gh-136006: fix Py_NAN expansion on Solaris systems

Include/pymath.h

@@ -57,9 +57,24 @@
 
 /* Py_NAN: Value that evaluates to a quiet Not-a-Number (NaN).  The sign is
  * undefined and normally not relevant, but e.g. fixed for float("nan").
+ *
+ * Note: On Solaris, NAN is a function address, hence arithmetic is impossible.
+ * For that reason, we instead use the built-in call if available or fallback
+ * to a generic NaN computed from strtod() as a last resort.
+ *
+ * See https://github.com/python/cpython/issues/136006 for details.
  */
 #if !defined(Py_NAN)
-#    define Py_NAN ((double)NAN)
+#  if defined(__sun)
+#    if _Py__has_builtin(__builtin_nanf)
+#       define Py_NAN   ((double)__builtin_nanf(""))
+#    else
+#       include <stdlib.h>
+#       define Py_NAN   (strtod("NAN", NULL))
+#    endif
+#  else
+#    define Py_NAN      ((double)NAN)
+#  endif
 #endif
 
 #endif /* Py_PYMATH_H */

Misc/NEWS.d/next/C_API/2025-07-08-22-07-54.gh-issue-136006.XRU5w4.rst

@@ -0,0 +1,2 @@
+On Solaris, the :c:macro:`!Py_NAN` macro now expands to a :c:type:`!double`
+instead of a function address. Patch by Bénédikt Tran.