+ comment on clang workaround

skirpichev · skirpichev · commit 45f8544205a6 · 2024-06-23T15:50:25.000+03:00
diff --git a/Modules/_ctypes/cfield.c b/Modules/_ctypes/cfield.c
@@ -1092,6 +1092,17 @@ d_get(void *ptr, Py_ssize_t size)
 }
 
 #ifdef __STDC_IEC_559_COMPLEX__
+/* 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+.  Unfortunatelly, it's impossible to emulate CMPLX(), using
+   only complex arithmetic, if implemented only mixed-mode rules
+   for "complex op real" cases (as most compilers do):
+   think about CMPLX(1, NAN).
+ */
 #if defined(__clang__) && __has_builtin(__builtin_complex) && \
     !defined(CMPLX)
 #  define CMPLX(x, y) __builtin_complex ((double) (x), (double) (y))
