bpo-41875: Use __builtin_unreachable when possible

Author: corona10

Include/pymacro.h

@@ -118,7 +118,9 @@
  "We've reached an unreachable state. Anything is possible.\n" \
  "The limits were in our heads all along. Follow your dreams.\n" \
  "https://xkcd.com/2200")
-#elif defined(__GNUC__) || defined(__clang__) || defined(__INTEL_COMPILER)
+#elif defined(__GNUC__) && (__GNUC__ > 4 || (__GNUC__ == 4 && __GNUC_MINOR__ >= 5))
+# define Py_UNREACHABLE() __builtin_unreachable()
+#elif defined(__clang__) || defined(__INTEL_COMPILER)
 # define Py_UNREACHABLE() __builtin_unreachable()
 #elif defined(_MSC_VER)
 # define Py_UNREACHABLE() __assume(0)

Misc/NEWS.d/next/C API/2020-09-28-21-56-51.bpo-38249.uzMCaZ.rst

@@ -0,0 +1,2 @@
+Update :c:macro:`Py_UNREACHABLE` to use __builtin_unreachable() if only the
+compiler is able to use it. Patch by Dong-hee Na.