Index: include/math.h
===================================================================
--- include/math.h
+++ include/math.h
@@ -8,16 +8,6 @@
 //
 //===----------------------------------------------------------------------===//
 
-// This include lives outside the header guard in order to support an MSVC
-// extension which allows users to do:
-//
-// #define _USE_MATH_DEFINES
-// #include <math.h>
-//
-// and receive the definitions of mathematical constants, even if <math.h>
-// has previously been included.
-#include_next <math.h>
-
 #ifndef _LIBCPP_MATH_H
 #define _LIBCPP_MATH_H
 
@@ -308,6 +298,8 @@
 #pragma GCC system_header
 #endif
 
+#include_next <math.h>
+
 #ifdef __cplusplus
 
 // We support including .h headers inside 'extern "C"' contexts, so switch
@@ -1494,4 +1486,18 @@
 
 #endif // __cplusplus
 
+#else // _LIBCPP_MATH_H
+
+// This include lives outside the header guard in order to support an MSVC
+// extension which allows users to do:
+//
+// #define _USE_MATH_DEFINES
+// #include <math.h>
+//
+// and receive the definitions of mathematical constants, even if <math.h>
+// has previously been included.
+#if defined(_LIBCPP_MSVCRT) && defined(_USE_MATH_DEFINES)
+#include_next <math.h>
+#endif
+
 #endif  // _LIBCPP_MATH_H