Change `tuple_size` from a `class` to a `struct` (see PR#39871)

[mclow.lists]

Summary

In the standard, it's a mix of things.
libc++ currently consistently implements this as a class.
I talked to the editors, and Jonathan believes that this can be fixed editorially.
I'm assuming that it will end up as a struct, and so here's a patch to implement that.

This will affect people who have specialized std::tuple_size for their own uses, and compile with -Wmismatched-tags.
It will also affect people who use libc++ with MS compilers, where the ABI is different between struct and class

I don't recommend landing this until the change goes into the standard - which will probably be a week or so.
But I was in the neighborhood, and I figured I'd write this up.

[EricWF]

LGTM.

Consider adding a *.fail.cpp test that declares a specialization as a class and checks for

// expected-error {{tuple_size' defined as a class template here but previously declared as a struct template}}

[ldionne]

Oh gosh I've been wondering forever why we defined them as classes in the standard. Glad to see we're fixing that.

[mclow.lists]

I'm waiting for the (editorial) change to go into the standard; once that happens, I will commit this.

[EricWF]

Should we commit this before the next release? Maybe ping Wakly to get the editorial fix landed?

[mclow.lists]

In D55466#1347491, @EricWF wrote:

Should we commit this before the next release? Maybe ping Wakly to get the editorial fix landed?

I will ping Jonathan about this today.

[mclow.lists]

Oh, look, this is https://github.com/cplusplus/draft/issues/534, where JW wrote:

Compilers do not compile the standard, they compile headers that come with their implementation. The implementation headers can use whatever class-keys they want to, they don't have to match the standard, and changing the standard doesn't mean implementations will change.

I say commit this.

[mclow.lists]

Committed as revision 350972.

include/__tuple

 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
-template <class _Tp> class _LIBCPP_TEMPLATE_VIS tuple_size;
+template <class _Tp> struct _LIBCPP_TEMPLATE_VIS tuple_size;
 
 #if !defined(_LIBCPP_CXX03_LANG)
 template <class _Tp, class...>

 using __enable_if_tuple_size_imp = _Tp;
 
 template <class _Tp>
-class _LIBCPP_TEMPLATE_VIS tuple_size<__enable_if_tuple_size_imp<
+struct _LIBCPP_TEMPLATE_VIS tuple_size<__enable_if_tuple_size_imp<
     const _Tp,
     typename enable_if<!is_volatile<_Tp>::value>::type,
     integral_constant<size_t, sizeof(tuple_size<_Tp>)>>>

     : public integral_constant<size_t, tuple_size<_Tp>::value> {};
 
 template <class _Tp>
-class _LIBCPP_TEMPLATE_VIS tuple_size<__enable_if_tuple_size_imp<
+struct _LIBCPP_TEMPLATE_VIS tuple_size<__enable_if_tuple_size_imp<
     volatile _Tp,
     typename enable_if<!is_const<_Tp>::value>::type,
     integral_constant<size_t, sizeof(tuple_size<_Tp>)>>>

     : public integral_constant<size_t, tuple_size<_Tp>::value> {};
 
 template <class _Tp>
-class _LIBCPP_TEMPLATE_VIS tuple_size<__enable_if_tuple_size_imp<
+struct _LIBCPP_TEMPLATE_VIS tuple_size<__enable_if_tuple_size_imp<
     const volatile _Tp,
     integral_constant<size_t, sizeof(tuple_size<_Tp>)>>>
     : public integral_constant<size_t, tuple_size<_Tp>::value> {};
 
 #else
-template <class _Tp> class _LIBCPP_TEMPLATE_VIS tuple_size<const _Tp> : public tuple_size<_Tp> {};
-template <class _Tp> class _LIBCPP_TEMPLATE_VIS tuple_size<volatile _Tp> : public tuple_size<_Tp> {};
-template <class _Tp> class _LIBCPP_TEMPLATE_VIS tuple_size<const volatile _Tp> : public tuple_size<_Tp> {};
+template <class _Tp> struct _LIBCPP_TEMPLATE_VIS tuple_size<const _Tp> : public tuple_size<_Tp> {};
+template <class _Tp> struct _LIBCPP_TEMPLATE_VIS tuple_size<volatile _Tp> : public tuple_size<_Tp> {};
+template <class _Tp> struct _LIBCPP_TEMPLATE_VIS tuple_size<const volatile _Tp> : public tuple_size<_Tp> {};
 #endif
 
 template <size_t _Ip, class _Tp> class _LIBCPP_TEMPLATE_VIS tuple_element;

 template <class... _Tp> struct __tuple_like<tuple<_Tp...> > : true_type {};
 
 template <class ..._Tp>
-class _LIBCPP_TEMPLATE_VIS tuple_size<tuple<_Tp...> >
+struct _LIBCPP_TEMPLATE_VIS tuple_size<tuple<_Tp...> >
     : public integral_constant<size_t, sizeof...(_Tp)>
 {
 };

