Use stdout for help output even in case of errors.

PiperOrigin-RevId: 522393331
Change-Id: Ia5f4ad6a2d16c033ea97f3c7e27e8eb7ee429242

absl/flags/internal/parse.h

@@ -16,6 +16,8 @@
 #ifndef ABSL_FLAGS_INTERNAL_PARSE_H_
 #define ABSL_FLAGS_INTERNAL_PARSE_H_
 
+#include <iostream>
+#include <ostream>
 #include <string>
 #include <vector>
 
@@ -40,9 +42,15 @@ enum class OnUndefinedFlag {
   kAbortIfUndefined
 };
 
-std::vector<char*> ParseCommandLineImpl(int argc, char* argv[],
-                                        UsageFlagsAction usage_flag_action,
-                                        OnUndefinedFlag undef_flag_action);
+// This is not a public interface. This interface exists to expose the ability
+// to change help output stream in case of parsing errors. This is used by
+// internal unit tests to validate expected outputs.
+// When this was written, `EXPECT_EXIT` only supported matchers on stderr,
+// but not on stdout.
+std::vector<char*> ParseCommandLineImpl(
+    int argc, char* argv[], UsageFlagsAction usage_flag_action,
+    OnUndefinedFlag undef_flag_action,
+    std::ostream& error_help_output = std::cout);
 
 // --------------------------------------------------------------------
 // Inspect original command line

absl/flags/parse.cc

@@ -22,6 +22,7 @@
 #include <cstdlib>
 #include <fstream>
 #include <iostream>
+#include <ostream>
 #include <string>
 #include <tuple>
 #include <utility>
@@ -693,7 +694,8 @@ std::vector<std::string> GetMisspellingHints(const absl::string_view flag) {
 
 std::vector<char*> ParseCommandLineImpl(int argc, char* argv[],
                                         UsageFlagsAction usage_flag_action,
-                                        OnUndefinedFlag undef_flag_action) {
+                                        OnUndefinedFlag undef_flag_action,
+                                        std::ostream& error_help_output) {
   std::vector<char*> positional_args;
   std::vector<UnrecognizedFlag> unrecognized_flags;
 
@@ -707,8 +709,8 @@ std::vector<char*> ParseCommandLineImpl(int argc, char* argv[],
 
     if (undef_flag_action == OnUndefinedFlag::kAbortIfUndefined) {
       if (!unrecognized_flags.empty()) {
-        flags_internal::HandleUsageFlags(std::cerr, ProgramUsageMessage());
-        std::exit(1);
+        flags_internal::HandleUsageFlags(error_help_output,
+        ProgramUsageMessage()); std::exit(1);
       }
     }
   }

absl/flags/parse_test.cc

@@ -18,6 +18,7 @@
 #include <stdlib.h>
 
 #include <fstream>
+#include <iostream>
 #include <string>
 #include <vector>
 
@@ -235,7 +236,9 @@ ABSL_RETIRED_FLAG(std::string, legacy_str, "l", "");
 namespace {
 
 namespace flags = absl::flags_internal;
+using testing::AllOf;
 using testing::ElementsAreArray;
+using testing::HasSubstr;
 
 class ParseTest : public testing::Test {
  public:
@@ -271,6 +274,15 @@ void InvokeParseAbslOnly(const char* (&in_argv)[N]) {
 // --------------------------------------------------------------------
 
 template <int N>
+std::vector<char*> InvokeParseCommandLineImpl(const char* (&in_argv)[N]) {
+  return flags::ParseCommandLineImpl(
+      N, const_cast<char**>(in_argv), flags::UsageFlagsAction::kHandleUsage,
+      flags::OnUndefinedFlag::kAbortIfUndefined, std::cerr);
+}
+
+// --------------------------------------------------------------------
+
+template <int N>
 std::vector<char*> InvokeParse(const char* (&in_argv)[N]) {
   return absl::ParseCommandLine(N, const_cast<char**>(in_argv));
 }
@@ -1066,8 +1078,9 @@ TEST_F(ParseDeathTest, ExitOnUnrecognizedFlagPrintsHelp) {
       "--help=int_flag",
   };
 
-  EXPECT_DEATH_IF_SUPPORTED(InvokeParse(in_args),
-                            "Try --helpfull to get a list of all flags");
+  EXPECT_EXIT(InvokeParseCommandLineImpl(in_args), testing::ExitedWithCode(1),
+              AllOf(HasSubstr("Unknown command line flag 'undef_flag1'"),
+                    HasSubstr("Try --helpfull to get a list of all flags")));
 }
 
 // --------------------------------------------------------------------