compile error with clang 5.0 -std=c++1z and no string_view

[phlptp]

Bug Report

Compile error on clang 5.0 with -std=c++1z

In file included from XXXX.cpp:32:
XXXX/json.hpp:9913:66: error: no type named
      'string_view' in namespace 'std'
  ...and not std::is_same<ValueType, typename std::string_view>::value
                                     ~~~~~~~~~~~~~~^~~~~~~~~~~

All other compilers tested thus far work fine.

my fix was to simply comment out that line since I wasn't using std::string_view, but but I am guessing there is a reason for including string_view in the enable_if statement so I don't think my fix is a valid solution and I am not familiar enough with the structures used to integrate a check for the existence of string_view before using it in an is_same template.

The other options was to include <string_view> if JSON_HAS_CPP_17 is enabled but that didn't seem proper either.

[nlohmann]

Related: #735.

[nlohmann]

We do not include <string_view>, but assume that it is included by <string> (see #735 (comment)). I cannot reproduce the issue, because Travis successfully runs a job with Clang 5 and -std=c++1z.

Please have a look at #735 - in particular, if you can include <string_view>.

[phlptp]

I am building with libc++ not libstdc++, on ubuntu 17.10. In the code I am using it in I use a string_view library that is not from the stl since it needed to work with string_view objects without requiring c++1z capability on all compilers so it is in a different namespace, I presume that would cause some interesting issues as well but the uses don't overlap in the code. I think I had the master branch in the code, but develop seems to fail in the same way. I suppose I should check the compile with libstdc++ and see if it fails in the same way.

[phlptp]

it compiles properly using libstdc++ and everything else the same, so using libc++ would appear to be the issue.

[nlohmann]

Thanks for the feedback. Does <string_view> work with libc++?

[phlptp]

No it doesn't, looks like the libc++ used in Ubuntu 17.10 is version 3.9 and string_view was not integrated until 4.0, but the compiler itself has c++17 support and the 3.9 library has partial support.

[nlohmann]

Thanks. Then this seems to be a duplicate of #735.