Can a C++ standard library header include a C standard header?

Question

I can only find in the standard draft N4582 that

[res.on.headers/1] A C++ header may include other C++ headers.

It seems not to specify that whether a C++ header can include a C standard header.

If it is permitted, is it unsafe to use a global name defined in a C standard header even if this header is not included (since the program may implicitly include the header through some C++ standard header)?

Answer 1

For the purpose of the question you follow up with (name conflicts), yes, it certainly can. The reason is that the C++ standard library includes the <c:::> C++ headers for the C standard library contents, and the standard explicitly allows these to provide the names in the global namespace as well (in addition to mandatorily providing them in ::std).

Further, based on Annex D [depr] of the C++ standard, the C standard library headers (the <:::.h> versions) are also part of the C++ standard library (albeit deprecated). This means [res.on.headers]/1 allows C++ headers to include them.