gutilsprivate: Factor out g_isnan() helper

There are a couple of places in the code which use `isnan()` and have
platform-specific workarounds for it. Unify those, and extend the
workaround to work for msys2-mingw32.

It seems that msys2-mingw32 can’t automatically use `isnan()` in a wider
mode than `float`:
```
In file included from ../glib/gdatetime.c:60:
../glib/gdatetime.c: In function 'g_date_time_new':
../glib/gdatetime.c:1648:14: error: conversion from 'gdouble' {aka 'double'} to 'float' may change value [-Werror=float-conversion]
 1648 |       isnan (seconds) ||
      |              ^~~~~~~
cc1.exe: all warnings being treated as errors
```

See: https://gitlab.gnome.org/pwithnall/glib/-/jobs/4022715

Using it in float mode on all platforms should not change behaviour, as
a conversion from `(double) NAN` to `float` should still give `NAN`.

Signed-off-by: Philip Withnall <pwithnall@gnome.org>

Helps: #3405

glib/gdatetime.c

@@ -82,15 +82,11 @@
 #include "gtestutils.h"
 #include "gthread.h"
 #include "gtimezone.h"
+#include "gutilsprivate.h"
 
 #ifndef G_OS_WIN32
 #include <sys/time.h>
 #include <time.h>
-#else
-#if defined (_MSC_VER) && (_MSC_VER < 1800)
-/* fallback implementation for isnan() on VS2012 and earlier */
-#define isnan _isnan
-#endif
 #endif /* !G_OS_WIN32 */
 
 struct _GDateTime
@@ -1645,7 +1641,7 @@ g_date_time_new (GTimeZone *tz,
       day < 1 || day > days_in_months[GREGORIAN_LEAP (year)][month] ||
       hour < 0 || hour > 23 ||
       minute < 0 || minute > 59 ||
-      isnan (seconds) ||
+      g_isnan (seconds) ||
       seconds < 0.0 || seconds >= 60.0)
     return NULL;
 

glib/gutilsprivate.h

@@ -26,6 +26,7 @@
 #include "gtypes.h"
 #include "gtestutils.h"
 
+#include <math.h>
 #include <time.h>
 
 G_BEGIN_DECLS
@@ -61,6 +62,23 @@ gboolean _g_localtime (time_t timet, struct tm *tm);
 
 gboolean g_set_prgname_once (const gchar *prgname);
 
+/* Although isnan() is defined as a type-independent macro in C99, mingw32
+ * doesn’t seem to support that (it defines `isnan (float d)` only). Older
+ * MSVC toolchains don’t support C99 either. So we provide an internal
+ * abstraction macro.
+ *
+ * This should not be made public; toolchains will soon enough catch up with
+ * C99, so third party code should just use isnan(). */
+static inline int
+g_isnan (double d)
+{
+#if (defined (_MSC_VER) && (_MSC_VER < 1800)) || defined(__MINGW32__)
+  return _isnan (d);
+#else
+  return isnan (d);
+#endif
+}
+
 G_END_DECLS
 
 #endif /* __G_UTILS_PRIVATE_H__ */

glib/tests/strfuncs.c

@@ -34,11 +34,11 @@
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
+
 #include "glib.h"
+#include "gutilsprivate.h"
 
 #if defined (_MSC_VER) && (_MSC_VER <= 1800)
-#define isnan(x) _isnan(x)
-
 #ifndef NAN
 static const unsigned long __nan[2] = {0xffffffff, 0x7fffffff};
 #define NAN (*(const float *) __nan)
@@ -1625,7 +1625,7 @@ check_strtod_string (gchar    *number,
 
       setlocale (LC_NUMERIC, locales[l]);
       d = g_ascii_strtod (number, &end);
-      g_assert_true (isnan (res) ? isnan (d) : (d == res));
+      g_assert_true (g_isnan (res) ? g_isnan (d) : (d == res));
       g_assert_true ((gsize) (end - number) ==
                      (check_end ? correct_len : strlen (number)));
     }
@@ -1660,7 +1660,7 @@ test_ascii_strtod (void)
   /* Do this before any call to setlocale.  */
   our_nan = atof ("NaN");
 #endif
-  g_assert_true (isnan (our_nan));
+  g_assert_true (g_isnan (our_nan));
 
 #ifdef INFINITY
   our_inf = INFINITY;