Conflicting inappropriate tags compile with the standard library, but not otherwise

Question

I looked at how it std::tuple_sizeis defined in the standard library of my system. I have a Mac OS and a compiler version Apple LLVM version 8.1.0 (clang-802.0.42). The standard library is in the location InstalledDirthat is displayedg++ --version

Given that the following code does not compile (because it clangdoesn’t like to specialize / re-declare something as structthat which was previously defined as classand vice versa)

#include <iostream>
#include <utility>
#include <array>
#include <tuple>
#include <type_traits>

using std::cout;
using std::endl;

template <class T>
class Something;
template <class One, class Two>
struct Something<std::pair<One, Two>>;
template <class One, class Two>
class Something<std::pair<One, Two>>
    : public std::integral_constant<int, 2> {};

int main() {
  cout << std::tuple_size<std::array<int, 2>>::value << endl;
  cout << std::tuple_size<std::tuple<int, int>>::value << endl;
  cout << std::tuple_size<std::pair<int, int>>::value << endl;
}

with the following error

error: struct template 'Something' was previously declared as a class template [-Werror,-Wmismatched-tags]
struct Something<std::pair<One, Two>>;
^
test.cpp:49:7: note: previous use is here
class Something;

I saw the following code in the header of the standard library <array>(with a special definition of tuple_sizelike classfollowing it later)

template <class T> class tuple_size;
template <size_t I, class T> class tuple_element;
template <class T1, class T2> struct tuple_size<pair<T1, T2> >;

Something? tuple_size, class, struct?

Answer 1

, ( clang / - , )

clang .

error: struct template 'Something' was previously declared as a class template [-Werror,-Wmismatched-tags]

, a) , , -Wmismatched-tags b), .

. ++ , , clang. , .

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Answer 2

,

#ifndef _something_h_
#define _something_h_

//#pragma GCC system_header

template <class T>
class Something;

template <class One, class Two>
struct Something<std::pair<One, Two>>;

template <class One, class Two>
class Something<std::pair<One, Two>>
 {};

#endif /* ndef _something_h_ */

cpp

#include <iostream>
#include <utility>
#include <array>
#include <tuple>
#include <type_traits>

#include "something.h"

using std::cout;
using std::endl;

int main() {
  cout << std::tuple_size<std::array<int, 2>>::value << endl;
  cout << std::tuple_size<std::tuple<int, int>>::value << endl;
  cout << std::tuple_size<std::pair<int, int>>::value << endl;
}

.

"something.h" .