libc++ `char_traits.h` assumes `EOF` is always available

[petrhosek]

libc++ assumes that EOF is always available and uses it unconditionally in the char_traits.h implementation.

In file included from /llvm_libcxx/include/chrono:830:                                                                                                                                                                                                                                                                                                                                 
In file included from /llvm_libcxx/include/__chrono/convert_to_tm.h:22:                                                                                                                                                                                                                                                                                                                
In file included from /llvm_libcxx/include/__chrono/statically_widen.h:17:                                                                                                                                                                                                                                                                                                             
In file included from /llvm_libcxx/include/__format/concepts.h:17:                                                                                                                                                                                                                                                                                                                     
In file included from /llvm_libcxx/include/__format/format_parse_context.h:16:                                                                                                                                                                                                                                                                                                         
In file included from /llvm_libcxx/include/string_view:225:                                                                                                                                                                                                                                                                                                                            
/llvm_libcxx/include/__string/char_traits.h:169:100: error: use of undeclared identifier 'EOF'                                                                                                                                                                                                                                                                                         
  169 |   static inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR int_type eof() _NOEXCEPT { return int_type(EOF); }                                                                                                                                                                                                                                                                                                                        
      |                                                                                                    ^                                                                                                                                                                                                                                                                                                                              
/llvm_libcxx/include/__string/char_traits.h:169:66: error: no return statement in constexpr function                                                                                                                                                                                                                                                                                   
  169 |   static inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR int_type eof() _NOEXCEPT { return int_type(EOF); }

LLVM libc doesn't currently define EOF on baremetal targets because there's no filesystem support (since baremetal targets typically don't have filesystem to begin with) which results in a compiler error.

We could extend LLVM libc to define EOF even on baremetal targets, but it's an open question whether libc++ should be able to handle the case where EOF is undefined.

This is related to issue #84879.

[philnik777]

I can't really think of a good reason a libc shouldn't provide it. Given that the main thing this might lead to are ODR violations, since libc++ would have to provide it's own if it can't find the libc's, IMO the libc should just provide it. It's not much harder than adding #define EOF (-1) to a header.

[ldionne]

I've come across this issue in a different setting and I had the same conclusion -- libc should provide it.

Basically, EOF is just a really unfortunately-named macro, but it is technically not tied specifically to input and output. It's literally just a macro defined to some integer, and that should be defined even when the underlying system has no notion of I/O or files. It feels really weird to have a header named <stdio.h> in a freestanding C library and to provide a macro named EOF but not any other IO related functionality, but I think that's the right thing to do.

[frederick-vs-ja]

I suspect that there should be an LWG issue - currently std::char_traits<char>::eof is required to be freestanding due to P2338R4, but it's also required to return the value of EOF which is not yet freestanding.

[jwakely]

The eof, not_eof, to_int_type and to_char_type members of char_traits are only needed by iostreams, so I see no reason for them to be present in freestanding.

In libstdc++ we make eof and not_eof depend on hosted, so they're absent for freestanding.