<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/65620>65620</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            libc++ makes wrong assumptions in std::pair constructor
        </td>
    </tr>

    <tr>
      <th>Labels</th>
      <td>
            libc++,
            new issue
      </td>
    </tr>

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

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

<pre>
    With latest libc++ this code doesn't compile in clang with  `-std=c++23`.

```c++
#include <type_traits>
#include <utility>

struct kvp {
  kvp() = default;
  explicit kvp(std::pair<int, int> const& p);
};

namespace std {

template <>
struct tuple_size<kvp> : std::integral_constant<std::size_t, 2> {};

template <>
struct tuple_element<0, kvp> {
  using type = int;
};

template <>
struct tuple_element<1, kvp> {
  using type = int;
};

} // namespace std


static_assert(std::is_nothrow_move_constructible<kvp>::value);
```

As far as I can tell, when checking wether `kvp` is nothrow move constructible, the compiler tries instantiating this `std::pair` constructor( https://github.com/llvm/llvm-project/blob/main/libcxx/include/__utility/pair.h#L286-L291 ).
That in turn tries to compile this code (https://github.com/llvm/llvm-project/blob/main/libcxx/include/__utility/pair.h#L279-L282), which assumes that `std::tuple_size<T>::value == 2` means that `std::get<0>(T)` and `std::get<1>(T)` will work.

For context, in the original code this type had `get` overloads in its own namespace, but not in namespace `std`.

</pre>
<img width="1px" height="1px" alt="" src="http://email.email.llvm.org/o/eJy8VUFr8zgQ_TWTy9CgjBPHPvjQJg0sfMcP9hhkW7W1lSUjjZt2f_0ixY0btsvywbJQ5MYazTy9eW8sQ9CdVaqC3RPsjis5ce981XkdpqH2Mqxq135Uv2vu0UhWgdHougF6AnpC7nXAxrUKW6eCBdozNm4YtVGoLTZG2g4v8SxCLh4Ct5Ad58OUQS7WII4gHuc1F9e_OWJ-S5m2jZlahZAd-GNUZ_ZSc4Ds-buIibXR_LHspjWwnxrG17cRYT9nxvgTqAAqEbIjtupFToYhu-2r99HoRvMcmPA_QvY4Su0hO2jLQAeMj-wZG2cDA-U4ApW3JLA_Lv-n1cpBhVE2CgO3C5rrymoYI8_xJrcrzOB5Go06B_2nguwQEWXPCNkj3mBpy6rz0pwTFBlhHW6b8dw54aV0cP_0d2j_Xl0ZNaiUWMRUnygWSqegbYexTYnTxM0_UvEr9Tb_QT3YHxHoBHTCuy7cxcwgJOvmLENQnr-2Xoezddx7dzkP7k1dqY6AdW2Wvlxj36SZ1J0YPhX-tdRjwBfpUQb8DRtpkZUx8a6XXllsetW8xhteFPfKRxvFCrlAHXAGghEI3gOhA3KvPs3okb1WAfVVFlpyIi26F3JxL-tcLKmcByqwZx5DjEjMdZr7qV43bgA6GfP2-XgYvftDNQx0qo2rgU6D1DZu6rp5fwc6zSYFOp3Pny6lUyy67oGyH1TkDz-o3CBQOQ-Gn73kOEl48na-A7vbiFnGD1Dxv6Hclw8_qKDY19Ql3fQoQ5iGCC7i_UrpnWd_3isjCjZqliLng5L2m_OdunotewYqfsaauUBp22-CNvdBF20MXpx_vZuxJ-dje1m9z6MrycR53WkrzZXMRGtyVC9ToZg_F-jelDdOtlFHqDmgu9jFRzFdPXEUZdxf_DUjvQ37VVtlbZmVcqWqTV5uN0IIUa76qty8iIZE3hbNyy7f1VvKi1o0onwpi6It65WuSFAmSrHf7EiIbL3fZ1KoQu7323LXbhVshRqkNuvY6rXz3UqHMKkq3-UkVkbWyoT0pSNavmJABHQAIqsumOLjm91x5aukmHrqAmyF0YHDkpg1G1V9-RYO8lUFvHhnu6scRtbOJq7uDPbVXavJm-qXhZswBqBTutZfAQAA__-T6V8a">