Compilation fails with clang 6.x-8.x in C++14 mode

[mmoelle1]

Problem

A fresh checkout of the develop branch of the library fails to build with Clang 6.0.1, 7.0.1, and 8.0.0 under CentOS 7 when compiled in C++14 mode with the following error:

json/single_include/nlohmann/json.hpp:13192:38: error: too few template arguments for class template 'less'
    using object_comparator_t = std::less<>;
                                     ^
/usr/lib/gcc/x86_64-redhat-linux/4.8.5/../../../../include/c++/4.8.5/bits/stl_function.h:231:12: note: template is declared here
    struct less : public binary_function<_Tp, _Tp, bool>
           ^
In file included from /home/mmoller/foo/json/test/src/unit-wstring.cpp:32:
/home/mmoller/foo/json/single_include/nlohmann/json.hpp:13282:11: error: use of undeclared identifier 'object_comparator_t'
          object_comparator_t,
          ^
/home/mmoller/foo/json/single_include/nlohmann/json.hpp:13674:9: error: unknown type name 'object_t'
        object_t* object;
        ^
/home/mmoller/foo/json/single_include/nlohmann/json.hpp:13776:26: error: unknown type name 'object_t'
        json_value(const object_t& value)
                         ^
/home/mmoller/foo/json/single_include/nlohmann/json.hpp:13782:20: error: unknown type name 'object_t'
        json_value(object_t&& value)
                   ^
/home/mmoller/foo/json/single_include/nlohmann/json.hpp:15181:5: error: unknown type name 'object_t'
    object_t* get_impl_ptr(object_t* /*unused*/) noexcept
    ^
/home/mmoller/foo/json/single_include/nlohmann/json.hpp:15181:28: error: unknown type name 'object_t'
    object_t* get_impl_ptr(object_t* /*unused*/) noexcept
                           ^
/home/mmoller/foo/json/single_include/nlohmann/json.hpp:15187:21: error: unknown type name 'object_t'
    constexpr const object_t* get_impl_ptr(const object_t* /*unused*/) const noexcept
                    ^
/home/mmoller/foo/json/single_include/nlohmann/json.hpp:15187:50: error: unknown type name 'object_t'
    constexpr const object_t* get_impl_ptr(const object_t* /*unused*/) const noexcept
                                                 ^
/home/mmoller/foo/json/single_include/nlohmann/json.hpp:15827:33: error: use of undeclared identifier 'object_t'
    reference at(const typename object_t::key_type& key)
                                ^
/home/mmoller/foo/json/single_include/nlohmann/json.hpp:15827:43: error: expected a qualified name after 'typename'
    reference at(const typename object_t::key_type& key)
                                          ^
/home/mmoller/foo/json/single_include/nlohmann/json.hpp:15827:51: error: expected ')'
    reference at(const typename object_t::key_type& key)
                                                  ^
/home/mmoller/foo/json/single_include/nlohmann/json.hpp:15827:17: note: to match this '('
    reference at(const typename object_t::key_type& key)
                ^
/home/mmoller/foo/json/single_include/nlohmann/json.hpp:15878:39: error: use of undeclared identifier 'object_t'
    const_reference at(const typename object_t::key_type& key) const
                                      ^
/home/mmoller/foo/json/single_include/nlohmann/json.hpp:15878:49: error: expected a qualified name after 'typename'
    const_reference at(const typename object_t::key_type& key) const
                                                ^
/home/mmoller/foo/json/single_include/nlohmann/json.hpp:15878:57: error: expected ')'
    const_reference at(const typename object_t::key_type& key) const
                                                        ^
/home/mmoller/foo/json/single_include/nlohmann/json.hpp:15878:23: note: to match this '('
    const_reference at(const typename object_t::key_type& key) const
                      ^
/home/mmoller/foo/json/single_include/nlohmann/json.hpp:16008:41: error: use of undeclared identifier 'object_t'
    reference operator[](const typename object_t::key_type& key)
                                        ^
/home/mmoller/foo/json/single_include/nlohmann/json.hpp:16008:51: error: expected a qualified name after 'typename'
    reference operator[](const typename object_t::key_type& key)
                                                  ^
/home/mmoller/foo/json/single_include/nlohmann/json.hpp:16008:59: error: expected ')'
    reference operator[](const typename object_t::key_type& key)
                                                          ^
/home/mmoller/foo/json/single_include/nlohmann/json.hpp:16008:25: note: to match this '('
    reference operator[](const typename object_t::key_type& key)
                        ^
/home/mmoller/foo/json/single_include/nlohmann/json.hpp:16057:47: error: use of undeclared identifier 'object_t'
    const_reference operator[](const typename object_t::key_type& key) const

Steps to reproduce:

$> CC=`which clang` CXX=`which clang++` cmake .. -DCMAKE_CXX_STANDARD=14
$> make

Remarks:

• The same error occurs when using the master branch.
• Compiling the same checkout GCC (tested with 4.8.5 and above) works fine.

[nlohmann]

I cannot reproduce this - at least with Clang 7.0.0 on Ubuntu, I can execute the exact commands you provided without error.

[mmoelle1]

The problem seems to be due to the fact that I compiled clang from the sources on my CentOS machine, whereby GCC 4.8.5 was used, which does not support C++14 to full extent. A properly compiled clang works without problems. Thanks for the quick help. Am 08.06.2019 um 18:55 schrieb Niels Lohmann <notifications@github.com<mailto:notifications@github.com>>: I cannot reproduce this - at least with Clang 7.0.0 on Ubuntu, I can execute the exact commands you provided without error. — You are receiving this because you authored the thread. Reply to this email directly, view it on GitHub<#1628?email_source=notifications&email_token=AAPGPIZOZZK3FBUA3CSU3LTPZPQAHA5CNFSM4HWHDPT2YY3PNVWWK3TUL52HS4DFVREXG43VMVBW63LNMVXHJKTDN5WW2ZLOORPWSZGODXHYSTI#issuecomment-500140365>, or mute the thread<https://github.com/notifications/unsubscribe-auth/AAPGPI3CUIKVCHG62JMN6BLPZPQAHANCNFSM4HWHDPTQ>.