<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/132668>132668</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            Non-constant tuple_size in structured binding is accepted
        </td>
    </tr>

    <tr>
      <th>Labels</th>
      <td>
            new issue
      </td>
    </tr>

    <tr>
      <th>Assignees</th>
      <td>
      </td>
    </tr>

    <tr>
      <th>Reporter</th>
      <td>
          halbi2
      </td>
    </tr>
</table>

<pre>
    ```
#include <tuple>
struct A {
 template<int> friend int get(A);
};
struct Conv {
  constexpr operator int() { return 2; }
};
template<> struct std::tuple_size<A> {
  static Conv& value; // no definition
};
template<int N> struct std::tuple_element<N, A> {
  using type = int;
};
auto [a,b] = A();
```

I think this TU is invalid, because `tuple_size<A>::value` is not an _integral constant expression_.
`clang++ -std=c++20 -c` rejects.
`clang++ -std=c++23 -c` accepts.
Is this intended for [P2280](https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2022/p2280r4.html)? But other compilers support P2280 too, and they all reject this `tuple_size` code.

I think #132463 could relate.
</pre>
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