[libc++] Only include_next C library headers when they exist

Some platforms don't provide all C library headers. In practice, libc++
only requires a few C library headers to exist, and only a few functions
on those headers. Missing functions that libc++ doesn't need for its own
implementation are handled properly by the using_if_exists attribute,
however a missing header is currently a hard error when we try to
do #include_next.

This patch should make libc++ more flexible on platforms that do not
provide C headers that libc++ doesn't actually require for its own
implementation. The only downside is that it may move some errors from
the #include_next point to later in the compilation if we actually try
to use something that isn't provided, which could be somewhat confusing.
However, these errors should be caught by folks trying to port libc++
over to a new platform (when running the libc++ test suite), not by end
users.

Differential Revision: https://reviews.llvm.org/D136683

18 files changed, 60 insertions, 32 deletions

diff --git a/libcxx/include/complex.h b/libcxx/include/complex.h
index a2814669c7d4..a3da21c843f3 100644
--- 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
index ba0925069c64..728173ed670f 100644
--- a/libcxx/include/ctype.h
+++ b/libcxx/include/ctype.h
@@ -35,7 +35,9 @@ int toupper(int c);
 #  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
index ea0559f0f5d1..7b02d2b47953 100644
--- a/libcxx/include/errno.h
+++ b/libcxx/include/errno.h
@@ -28,7 +28,9 @@ Macros:
 #  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
index a9ba6800245b..15e41568cbf7 100644
--- a/libcxx/include/fenv.h
+++ b/libcxx/include/fenv.h
@@ -56,7 +56,9 @@ int feupdateenv(const fenv_t* envp);
 #  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
index 606081251dfb..9e5f711fab83 100644
--- a/libcxx/include/float.h
+++ b/libcxx/include/float.h
@@ -76,7 +76,9 @@ Macros:
 #  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
index e0fd71f9b65d..b755526b8492 100644
--- a/libcxx/include/inttypes.h
+++ b/libcxx/include/inttypes.h
@@ -248,7 +248,9 @@ uintmax_t wcstoumax(const wchar_t* restrict nptr, wchar_t** restrict endptr, int
 #   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
index 3e1e85a8a0b4..032a90967d0c 100644
--- a/libcxx/include/limits.h
+++ b/libcxx/include/limits.h
@@ -44,7 +44,11 @@ Macros:
 #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
index 17c0a705a009..3fb812035a52 100644
--- a/libcxx/include/locale.h
+++ b/libcxx/include/locale.h
@@ -43,6 +43,8 @@ Functions:
 #  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
index d5a7095a596a..d60943086036 100644
--- a/libcxx/include/math.h
+++ b/libcxx/include/math.h
@@ -297,7 +297,9 @@ long double    truncl(long double x);
 #  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
index de4f9edf4886..f4a2bbcb0bd3 100644
--- a/libcxx/include/setjmp.h
+++ b/libcxx/include/setjmp.h
@@ -31,7 +31,9 @@ void longjmp(jmp_buf env, int val);
 #  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
index 0bc1aa8304ad..5bba00568dff 100644
--- a/libcxx/include/stdbool.h
+++ b/libcxx/include/stdbool.h
@@ -24,7 +24,9 @@ Macros:
 #  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
index 19e344f2a000..f1725db06212 100644
--- a/libcxx/include/stddef.h
+++ b/libcxx/include/stddef.h
@@ -42,7 +42,9 @@ Types:
 #  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
index ee71f62ab095..fa7b011c71ae 100644
--- a/libcxx/include/stdint.h
+++ b/libcxx/include/stdint.h
@@ -120,6 +120,8 @@ Macros:
 #   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
index ad1b4c05f1fa..cea43aa7680b 100644
--- a/libcxx/include/stdio.h
+++ b/libcxx/include/stdio.h
@@ -104,7 +104,9 @@ void perror(const char* s);
 #  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
index e4dce9c54546..64581b67f245 100644
--- a/libcxx/include/stdlib.h
+++ b/libcxx/include/stdlib.h
@@ -90,7 +90,9 @@ void *aligned_alloc(size_t alignment, size_t size);                       // C11
 #  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
index 082c632a29bc..627cbae781c1 100644
--- a/libcxx/include/string.h
+++ b/libcxx/include/string.h
@@ -57,7 +57,9 @@ size_t strlen(const char* s);
 #  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
index c65091708a9a..e6f0a4ab2611 100644
--- 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
index 0fba53b268ae..c684508dc2cc 100644
--- a/libcxx/include/wchar.h
+++ b/libcxx/include/wchar.h
@@ -120,7 +120,9 @@ size_t wcsrtombs(char* restrict dst, const wchar_t** restrict src, size_t len,
 #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_)