 
 
 template <class ..._Tp>
-class _LIBCPP_TEMPLATE_VIS tuple_size<__tuple_types<_Tp...> >
+struct _LIBCPP_TEMPLATE_VIS tuple_size<__tuple_types<_Tp...> >
     : public integral_constant<size_t, sizeof...(_Tp)>
 {
 };

include/array

 template <class T, size_t N >
   void swap(array<T,N>& x, array<T,N>& y) noexcept(noexcept(x.swap(y))); // C++17
 
-template <class T> class tuple_size;
+template <class T> struct 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>>;

 }
 
 template <class _Tp, size_t _Size>
-class _LIBCPP_TEMPLATE_VIS tuple_size<array<_Tp, _Size> >
+struct _LIBCPP_TEMPLATE_VIS tuple_size<array<_Tp, _Size> >
     : public integral_constant<size_t, _Size> {};
 
 template <size_t _Ip, class _Tp, size_t _Size>

include/tuple

   constexpr T make_from_tuple(Tuple&& t); // C++17
 
 // 20.4.1.4, tuple helper classes:
-template <class T> class tuple_size; // undefined
-template <class... T> class tuple_size<tuple<T...>>;
+template <class T> struct tuple_size; // undefined
+template <class... T> struct tuple_size<tuple<T...>>;
 template <class T>
  inline constexpr size_t tuple_size_v = tuple_size<T>::value; // C++17
 template <size_t I, class T> class tuple_element; // undefined

include/utility

 struct piecewise_construct_t { };
 inline constexpr piecewise_construct_t piecewise_construct = piecewise_construct_t();
 
-template <class T> class tuple_size;
+template <class T> struct tuple_size;
 template <size_t I, class T> class tuple_element;
 
 template <class T1, class T2> struct tuple_size<pair<T1, T2> >;

 #endif  // _LIBCPP_CXX03_LANG
 
 template <class _T1, class _T2>
-  class _LIBCPP_TEMPLATE_VIS tuple_size<pair<_T1, _T2> >
+  struct _LIBCPP_TEMPLATE_VIS tuple_size<pair<_T1, _T2> >
     : public integral_constant<size_t, 2> {};
 
 template <size_t _Ip, class _T1, class _T2>

test/std/utilities/tuple/tuple.tuple/tuple.helper/tuple_size_incomplete.fail.cpp

 // template <class... Types> class tuple;
 
 // template <class... Types>
-//   class tuple_size<tuple<Types...>>
+//   struct tuple_size<tuple<Types...>>
 //     : public integral_constant<size_t, sizeof...(Types)> { };
 
 // UNSUPPORTED: c++98, c++03

 struct Dummy3 {};
 
 template <>
-class std::tuple_size<Dummy1> {
+struct std::tuple_size<Dummy1> {
 public:
   static size_t value;
 };
 
 template <>
-class std::tuple_size<Dummy2> {
+struct std::tuple_size<Dummy2> {
 public:
   static void value() {}
 };
 
 template <>
-class std::tuple_size<Dummy3> {};
+struct std::tuple_size<Dummy3> {};
 
 int main()
 {

test/std/utilities/tuple/tuple.tuple/tuple.helper/tuple_size_incomplete.pass.cpp

 // template <class... Types> class tuple;
 
 // template <class... Types>
-//   class tuple_size<tuple<Types...>>
+//   struct tuple_size<tuple<Types...>>
 //     : public integral_constant<size_t, sizeof...(Types)> { };
 
 // XFAIL: gcc-4.9

 struct Dummy2 {};
 
 namespace std {
-template <> class tuple_size<Dummy1> : public integral_constant<size_t, 0> {};
+template <> struct tuple_size<Dummy1> : public integral_constant<size_t, 0> {};
 }
 
 template <class T>

test/std/utilities/tuple/tuple.tuple/tuple.helper/tuple_size_structured_bindings.pass.cpp

 // template <class... Types> class tuple;
 
 // template <class... Types>
-//   class tuple_size<tuple<Types...>>
+//   struct tuple_size<tuple<Types...>>
 //     : public integral_constant<size_t, sizeof...(Types)> { };
 
 // UNSUPPORTED: c++98, c++03, c++11, c++14

 }
 
 template <>
-class std::tuple_size<Test> {
+struct std::tuple_size<Test> {
 public:
   static const size_t value = 1;
 };