diff --git a/clang/tools/diagtool/DiagnosticNames.h b/clang/tools/diagtool/DiagnosticNames.h
index d8fd6401ef6b8..f541e88577cc5 100644
--- a/clang/tools/diagtool/DiagnosticNames.h
+++ b/clang/tools/diagtool/DiagnosticNames.h
@@ -76,11 +76,11 @@ namespace diagtool {
         return *this;
       }
 
-      bool operator==(group_iterator &Other) const {
+      bool operator==(const group_iterator &Other) const {
         return CurrentID == Other.CurrentID;
       }
 
-      bool operator!=(group_iterator &Other) const {
+      bool operator!=(const group_iterator &Other) const {
         return CurrentID != Other.CurrentID;
       }
     };
diff --git a/clang/tools/diagtool/TreeView.cpp b/clang/tools/diagtool/TreeView.cpp
index 96951287cc4e8..843bd377e574c 100644
--- a/clang/tools/diagtool/TreeView.cpp
+++ b/clang/tools/diagtool/TreeView.cpp
@@ -36,6 +36,17 @@ class TreePrinter {
     return Diags.isIgnored(DiagID, SourceLocation());
   }
 
+  static bool unimplemented(const GroupRecord &Group) {
+    if (!Group.diagnostics().empty())
+      return false;
+
+    for (const GroupRecord &GR : Group.subgroups())
+      if (!unimplemented(GR))
+        return false;
+
+    return true;
+  }
+
   static bool enabledByDefault(const GroupRecord &Group) {
     for (const DiagnosticRecord &DR : Group.diagnostics()) {
       if (isIgnored(DR.DiagID))
@@ -53,7 +64,9 @@ class TreePrinter {
   void printGroup(const GroupRecord &Group, unsigned Indent = 0) {
     out.indent(Indent * 2);
 
-    if (enabledByDefault(Group))
+    if (unimplemented(Group))
+      out << Colors::RED;
+    else if (enabledByDefault(Group))
       out << Colors::GREEN;
     else
       out << Colors::YELLOW;
@@ -117,7 +130,9 @@ class TreePrinter {
 
   void showKey() {
     out << '\n' << Colors::GREEN << "GREEN" << Colors::RESET
-        << " = enabled by default\n\n";
+        << " = enabled by default";
+    out << '\n' << Colors::RED << "RED" << Colors::RESET
+        << " = unimplemented (accepted for GCC compatibility)\n\n";
   }
 };