diff --git a/clang/lib/Driver/Driver.cpp b/clang/lib/Driver/Driver.cpp
--- a/clang/lib/Driver/Driver.cpp
+++ b/clang/lib/Driver/Driver.cpp
@@ -223,7 +223,6 @@
   InputArgList Args =
       getOpts().ParseArgs(ArgStrings, MissingArgIndex, MissingArgCount,
                           IncludedFlagsBitmask, ExcludedFlagsBitmask);
-
   // Check for missing argument error.
   if (MissingArgCount) {
     Diag(diag::err_drv_missing_argument)
@@ -263,20 +262,43 @@
   }
 
   for (const Arg *A : Args.filtered(options::OPT_UNKNOWN)) {
-    unsigned DiagID;
+    // The default diagnostic when the option is unknow
+    unsigned DiagID = diag::err_drv_unknown_argument;
+
+    // Try to find a good hint for the user
     auto ArgString = A->getAsString(Args);
     std::string Nearest;
-    if (getOpts().findNearest(
-          ArgString, Nearest, IncludedFlagsBitmask, ExcludedFlagsBitmask) > 1) {
-      DiagID = IsCLMode() ? diag::warn_drv_unknown_argument_clang_cl
-                          : diag::err_drv_unknown_argument;
-      Diags.Report(DiagID) << ArgString;
-    } else {
+    unsigned Distance = getOpts().findNearest(
+        ArgString, Nearest, IncludedFlagsBitmask, ExcludedFlagsBitmask);
+    assert(Distance != 0 && "This option should not be 'unknown'");
+
+    if (Distance == 1) {
+      // Found a good suggestion - propose that in the diagnostic
       DiagID = IsCLMode()
                    ? diag::warn_drv_unknown_argument_clang_cl_with_suggestion
                    : diag::err_drv_unknown_argument_with_suggestion;
       Diags.Report(DiagID) << ArgString << Nearest;
+    } else {
+      // No good suggestion was found - the diagnostic depends on the mode in
+      // which the driver operates
+      if (IsCLMode())
+        // In CL mode just warn the user (MSVC consumes everything anyway)
+        DiagID = diag::warn_drv_unknown_argument_clang_cl;
+      else if (!IsFlangMode()) {
+        // In non-Flang mode, check whether this is a Flang flag and
+        // communicate accordingly.
+        ExcludedFlagsBitmask &= ~options::FlangOnlyOption;
+        if (getOpts().findNearest(ArgString, Nearest, IncludedFlagsBitmask,
+                                  ExcludedFlagsBitmask) == 0)
+          DiagID =
+              Diags.getCustomDiagID(clang::DiagnosticsEngine::Warning,
+                                    "command line option ‘%0’ is only valid "
+                                    "in Flang mode (i.e. for Fortran input)");
+      } // TODO: Check whether this is a C-mode flag and report accordingly.
+
+      Diags.Report(DiagID) << ArgString;
     }
+
     ContainsError |= Diags.getDiagnosticLevel(DiagID, SourceLocation()) >
                      DiagnosticsEngine::Warning;
   }