+ comment on clang workaround

Author: skirpichev

Modules/_ctypes/cfield.c

@@ -1092,10 +1092,32 @@ d_get(void *ptr, Py_ssize_t size)
 }
 
 #ifdef __STDC_IEC_559_COMPLEX__
-#if defined(__clang__) && __has_builtin(__builtin_complex) && \
-    !defined(CMPLX)
-#  define CMPLX(x, y) __builtin_complex ((double) (x), (double) (y))
-#endif
+/* Other compilers (than clang), that claims to
+   implement C11 *and* define __STDC_IEC_559_COMPLEX__ don't have
+   issue with CMPLX().  This is specific to glibc & clang combination:
+   https://sourceware.org/bugzilla/show_bug.cgi?id=26287
+
+   Here we fallback to using __builtin_complex(), available in clang
+   v12+.  Else CMPLX implemented following C11 6.2.5p13: "Each complex type
+   has the same representation and alignment requirements as an array
+   type containing exactly two elements of the corresponding real type;
+   the first element is equal to the real part, and the second element
+   to the imaginary part, of the complex number.
+ */
+#  if !defined(CMPLX)
+#    if defined(__clang__) && __has_builtin(__builtin_complex)
+#      define CMPLX(x, y) __builtin_complex ((double) (x), (double) (y))
+#    else
+inline double complex
+CMPLX(double real, imag)
+{
+    double complex z;
+    ((double *)(&z))[0] = real;
+    ((double *)(&z))[1] = imag;
+    return z;
+}
+#    endif
+#  endif
 static PyObject *
 C_set(void *ptr, PyObject *value, Py_ssize_t size)
 {