From 85cffe268e2c4d7585e589ddb4e38bf7f61f5fb5 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?B=C3=A9n=C3=A9dikt=20Tran?=
 <10796600+picnixz@users.noreply.github.com>
Date: Tue, 8 Jul 2025 22:06:25 +0200
Subject: [PATCH] fix `Py_NAN` on Solaris systems

---
 Include/pymath.h                                | 17 ++++++++++++++++-
 ...25-07-08-22-07-54.gh-issue-136006.XRU5w4.rst |  2 ++
 2 files changed, 18 insertions(+), 1 deletion(-)
 create mode 100644 Misc/NEWS.d/next/C_API/2025-07-08-22-07-54.gh-issue-136006.XRU5w4.rst

diff --git a/Include/pymath.h b/Include/pymath.h
index 0ead1f95670fde..e2919c7b527267 100644
--- a/Include/pymath.h
+++ b/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 */
diff --git a/Misc/NEWS.d/next/C_API/2025-07-08-22-07-54.gh-issue-136006.XRU5w4.rst b/Misc/NEWS.d/next/C_API/2025-07-08-22-07-54.gh-issue-136006.XRU5w4.rst
new file mode 100644
index 00000000000000..2165e535b12245
--- /dev/null
+++ b/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.