diff --git a/libcxx/include/math.h b/libcxx/include/math.h
--- a/libcxx/include/math.h
+++ b/libcxx/include/math.h
@@ -301,6 +301,10 @@
 
 #ifdef __cplusplus
 
+#    ifdef fpclassify
+#      undef fpclassify
+#    endif // fpclassify
+
 // We support including .h headers inside 'extern "C"' contexts, so switch
 // back to C++ linkage before including these C++ headers.
 extern "C++" {
@@ -310,12 +314,52 @@
 #include <stdlib.h>
 #include <type_traits>
 
-// signbit
-
 #    ifdef signbit
 #      undef signbit
 #    endif // signbit
 
+#    ifdef isfinite
+#      undef isfinite
+#    endif // isfinite
+
+#    ifdef isinf
+#      undef isinf
+#    endif // isinf
+
+#    ifdef isnan
+#      undef isnan
+#    endif // isnan
+
+#    ifdef isnormal
+#      undef isnormal
+#    endif // isnormal
+
+#    ifdef isgreater
+#      undef isgreater
+#    endif // isgreater
+
+#    ifdef isgreaterequal
+#      undef isgreaterequal
+#    endif // isgreaterequal
+
+#    ifdef isless
+#      undef isless
+#    endif // isless
+
+#    ifdef islessequal
+#      undef islessequal
+#    endif // islessequal
+
+#    ifdef islessgreater
+#      undef islessgreater
+#    endif // islessgreater
+
+#    ifdef isunordered
+#      undef isunordered
+#    endif // isunordered
+
+// signbit
+
 template <class _A1, std::__enable_if_t<std::is_floating_point<_A1>::value, int> = 0>
 inline _LIBCPP_HIDE_FROM_ABI bool signbit(_A1 __x) _NOEXCEPT {
   return __builtin_signbit(__x);
@@ -333,10 +377,6 @@
 
 // fpclassify
 
-#    ifdef fpclassify
-#      undef fpclassify
-#    endif // fpclassify
-
 template <class _A1, std::__enable_if_t<std::is_floating_point<_A1>::value, int> = 0>
 inline _LIBCPP_HIDE_FROM_ABI int fpclassify(_A1 __x) _NOEXCEPT {
   return __builtin_fpclassify(FP_NAN, FP_INFINITE, FP_NORMAL, FP_SUBNORMAL, FP_ZERO, __x);
@@ -347,10 +387,10 @@
   return __x == 0 ? FP_ZERO : FP_NORMAL;
 }
 
-// isfinite
+// The MSVC runtime already provides these functions as templates
+#ifndef _LIBCPP_MSVCRT
 
-#    ifdef isfinite
-#      undef isfinite
+// isfinite
 
 template <class _A1,
           std::__enable_if_t<std::is_arithmetic<_A1>::value && std::numeric_limits<_A1>::has_infinity, int> = 0>
@@ -364,13 +404,8 @@
   return true;
 }
 
-#    endif // isfinite
-
 // isinf
 
-#    ifdef isinf
-#      undef isinf
-
 template <class _A1,
           std::__enable_if_t<std::is_arithmetic<_A1>::value && std::numeric_limits<_A1>::has_infinity, int> = 0>
 inline _LIBCPP_HIDE_FROM_ABI bool isinf(_A1 __x) _NOEXCEPT {
@@ -392,13 +427,8 @@
 inline _LIBCPP_HIDE_FROM_ABI bool isinf(long double __x) _NOEXCEPT { return __builtin_isinf(__x); }
 #      endif
 
-#    endif // isinf
-
 // isnan
 
-#    ifdef isnan
-#      undef isnan
-
 template <class _A1, std::__enable_if_t<std::is_floating_point<_A1>::value, int> = 0>
 inline _LIBCPP_HIDE_FROM_ABI bool isnan(_A1 __x) _NOEXCEPT {
   return __builtin_isnan(__x);
@@ -417,13 +447,8 @@
 inline _LIBCPP_HIDE_FROM_ABI bool isnan(long double __x) _NOEXCEPT { return __builtin_isnan(__x); }
 #      endif
 
-#    endif // isnan
-
 // isnormal
 
-#    ifdef isnormal
-#      undef isnormal
-
 template <class _A1, std::__enable_if_t<std::is_floating_point<_A1>::value, int> = 0>
 inline _LIBCPP_HIDE_FROM_ABI bool isnormal(_A1 __x) _NOEXCEPT {
   return __builtin_isnormal(__x);
@@ -434,13 +459,8 @@
   return __x != 0;
 }
 
-#    endif // isnormal
-
 // isgreater
 
-#    ifdef isgreater
-#      undef isgreater
-
 template <class _A1,
           class _A2,
           std::__enable_if_t<std::is_arithmetic<_A1>::value && std::is_arithmetic<_A2>::value, int> = 0>
@@ -449,13 +469,8 @@
   return __builtin_isgreater((type)__x, (type)__y);
 }
 
-#    endif // isgreater
-
 // isgreaterequal
 
-#    ifdef isgreaterequal
-#      undef isgreaterequal
-
 template <class _A1,
           class _A2,
           std::__enable_if_t<std::is_arithmetic<_A1>::value && std::is_arithmetic<_A2>::value, int> = 0>
@@ -464,13 +479,8 @@
   return __builtin_isgreaterequal((type)__x, (type)__y);
 }
 
-#    endif // isgreaterequal
-
 // isless
 
-#    ifdef isless
-#      undef isless
-
 template <class _A1,
           class _A2,
           std::__enable_if_t<std::is_arithmetic<_A1>::value && std::is_arithmetic<_A2>::value, int> = 0>
@@ -479,13 +489,8 @@
   return __builtin_isless((type)__x, (type)__y);
 }
 
-#    endif // isless
-
 // islessequal
 
-#    ifdef islessequal
-#      undef islessequal
-
 template <class _A1,
           class _A2,
           std::__enable_if_t<std::is_arithmetic<_A1>::value && std::is_arithmetic<_A2>::value, int> = 0>
@@ -494,13 +499,8 @@
   return __builtin_islessequal((type)__x, (type)__y);
 }
 
-#    endif // islessequal
-
 // islessgreater
 
-#    ifdef islessgreater
-#      undef islessgreater
-
 template <class _A1,
           class _A2,
           std::__enable_if_t<std::is_arithmetic<_A1>::value && std::is_arithmetic<_A2>::value, int> = 0>
@@ -509,13 +509,8 @@
   return __builtin_islessgreater((type)__x, (type)__y);
 }
 
-#    endif // islessgreater
-
 // isunordered
 
-#    ifdef isunordered
-#      undef isunordered
-
 template <class _A1,
           class _A2,
           std::__enable_if_t<std::is_arithmetic<_A1>::value && std::is_arithmetic<_A2>::value, int> = 0>
@@ -524,7 +519,7 @@
   return __builtin_isunordered((type)__x, (type)__y);
 }
 
-#    endif // isunordered
+#endif // _LIBCPP_MSVCRT
 
 // abs
 //