Use stdout for help output even in case of errors.

PiperOrigin-RevId: 522393331
Change-Id: Ia5f4ad6a2d16c033ea97f3c7e27e8eb7ee429242

1 files changed, 11 insertions, 3 deletions

diff --git a/absl/flags/internal/parse.h b/absl/flags/internal/parse.h
index e4352049..10c531b8 100644
--- a/absl/flags/internal/parse.h
+++ b/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