diff --git a/libcxx/include/complex.h b/libcxx/include/complex.h
--- a/libcxx/include/complex.h
+++ b/libcxx/include/complex.h
@@ -24,13 +24,9 @@
 #endif
 
 #ifdef __cplusplus
-
-#include <ccomplex>
-
-#else  // __cplusplus
-
-#include_next <complex.h>
-
-#endif // __cplusplus
+#  include <ccomplex>
+#elif __has_include_next(<complex.h>)
+#  include_next <complex.h>
+#endif
 
 #endif // _LIBCPP_COMPLEX_H
diff --git a/libcxx/include/ctype.h b/libcxx/include/ctype.h
--- a/libcxx/include/ctype.h
+++ b/libcxx/include/ctype.h
@@ -35,7 +35,9 @@
 #  pragma GCC system_header
 #endif
 
-#include_next <ctype.h>
+#if __has_include_next(<ctype.h>)
+#  include_next <ctype.h>
+#endif
 
 #ifdef __cplusplus
 
diff --git a/libcxx/include/errno.h b/libcxx/include/errno.h
--- a/libcxx/include/errno.h
+++ b/libcxx/include/errno.h
@@ -28,7 +28,9 @@
 #  pragma GCC system_header
 #endif
 
-#include_next <errno.h>
+#if __has_include_next(<errno.h>)
+#  include_next <errno.h>
+#endif
 
 #ifdef __cplusplus
 
diff --git a/libcxx/include/fenv.h b/libcxx/include/fenv.h
--- a/libcxx/include/fenv.h
+++ b/libcxx/include/fenv.h
@@ -56,7 +56,9 @@
 #  pragma GCC system_header
 #endif
 
-#include_next <fenv.h>
+#if __has_include_next(<fenv.h>)
+#  include_next <fenv.h>
+#endif
 
 #ifdef __cplusplus
 
diff --git a/libcxx/include/float.h b/libcxx/include/float.h
--- a/libcxx/include/float.h
+++ b/libcxx/include/float.h
@@ -76,7 +76,9 @@
 #  pragma GCC system_header
 #endif
 
-#include_next <float.h>
+#if __has_include_next(<float.h>)
+#  include_next <float.h>
+#endif
 
 #ifdef __cplusplus
 
diff --git a/libcxx/include/inttypes.h b/libcxx/include/inttypes.h
--- a/libcxx/include/inttypes.h
+++ b/libcxx/include/inttypes.h
@@ -248,7 +248,9 @@
 #   define __STDC_FORMAT_MACROS
 #endif
 
-#include_next <inttypes.h>
+#if __has_include_next(<inttypes.h>)
+#  include_next <inttypes.h>
+#endif
 
 #ifdef __cplusplus
 
diff --git a/libcxx/include/limits.h b/libcxx/include/limits.h
--- a/libcxx/include/limits.h
+++ b/libcxx/include/limits.h
@@ -44,7 +44,11 @@
 #endif
 
 #ifndef __GNUC__
-#include_next <limits.h>
+
+#  if __has_include_next(<limits.h>)
+#    include_next <limits.h>
+#  endif
+
 #else
 // GCC header limits.h recursively includes itself through another header called
 // syslimits.h for some reason. This setup breaks down if we directly
diff --git a/libcxx/include/locale.h b/libcxx/include/locale.h
--- a/libcxx/include/locale.h
+++ b/libcxx/include/locale.h
@@ -43,6 +43,8 @@
 #  pragma GCC system_header
 #endif
 
-#include_next <locale.h>
+#if __has_include_next(<locale.h>)
+#  include_next <locale.h>
+#endif
 
 #endif // _LIBCPP_LOCALE_H
diff --git a/libcxx/include/math.h b/libcxx/include/math.h
--- a/libcxx/include/math.h
+++ b/libcxx/include/math.h
@@ -297,7 +297,9 @@
 #  pragma GCC system_header
 #endif
 
-#include_next <math.h>
+#  if __has_include_next(<math.h>)
+#    include_next <math.h>
+#  endif
 
 #ifdef __cplusplus
 
diff --git a/libcxx/include/setjmp.h b/libcxx/include/setjmp.h
--- a/libcxx/include/setjmp.h
+++ b/libcxx/include/setjmp.h
@@ -31,7 +31,9 @@
 #  pragma GCC system_header
 #endif
 
