[libcxx] [test] Detect the UCRT printf("%a") formatting bug (llvm#99846)

This fixes testing with MinGW, if built without
__USE_MINGW_ANSI_STDIO=1.

On x86 MinGW, such a configuration fails printf tests with long doubles
due to mismatches between 80 and 64 bit long doubles - but on ARM,
there's no such issue, so building without __USE_MINGW_ANSI_STDIO=1 is
perfectly valid there.

Add another similar XFAIL to a libcxxabi test; this test isn't executed
in MSVC environments, so no XFAIL has been needed so far.

libcxx/test/std/localization/locale.categories/category.numeric/locale.nm.put/facet.num.put.members/put_double.hex.pass.cpp

@@ -12,10 +12,7 @@
 
 // iter_type put(iter_type s, ios_base& iob, char_type fill, double v) const;
 
-// With the Microsoft UCRT, printf("%a", 0.0) produces "0x0.0000000000000p+0"
-// while other C runtimes produce just "0x0p+0".
-// https://developercommunity.visualstudio.com/t/Printf-formatting-of-float-as-hex-prints/1660844
-// XFAIL: msvc
+// XFAIL: win32-broken-printf-a-precision
 
 // XFAIL: LIBCXX-AIX-FIXME
 

libcxx/test/std/localization/locale.categories/category.numeric/locale.nm.put/facet.num.put.members/put_long_double.hex.pass.cpp

@@ -12,10 +12,7 @@
 
 // iter_type put(iter_type s, ios_base& iob, char_type fill, long double v) const;
 
-// With the Microsoft UCRT, printf("%a", 0.0) produces "0x0.0000000000000p+0"
-// while other C runtimes produce just "0x0p+0".
-// https://developercommunity.visualstudio.com/t/Printf-formatting-of-float-as-hex-prints/1660844
-// XFAIL: msvc
+// XFAIL: win32-broken-printf-a-precision
 
 // XFAIL: LIBCXX-AIX-FIXME
 

libcxx/utils/libcxx/test/features.py

@@ -263,6 +263,26 @@ def _mingwSupportsModules(cfg):
  """,
  ),
  ),
+ # Check for a Windows UCRT bug (not fixed upstream yet).
+ # With UCRT, printf("%a", 0.0) produces "0x0.0000000000000p+0",
+ # while other C runtimes produce just "0x0p+0".
+ # https://developercommunity.visualstudio.com/t/Printf-formatting-of-float-as-hex-prints/1660844
+ Feature(
+ name="win32-broken-printf-a-precision",
+ when=lambda cfg: "_WIN32" in compilerMacros(cfg)
+ and not programSucceeds(
+ cfg,
+ """
+ #include <stdio.h>
+ #include <string.h>
+ int main(int, char**) {
+ char buf[100];
+ snprintf(buf, sizeof(buf), "%a", 0.0);
+ return strcmp(buf, "0x0p+0");
+ }
+ """,
+ ),
+ ),
  # Check for Glibc < 2.27, where the ru_RU.UTF-8 locale had
  # mon_decimal_point == ".", which our tests don't handle.
  Feature(

libcxxabi/test/test_demangle.pass.cpp

@@ -17,6 +17,8 @@
 // 80-bit format, and this demangling test is failing on it.
 // XFAIL: LIBCXX-ANDROID-FIXME && target={{i686|x86_64}}-{{.+}}-android{{.*}}
 
+// XFAIL: win32-broken-printf-a-precision
+
 #include "support/timer.h"
 #include <algorithm>
 #include <cassert>