--- a/clang/lib/Basic/Warnings.cpp	Mon May 25 11:55:54 2020 +0900
+++ b/clang/lib/Basic/Warnings.cpp	Tue Jun 08 06:07:14 2021 +0900
@@ -130,11 +130,14 @@
       }
 
       // -Werror/-Wno-error is a special case, not controlled by the option
-      // table. It also has the "specifier" form of -Werror=foo and -Werror-foo.
+      // table. It also has the "specifier" form of -Werror=foo. GCC supports
+      // the deprecated -Werror-implicit-function-declaration which is used by
+      // a few projects.
       if (Opt.startswith("error")) {
         StringRef Specifier;
         if (Opt.size() > 5) {  // Specifier must be present.
-          if ((Opt[5] != '=' && Opt[5] != '-') || Opt.size() == 6) {
+          if (Opt[5] != '=' &&
+              Opt.substr(5) != "-implicit-function-declaration") {
             if (Report)
               Diags.Report(diag::warn_unknown_warning_specifier)
                 << "-Werror" << ("-W" + OrigOpt.str());