Support Py_UNUSED() on clang (GH-13544)

Author: vstinner

Doc/c-api/intro.rst

@@ -156,7 +156,7 @@ complete listing.
 .. c:macro:: Py_UNUSED(arg)
 
    Use this for unused arguments in a function definition to silence compiler
-   warnings, e.g. ``PyObject* func(PyObject *Py_UNUSED(ignored))``.
+   warnings. Example: ``int func(int a, int Py_UNUSED(b)) { return a; }``.
 
    .. versionadded:: 3.4
 

Include/pymacro.h

@@ -89,10 +89,15 @@
 /* Check if pointer "p" is aligned to "a"-bytes boundary. */
 #define _Py_IS_ALIGNED(p, a) (!((uintptr_t)(p) & (uintptr_t)((a) - 1)))
 
-#ifdef __GNUC__
-#define Py_UNUSED(name) _unused_ ## name __attribute__((unused))
+/* Use this for unused arguments in a function definition to silence compiler
+ * warnings. Example:
+ *
+ * int func(int a, int Py_UNUSED(b)) { return a; }
+ */
+#if defined(__GNUC__) || defined(__clang__)
+#  define Py_UNUSED(name) _unused_ ## name __attribute__((unused))
 #else
-#define Py_UNUSED(name) _unused_ ## name
+#  define Py_UNUSED(name) _unused_ ## name
 #endif
 
 #define Py_UNREACHABLE() abort()