diff --git a/libcxx/include/math.h b/libcxx/include/math.h
index 4e6304a753984..2695be9a497bc 100644
--- a/libcxx/include/math.h
+++ b/libcxx/include/math.h
@@ -307,53 +307,79 @@ long double    truncl(long double x);
 // back to C++ linkage before including these C++ headers.
 extern "C++" {
 
-#    ifdef fpclassify
-#      undef fpclassify
-#    endif
-
-#    ifdef signbit
-#      undef signbit
-#    endif
-
-#    ifdef isfinite
-#      undef isfinite
-#    endif
-
-#    ifdef isinf
-#      undef isinf
-#    endif
-
-#    ifdef isnan
-#      undef isnan
-#    endif
-
-#    ifdef isnormal
-#      undef isnormal
-#    endif
-
-#    ifdef isgreater
-#      undef isgreater
-#    endif
-
-#    ifdef isgreaterequal
-#      undef isgreaterequal
-#    endif
-
-#    ifdef isless
-#      undef isless
-#    endif
-
-#    ifdef islessequal
-#      undef islessequal
-#    endif
-
-#    ifdef islessgreater
-#      undef islessgreater
-#    endif
-
-#    ifdef isunordered
-#      undef isunordered
-#    endif
+// According to section 7.1.4 Use of library functions of the C standard, any
+// function declared in a header may be additionally implemented as a
+// function-like macro defined in the header, so if a library function is
+// declared explicitly when the C standard library header is included, as is
+// the case here, we need to use #undef to remove any macro definition.
+#    undef acos
+#    undef acosh
+#    undef asin
+#    undef asinh
+#    undef atan
+#    undef atan2
+#    undef atanh
+#    undef cbrt
+#    undef ceil
+#    undef copysign
+#    undef cos
+#    undef cosh
+#    undef erf
+#    undef erfc
+#    undef exp
+#    undef exp2
+#    undef expm1
+#    undef fabs
+#    undef fdim
+#    undef floor
+#    undef fma
+#    undef fmax
+#    undef fmin
+#    undef fmod
+#    undef fpclassify
+#    undef frexp
+#    undef hypot
+#    undef ilogb
+#    undef isfinite
+#    undef isgreater
+#    undef isgreaterequal
+#    undef isinf
+#    undef isless
+#    undef islessequal
+#    undef islessgreater
+#    undef isnan
+#    undef isnormal
+#    undef isunordered
+#    undef ldexp
+#    undef lgamma
+#    undef llrint
+#    undef llround
+#    undef log
+#    undef log10
+#    undef log1p
+#    undef log2
+#    undef logb
+#    undef lrint
+#    undef lround
+#    undef modf
+#    undef nearbyint
+#    undef nextafter
+#    undef nexttoward
+#    undef pow
+#    undef remainder
+#    undef remquo
+#    undef rint
+#    undef round
+#    undef scalbln
+#    undef scalbn
+#    undef signbit
+#    undef sin
+#    undef sinh
+#    undef sqrt
+#    undef tan
+#    undef tanh
+#    undef tgamma
+#    undef trunc
 
 #    include <__math/abs.h>
 #    include <__math/copysign.h>
diff --git a/libcxx/include/stdlib.h b/libcxx/include/stdlib.h
index a74344d49150c..b1151b58a356f 100644
--- a/libcxx/include/stdlib.h
+++ b/libcxx/include/stdlib.h
@@ -98,15 +98,9 @@ void *aligned_alloc(size_t alignment, size_t size);                       // C11
 extern "C++" {
 // abs
 
-#    ifdef abs
-#      undef abs
-#    endif
-#    ifdef labs
-#      undef labs
-#    endif
-#    ifdef llabs
-#      undef llabs
-#    endif
+#    undef abs
+#    undef labs
+#    undef llabs
 
 // MSVCRT already has the correct prototype in <stdlib.h> if __cplusplus is defined
 #    if !defined(_LIBCPP_MSVCRT)
@@ -128,15 +122,9 @@ _LIBCPP_NODISCARD inline _LIBCPP_HIDE_FROM_ABI long double abs(long double __lcp
 
 // div
 
-#    ifdef div
-#      undef div
-#    endif
-#    ifdef ldiv
-#      undef ldiv
-#    endif
-#    ifdef lldiv
-#      undef lldiv
-#    endif
+#    undef div
+#    undef ldiv
+#    undef lldiv
 
 // MSVCRT already has the correct prototype in <stdlib.h> if __cplusplus is defined
 #    if !defined(_LIBCPP_MSVCRT)