[llvm-dev] Compiling LLVM with GCC 5/6/7 and -std=c++1z fails due to use of std::char_traits constexpr

[Dan Bailey via llvm-dev]

Hi,

When trying to compile LLVM using GCC 5.1 => GCC 7.2 (inclusive) with
-std=c++1z, using StringRef errors due to the lack of  a constexpr
std::char_traits<char>::length():

.../include/llvm/ADT/StringRef.h:85:44: *error*: call to non-constexpr
function ‘static std::size_t std::char_traits<char>::length(const
char_type*)’
       return std::char_traits<char>::length(Str);
              ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~

Looking into the issue, StringRef.h uses this logic to gate against using
constexpr char_traits:

#if *__cplusplus* > 201402L
      return std::char_traits<char>::length(Str);
#elif ...

However for the GCC version range mentioned above, the -std=c++1z flag
generates __cplusplus 201500L or higher despite char_traits not being
constexpr.

I think the correct thing to do here might be to use the feature macro
instead:

#if *__cpp_lib_constexpr_char_traits*
      return std::char_traits<char>::length(Str);
#elif ...

Any thoughts?

Thanks,
-- 

Dan Bailey        | R&D               | ILM Vancouver
danbailey at ilm.com | 415 746-XXXX Desk | 778 882-XXXX Cell
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