<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/105248>105248</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            Redeclaration of explicitly defaulted comparison operator makes it undefined
        </td>
    </tr>

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

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

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

<pre>
    This program
```
struct A {
    friend constexpr bool operator ==(const A &, const A &) noexcept = default;
};

constexpr bool operator ==(const A &, const A &) noexcept;
static_assert( A{} == A{} );
```
looks valid and it is accepted by EDG and MSVC. But Clang rejects it with the error

> undefined function 'operator==' cannot be used in a constant expression

GCC unfortunately has a similar issue.

Online demo: https://gcc.godbolt.org/z/azqbosPTd

Related discussion: https://stackoverflow.com/q/78889523/7325599
</pre>
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