[gcc r12-3859] libstdc++: std::system_category should know meaning of zero [PR102425]

[Jonathan Wakely <redi@gcc.gnu.org>]

https://gcc.gnu.org/g:ce01e2e64c340dadb55a8a24c545a13e654804d4

commit r12-3859-gce01e2e64c340dadb55a8a24c545a13e654804d4
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Wed Sep 22 11:58:20 2021 +0100

    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.

Diff:
---
 libstdc++-v3/src/c++11/system_error.cc                 |  3 +++
 .../testsuite/19_diagnostics/error_category/102425.cc  | 18 ++++++++++++++++++
 2 files changed, 21 insertions(+)

diff --git a/libstdc++-v3/src/c++11/system_error.cc b/libstdc++-v3/src/c++11/system_error.cc
index f12290adaee..6c79202eb0e 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 00000000000..069b5e284e1
--- /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();
+}