Add a call to handle usage flags in case of unrecognized flags

PiperOrigin-RevId: 519090305
Change-Id: Ic97a36da33e1e0681765a913f3c54c3d818eebdc

absl/flags/parse.cc

@@ -706,7 +706,10 @@ std::vector<char*> ParseCommandLineImpl(int argc, char* argv[],
         (undef_flag_action == OnUndefinedFlag::kAbortIfUndefined));
 
     if (undef_flag_action == OnUndefinedFlag::kAbortIfUndefined) {
-      if (!unrecognized_flags.empty()) { std::exit(1); }
+      if (!unrecognized_flags.empty()) {
+        flags_internal::HandleUsageFlags(std::cerr, ProgramUsageMessage());
+        std::exit(1);
+      }
     }
   }
 
@@ -891,7 +894,7 @@ HelpMode ParseAbseilFlagsOnlyImpl(
     flags_internal::ReportUsageError(
         "NOTE: command line flags are disabled in this build", true);
 #else
-    flags_internal::HandleUsageFlags(std::cout, ProgramUsageMessage());
+    flags_internal::HandleUsageFlags(std::cerr, ProgramUsageMessage());
 #endif
     return HelpMode::kFull;  // We just need to make sure the exit with
                              // code 1.

absl/flags/parse_test.cc

@@ -17,10 +17,8 @@
 
 #include <stdlib.h>
 
-#include <cstddef>
 #include <fstream>
 #include <string>
-#include <utility>
 #include <vector>
 
 #include "gmock/gmock.h"
@@ -1061,4 +1059,17 @@ TEST_F(ParseTest, AllUndefOkFlagsAreIgnored) {
 
 // --------------------------------------------------------------------
 
+TEST_F(ParseDeathTest, ExitOnUnrecognizedFlagPrintsHelp) {
+  const char* in_args[] = {
+      "testbin",
+      "--undef_flag1",
+      "--help=int_flag",
+  };
+
+  EXPECT_DEATH_IF_SUPPORTED(InvokeParse(in_args),
+                            "Try --helpfull to get a list of all flags");
+}
+
+// --------------------------------------------------------------------
+
 }  // namespace