-#include_next <setjmp.h>
+#if __has_include_next(<setjmp.h>)
+#  include_next <setjmp.h>
+#endif
 
 #ifdef __cplusplus
 
diff --git a/libcxx/include/stdbool.h b/libcxx/include/stdbool.h
--- a/libcxx/include/stdbool.h
+++ b/libcxx/include/stdbool.h
@@ -24,7 +24,9 @@
 #  pragma GCC system_header
 #endif
 
-#include_next <stdbool.h>
+#if __has_include_next(<stdbool.h>)
+#  include_next <stdbool.h>
+#endif
 
 #ifdef __cplusplus
 #undef bool
diff --git a/libcxx/include/stddef.h b/libcxx/include/stddef.h
--- a/libcxx/include/stddef.h
+++ b/libcxx/include/stddef.h
@@ -42,7 +42,9 @@
 #  pragma GCC system_header
 #endif
 
-#include_next <stddef.h>
+#  if __has_include_next(<stddef.h>)
+#    include_next <stddef.h>
+#  endif
 
 #ifdef __cplusplus
     typedef decltype(nullptr) nullptr_t;
diff --git a/libcxx/include/stdint.h b/libcxx/include/stdint.h
--- a/libcxx/include/stdint.h
+++ b/libcxx/include/stdint.h
@@ -120,6 +120,8 @@
 #   define __STDC_CONSTANT_MACROS
 #endif
 
-#include_next <stdint.h>
+#if __has_include_next(<stdint.h>)
+#  include_next <stdint.h>
+#endif
 
 #endif // _LIBCPP_STDINT_H
diff --git a/libcxx/include/stdio.h b/libcxx/include/stdio.h
--- a/libcxx/include/stdio.h
+++ b/libcxx/include/stdio.h
@@ -104,7 +104,9 @@
 #  pragma GCC system_header
 #endif
 
-#include_next <stdio.h>
+#  if __has_include_next(<stdio.h>)
+#    include_next <stdio.h>
+#  endif
 
 #ifdef __cplusplus
 
diff --git a/libcxx/include/stdlib.h b/libcxx/include/stdlib.h
--- a/libcxx/include/stdlib.h
+++ b/libcxx/include/stdlib.h
@@ -90,7 +90,9 @@
 #  pragma GCC system_header
 #endif
 
-#include_next <stdlib.h>
+#  if __has_include_next(<stdlib.h>)
+#    include_next <stdlib.h>
+#  endif
 
 #ifdef __cplusplus
 extern "C++" {
diff --git a/libcxx/include/string.h b/libcxx/include/string.h
--- a/libcxx/include/string.h
+++ b/libcxx/include/string.h
@@ -57,7 +57,9 @@
 #  pragma GCC system_header
 #endif
 
-#include_next <string.h>
+#if __has_include_next(<string.h>)
+#  include_next <string.h>
+#endif
 
 // MSVCRT, GNU libc and its derivates may already have the correct prototype in
 // <string.h>. This macro can be defined by users if their C library provides
diff --git a/libcxx/include/tgmath.h b/libcxx/include/tgmath.h
--- a/libcxx/include/tgmath.h
+++ b/libcxx/include/tgmath.h
@@ -24,13 +24,11 @@
 #endif
 
 #ifdef __cplusplus
-
-#include <ctgmath>
-
-#else  // __cplusplus
-
-#include_next <tgmath.h>
-
-#endif // __cplusplus
+#  include <ctgmath>
+#else
+#  if __has_include_next(<tgmath.h>)
+#    include_next <tgmath.h>
+#  endif
+#endif
 
 #endif // _LIBCPP_TGMATH_H
diff --git a/libcxx/include/wchar.h b/libcxx/include/wchar.h
--- a/libcxx/include/wchar.h
+++ b/libcxx/include/wchar.h
@@ -120,7 +120,9 @@
 #define __CORRECT_ISO_CPP_WCHAR_H_PROTO
 #endif
 
-#include_next <wchar.h>
+#  if __has_include_next(<wchar.h>)
+#    include_next <wchar.h>
+#  endif
 
 // Determine whether we have const-correct overloads for wcschr and friends.
 #if defined(_WCHAR_H_CPLUSPLUS_98_CONFORMANCE_)