std::tuple_size should be a struct

[llvmbot]

Bugzilla Link	39871
Resolution	FIXED
Resolved on	Jan 11, 2019 15:19
Version	unspecified
OS	All
Reporter	LLVM Bugzilla Contributor
CC	@mclow,@zygoloid

Extended Description

Currently std::tuple_size is declared as class tuple_size. According to the C++ standard, tuple_size is a struct (http://eel.is/c++draft/tuple.helper#lib:tuple_size).

This creates portability problems between libcxx and libstdc++, which correctly declares it as a struct. Currently using either will result in -Wmismatched-tags.

[llvmbot]

assigned to @mclow

[llvmbot]

Further research revealed this might've changed recently? The C++14 draft I have access to (N3936, Page 491) lists them as class , same as the C++11 draft (N3337, Page 473).

Since the only ABI difference will be on MSVC, and MSVC is using struct, I think it could be possible to change it to a struct. Which will possibly break programs compiled against an old version of libcxx on Windows, but will provide ABI compatibility with MSVC in the future.

[mclow]

[ Short answer, it's not libc++, it's the standard ]

In [utility.syn], we've got (this is all post-2017):

// 19.4.4, tuple-like access to pair
template class tuple_size;
template<size_t I, class T> class tuple_element;
template<class T1, class T2> struct tuple_size<pair<T1, T2>>;
template<size_t I, class T1, class T2> struct tuple_element<I, pair<T1, T2>>;

In [pair.astuple]:
template<class T1, class T2>
struct tuple_size<pair<T1, T2>> : integral_constant<size_t, 2> {};

In [tuple.syn]
// 19.5.3.6, tuple helper classes
template class tuple_size;
template class tuple_size;
template class tuple_size;
template class tuple_size;

template<class... Types> class tuple_size<tuple<Types...>>;

and in [tuple.helper]:

template struct tuple_size;
template<class... Types>
class tuple_size<tuple<Types...>> : public integral_constant<size_t, sizeof...(Types)> {};

In [array.syn]:
template class tuple_size;
template<size_t I, class T> class tuple_element;

template<class T, size_t N>
struct tuple_size<array<T, N>>;
template<size_t I, class T, size_t N>
struct tuple_element<I, array<T, N>>;

in [array.tuple]:
template<class T, size_t N>
struct tuple_size<array<T, N>> : integral_constant<size_t, N> { };
tuple_element<I, array<T, N>>::type;

I'll have to clean this up in the standard.

[llvmbot]

Ouch, alright, that's unfortunate.
Is libcxx actually defining std::tuple_size differently, in different places?

[mclow]

It appears that libc++ is consistent; it uses 'class' everywhere.

[mclow]

The standard isn't going to change; see cplusplus/draft#534

Committed revision 350972 to make tuple_size a struct everywhere.

[zygoloid]

As of r348233, Clang ignores declarations in system headers when deciding whether to issue -Wmismatched-tags, which should cover this more generally (but only for new versions of Clang, of course).