[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
NOKEYCHECK=True
GitOrigin-RevId: 226409c62879bf5ff9928cd23a4255cd7c614fe0
diff --git a/include/stdlib.h b/include/stdlib.h
index e4dce9c..64581b6 100644
--- a/include/stdlib.h
+++ b/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++" {