[PATCH] libstdc++: Fix build failure due to missing <langinfo.h> [PR98374]

[Patrick Palka]

This fixes a build failure on Windows which lacks <langinfo.h>.
(We can't use the more portable localeconv() from <clocale> to obtain
the radix character of the current locale here because it's not
thread-safe, unfortunately.)

This change means that on Windows and other such systems, we'll just
always assume the radix character used by printf is '.' when formatting
a long double through it.

libstdc++-v3/ChangeLog:

	PR libstdc++/98374
	* src/c++17/floating_to_chars.cc: Guard include of <langinfo.h>
	with __has_include.
	(__floating_to_chars_precision) [!defined(RADIXCHAR)]: Don't
	attempt to obtain the radix character of the current locale,
	just assume it's '.'.
---
 libstdc++-v3/src/c++17/floating_to_chars.cc | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/libstdc++-v3/src/c++17/floating_to_chars.cc b/libstdc++-v3/src/c++17/floating_to_chars.cc
index 474e791e717..6470fbb0b95 100644
--- a/libstdc++-v3/src/c++17/floating_to_chars.cc
+++ b/libstdc++-v3/src/c++17/floating_to_chars.cc
@@ -33,7 +33,9 @@
 #include <cmath>
 #include <cstdio>
 #include <cstring>
-#include <langinfo.h>
+#if __has_include(<langinfo.h>)
+# include <langinfo.h> // for nl_langinfo
+#endif
 #include <optional>
 #include <string_view>
 #include <type_traits>
@@ -1113,6 +1115,7 @@ template<typename T>
 	// to handle a radix point that's different from '.'.
 	char radix[6] = {'.', '\0', '\0', '\0', '\0', '\0'};
 	if (effective_precision > 0)
+#ifdef RADIXCHAR
 	  // ???: Can nl_langinfo() ever return null?
 	  if (const char* const radix_ptr = nl_langinfo(RADIXCHAR))
 	    {
@@ -1121,6 +1124,7 @@ template<typename T>
 	      // UTF-8 character) wide.
 	      __glibcxx_assert(radix[4] == '\0');
 	    }
+#endif
 
 	// Compute straightforward upper bounds on the output length.
 	int output_length_upper_bound;
-- 
2.30.0.rc0