diff --git a/src/include/port.h b/src/include/port.h
index 74a9dc4d4d5..0ce72e50e5e 100644
--- a/src/include/port.h
+++ b/src/include/port.h
@@ -347,15 +347,18 @@ extern int	isinf(double x);
 /*
  * Glibc doesn't use the builtin for clang due to a *gcc* bug in a version
  * newer than the gcc compatibility clang claims to have. This would cause a
- * *lot* of superfluous function calls, therefore revert when using clang.
+ * *lot* of superfluous function calls, therefore revert when using clang. In
+ * C++ there's issues with libc++ (not libstdc++), so disable as well.
  */
-#ifdef __clang__
+#if defined(__clang__) && !defined(__cplusplus)
 /* needs to be separate to not confuse other compilers */
 #if __has_builtin(__builtin_isinf)
+/* need to include before, to avoid getting overwritten */
+#include <math.h>
 #undef isinf
 #define isinf __builtin_isinf
 #endif							/* __has_builtin(isinf) */
-#endif							/* __clang__ */
+#endif							/* __clang__ && !__cplusplus*/
 #endif							/* !HAVE_ISINF */
 
 #ifndef HAVE_MKDTEMP