compilation error on clang-8 + C++17

[janwilmans]

json.h:19799:62: error: no type named 'string_view' in namespace 'std'
                   && !std::is_same<ValueType, typename std::string_view>::value

I think an include of <string_view> is missing ? This only occurs on debian-8 + clang-8 + C++17 here.

[nlohmann]

Which version are you using? The filename seems odd.

[janwilmans]

The file is just renamed, its this version, taken from the tag 3.9.1

    __ _____ _____ _____
 __|  |   __|     |   | |  JSON for Modern C++
|  |  |__   |  |  | | | |  version 3.9.1
|_____|_____|_____|_|___|  https://github.com/nlohmann/json

Licensed under the MIT License <http://opensource.org/licenses/MIT>.
SPDX-License-Identifier: MIT
Copyright (c) 2013-2019 Niels Lohmann <http://nlohmann.me>.

The reason why I got this problem it that I installed a binary distribution of clang-8 on debian-8, but apparently, if I do not explicitly specify -stdlib=libc++ it still uses the stock version of libstd++, which doesn't support C++17.

Long story short, I have a weird setup, but looking at the code, I didnt't see an include of <string_view>, I see that on head it is present (https://github.com/nlohmann/json/blob/develop/include/nlohmann/json.hpp#L77)