[gcc r16-6680] libstdc++: Fix std::system_category().message(int) on mingw32 target

[Jonathan Wakely]

https://gcc.gnu.org/g:972be84265c2f8a57ca651256c307486482582c6

commit r16-6680-g972be84265c2f8a57ca651256c307486482582c6
Author: Wang Jinghao <zheng.xianyuwang@gmail.com>
Date:   Sat Jan 10 18:43:14 2026 +0800

    libstdc++: Fix std::system_category().message(int) on mingw32 target
    
    On the mingw32 target, std::system_category().message(int) uses
    FormatMessage api to format error messages. When the error message
    contains insert sequences, it is unsafe not to use the
    FORMAT_MESSAGE_OGNORE_INSERTS flag, as seen at:
    https://devblogs.microsoft.com/oldnewthing/20071128-00/?p=24353
    
    The output of FormatMessage ends with "\r\n" and includes a Full stop
    character used by the current thread's UI language. Now, we will remove
    "\r\n" and any trailing '.' from the output in any language environment.
    
    In the testsuite for std::system_category().message(int), we first
    switch the thread UI language to en-US to meet expectations in any
    language environment.
    
    libstdc++-v3/ChangeLog:
    
            * src/c++11/system_error.cc (system_error_category) [_WIN32]:
            Use FormatMessageA function instead of FormatMessage macro.
            * testsuite/19_diagnostics/error_category/system_category.cc:
            Fix typo in __MINGW32__ macro name.  Adjust behavior on the
            mingw32 target.

Diff:
---
 libstdc++-v3/src/c++11/system_error.cc              | 21 +++++++++++----------
 .../error_category/system_category.cc               | 19 ++++++++++++++++---
 2 files changed, 27 insertions(+), 13 deletions(-)

diff --git a/libstdc++-v3/src/c++11/system_error.cc b/libstdc++-v3/src/c++11/system_error.cc
index 548c88fcfdea..851f51acd404 100644
--- a/libstdc++-v3/src/c++11/system_error.cc
+++ b/libstdc++-v3/src/c++11/system_error.cc
@@ -161,22 +161,23 @@ namespace
 #if defined(_WIN32) && !defined(__CYGWIN__)
       char* buf = nullptr;
       auto len
-	= FormatMessage(FORMAT_MESSAGE_FROM_SYSTEM
-			| FORMAT_MESSAGE_ALLOCATE_BUFFER,
-			nullptr,
-			i,
-			LANG_USER_DEFAULT,
-			reinterpret_cast<LPTSTR>(&buf),
-			0,
-			nullptr);
+	= FormatMessageA(FORMAT_MESSAGE_FROM_SYSTEM
+			 | FORMAT_MESSAGE_ALLOCATE_BUFFER
+			 | FORMAT_MESSAGE_IGNORE_INSERTS,
+			 nullptr,
+			 i,
+			 LANG_USER_DEFAULT,
+			 reinterpret_cast<LPTSTR>(&buf),
+			 0,
+			 nullptr);
       if (len > 0)
       {
 	struct deleter {
 	  void operator()(void* p) const { ::LocalFree(p); }
 	};
 	std::unique_ptr<char[], deleter> guard(buf);
-	if (len > 3 && !__builtin_memcmp(buf + len - 3, ".\r\n", 3)) [[likely]]
-	  len -= 3;
+	if (len > 2 && !__builtin_memcmp (buf + len - 2, "\r\n", 2)) [[likely]]
+	  len -= 2 + (buf[len - 3] == '.');
 	return string(buf, len);
       }
       return string("Unknown error code");
diff --git a/libstdc++-v3/testsuite/19_diagnostics/error_category/system_category.cc b/libstdc++-v3/testsuite/19_diagnostics/error_category/system_category.cc
index 250d34f9bb5e..b314884ef698 100644
--- a/libstdc++-v3/testsuite/19_diagnostics/error_category/system_category.cc
+++ b/libstdc++-v3/testsuite/19_diagnostics/error_category/system_category.cc
@@ -21,6 +21,11 @@
 #include <locale>
 #include <testsuite_hooks.h>
 
+#if defined __MINGW32__ || defined __MINGW64__
+#define WIN32_LEAN_AND_MEAN
+#include <windows.h>
+#endif
+
 void
 test01()
 {
@@ -34,7 +39,7 @@ test02()
   const std::error_category& cat = std::system_category();
   std::error_condition cond;
 
-#if defined __MING32__ || defined __MINGW64__
+#if defined __MINGW32__ || defined __MINGW64__
   cond = cat.default_error_condition(8); // ERROR_NOT_ENOUGH_MEMORY
   VERIFY( cond.value() == ENOMEM );
   VERIFY( cond.category() == std::generic_category() );
@@ -112,9 +117,17 @@ test03()
   // set "C" locale to get expected message
   auto loc = std::locale::global(std::locale::classic());
 
-#if defined __MING32__ || defined __MINGW64__
+#if defined __MINGW32__ || defined __MINGW64__
+  // On Windows, set thread preferred UI languages to "en-US"
+  // to get expected message
+  ULONG num_langs = 1;
+  SetThreadPreferredUILanguages(MUI_LANGUAGE_NAME, L"en-US\0", &num_langs);
+
   std::string msg = std::system_category().message(5); // ERROR_ACCESS_DENIED
-  VERIFY(msg == "Access denied");
+  // Windows returns "Access is denied" but Wine returns "Access denied".
+  VERIFY(msg == "Access is denied" || msg == "Access denied");
+
+  SetThreadPreferredUILanguages(MUI_RESET_FILTERS, nullptr, nullptr);
 #else
   std::string msg = std::system_category().message(EBADF);
   VERIFY( msg.find("file") != std::string::npos );