatal error that prevents code and module file generation
+  Warning, // likely problem
+  Portability, // nonstandard or obsolete features
+  Because, // for AttachTo(), explanatory attachment to support another message
+  Context, // (internal): attachment from SetContext()
+  Todo, // a feature that's not yet implemented, a fatal error
+  None // everything else, common for attachments with source locations
+};
 
 class MessageFixedText {
 public:
@@ -57,7 +59,9 @@
     severity_ = severity;
     return *this;
   }
-  bool isFatal() const { return severity_ == Severity::Error; }
+  bool isFatal() const {
+    return severity_ == Severity::Error || severity_ == Severity::Todo;
+  }
 
 private:
   CharBlock text_;
@@ -77,6 +81,10 @@
     const char str[], std::size_t n) {
   return MessageFixedText{str, n, Severity::Portability};
 }
+constexpr MessageFixedText operator""_todo_en_US(
+    const char str[], std::size_t n) {
+  return MessageFixedText{str, n, Severity::Todo};
+}
 constexpr MessageFixedText operator""_en_US(const char str[], std::size_t n) {
   return MessageFixedText{str, n, Severity::None};
 }
@@ -99,7 +107,9 @@
   MessageFormattedText &operator=(const MessageFormattedText &) = default;
   MessageFormattedText &operator=(MessageFormattedText &&) = default;
   const std::string &string() const { return string_; }
-  bool isFatal() const { return severity_ == Severity::Error; }
+  bool isFatal() const {
+    return severity_ == Severity::Error || severity_ == Severity::Todo;
+  }
   Severity severity() const { return severity_; }
   MessageFormattedText &set_severity(Severity severity) {
     severity_ = severity;
diff --git a/flang/lib/Parser/message.cpp b/flang/lib/Parser/message.cpp
--- a/flang/lib/Parser/message.cpp
+++ b/flang/lib/Parser/message.cpp
@@ -155,7 +155,9 @@
       location_, that.location_);
 }
 
-bool Message::IsFatal() const { return severity() == Severity::Error; }
+bool Message::IsFatal() const {
+  return severity() == Severity::Error || severity() == Severity::Todo;
+}
 
 Severity Message::severity() const {
   return common::visit(
@@ -224,6 +226,8 @@
     return "because: ";
   case Severity::Context:
     return "in the context: ";
+  case Severity::Todo:
+    return "error: not yet implemented: ";
   case Severity::None:
     break;
   }
diff --git a/flang/lib/Semantics/expression.cpp b/flang/lib/Semantics/expression.cpp
--- a/flang/lib/Semantics/expression.cpp
+++ b/flang/lib/Semantics/expression.cpp
@@ -3227,12 +3227,10 @@
             actual = ActualArgument(label.v);
           },
           [&](const parser::ActualArg::PercentRef &) {
-            context_.Say(
-                "not yet implemented: %REF() intrinsic for arguments"_err_en_US);
+            context_.Say("%REF() intrinsic for arguments"_todo_en_US);
           },
           [&](const parser::ActualArg::PercentVal &) {
-            context_.Say(
-                "not yet implemetned: %VAL() intrinsic for arguments"_err_en_US);
+            context_.Say("%VAL() intrinsic for arguments"_todo_en_US);
           },
       },
       std::get<parser::ActualArg>(arg.t).u);
diff --git a/flang/lib/Semantics/resolve-names.cpp b/flang/lib/Semantics/resolve-names.cpp
--- a/flang/lib/Semantics/resolve-names.cpp
+++ b/flang/lib/Semantics/resolve-names.cpp
@@ -774,9 +774,7 @@
   bool isAbstract() const;
 
 protected:
-  Symbol &GetGenericSymbol() {
-    return DEREF(genericInfo_.top().symbol);
-  }
+  Symbol &GetGenericSymbol() { return DEREF(genericInfo_.top().symbol); }
   // Add to generic the symbol for the subprogram with the same name
   void CheckGenericProcedures(Symbol &);
 
@@ -4895,7 +4893,7 @@
 }
 
 bool DeclarationVisitor::Pre(const parser::Union::UnionStmt &) {
-  Say("not yet implemented: support for UNION"_err_en_US); // TODO
+  Say("support for UNION"_todo_en_US); // TODO
   return true;
 }