From da206878f64ef66b1f7527c4d87e8baf9a5c5bfd Mon Sep 17 00:00:00 2001
From: Jonathan Wakely <jwakely@redhat.com>
Date: Wed, 22 Sep 2021 11:58:20 +0100
Subject: [PATCH] libstdc++: std::system_category should know meaning of zero
 [PR102425]

Although 0 is not an errno value, it should still be recognized as
corresponding to a value belonging to the generic_category().

Signed-off-by: Jonathan Wakely <jwakely@redhat.com>

libstdc++-v3/ChangeLog:

	PR libstdc++/102425
	* src/c++11/system_error.cc
	(system_error_category::default_error_condition): Add 0 to
	switch.
	* testsuite/19_diagnostics/error_category/102425.cc: New test.

(cherry picked from commit ce01e2e64c340dadb55a8a24c545a13e654804d4)
---
 libstdc++-v3/src/c++11/system_error.cc         |  3 +++
 .../19_diagnostics/error_category/102425.cc    | 18 ++++++++++++++++++
 2 files changed, 21 insertions(+)
 create mode 100644 libstdc++-v3/testsuite/19_diagnostics/error_category/102425.cc

diff --git a/libstdc++-v3/src/c++11/system_error.cc b/libstdc++-v3/src/c++11/system_error.cc
index 23fb6182825d..0ee3ed81961a 100644
--- a/libstdc++-v3/src/c++11/system_error.cc
+++ b/libstdc++-v3/src/c++11/system_error.cc
@@ -70,6 +70,8 @@ namespace
     virtual std::error_condition
     default_error_condition(int ev) const noexcept
     {
+      // Use generic category for all known POSIX errno values (including zero)
+      // and system category otherwise.
       switch (ev)
       {
       // List of errno macros from [cerrno.syn].
@@ -310,6 +312,7 @@ namespace
 #ifdef EXDEV
       case EXDEV:
 #endif
+      case 0:
         return std::error_condition(ev, std::generic_category());
 
       /* Additional system-dependent mappings from non-standard error codes
diff --git a/libstdc++-v3/testsuite/19_diagnostics/error_category/102425.cc b/libstdc++-v3/testsuite/19_diagnostics/error_category/102425.cc
new file mode 100644
index 000000000000..069b5e284e1c
--- /dev/null
+++ b/libstdc++-v3/testsuite/19_diagnostics/error_category/102425.cc
@@ -0,0 +1,18 @@
+// { dg-do run { target c++11 } }
+#include <system_error>
+#include <testsuite_hooks.h>
+
+void test01()
+{
+  // PR libstdc++/102425
+  VERIFY( std::error_code() == std::error_condition() );
+
+  auto zero = std::system_category().default_error_condition(0);
+  // This is the condition that the equality above relies on:
+  VERIFY( zero.category() == std::generic_category() );
+}
+
+int main()
+{
+  test01();
+}
-- 
2.47.2