-Wmismatched-tags in 3.5.0?

[past-due]

While I'm aware of the workarounds, I thought you might find the following interesting.

• What is the issue you have?

-Wmismatched-tags using Clang v6.0 on Ubuntu 18.04:

json.hpp:1799:1: error: 'tuple_size' defined as a class template here but previously declared as a struct template [-Werror,-Wmismatched-tags]
class tuple_size<::nlohmann::detail::iteration_proxy_value<IteratorType>>
^
/usr/include/c++/7.3.0/utility:84:5: note: did you mean class here?
    struct tuple_size;
    ^
[...]
json.hpp:1803:1: error: 'tuple_element' defined as a class template here but previously declared as a struct template [-Werror,-Wmismatched-tags]
class tuple_element<N, ::nlohmann::detail::iteration_proxy_value<IteratorType >>
^
/usr/include/c++/7.3.0/utility:129:5: note: did you mean class here?
    struct tuple_element;
    ^
2 errors generated.

• Please describe the steps to reproduce the issue. Can you provide a small but working code example?

Attempted to update Warzone 2100 from nlohmann/json 3.4.0 to 3.5.0.
(Compilation succeeds with 3.4.0. CI treats many warnings as errors. -std=gnu++11)

• What is the expected behavior?

Compilation succeeds without error / warnings.

• And what is the actual behavior instead?

[-Wmismatched-tags] is triggered

• Which compiler and operating system are you using? Is it a supported compiler?

The CXX compiler identification is Clang 6.0.0
on Ubuntu 18.04

• Did you use a released version of the library or the version from the develop branch?

3.5.0 release

[Naios]

I'm encountering the same warning, note that some standard libraries are defining std::tuple_size and std::tuple_element wrongly as class whereas struct would be right.

In a library of mine, I came up with the following workarround to suppress the warnings on my own std::tuple_size and std::tuple_element specialization, since one can't do much if some standard library versions define it differently:

namespace std {
// The GCC standard library defines tuple_size as class and struct
// which triggers a warning here.
#if defined(__clang__)
#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wmismatched-tags"
#endif
template <typename... Args>
struct tuple_size<cti::result<Args...>>
    : std::integral_constant<size_t, sizeof...(Args)> {};

template <std::size_t I, typename... Args>
struct tuple_element<I, cti::result<Args...>>
    : tuple_element<I, tuple<Args...>> {};
#if defined(__clang__)
#pragma GCC diagnostic pop
#endif
} // namespace std

[past-due]

An additional reference on this issue:

• struct vs. class when specialising tuple_element/tuple_size

[nlohmann]

Thanks for reporting. I had a warning before I released and adjusted the code wrt. to the examples from cppreference.com. Now, I do not get any warning with GCC 9 and Clang 8.

[past-due]

It looks like this might be fixed in Clang 8, but Clang 7 may also trigger it. (Was playing around with a small example linked in the isocpp group post above.)

@nlohmann: Would disabling the -Wmismatched-tags warning for Clang around the tuple_size and tuple_element specializations (similar to what @Naios suggested) be an acceptable PR?

[nlohmann]

@past-due Yes, of course are PRs welcome!

[theodelrieu]

Note that the C++ standard declares tuple_size and tuple_element as classes, but defines them as structs, see paragraph 19.4.4 of the current draft.

I'm also for explicitly disabling those warnings.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[past-due]

Please don't close, stale bot.