<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/133056>133056</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            [libc++] Instantiation of `pair` in class template argument deduction is a bit too eager
        </td>
    </tr>

    <tr>
      <th>Labels</th>
      <td>
            libc++
      </td>
    </tr>

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

    <tr>
      <th>Reporter</th>
      <td>
          frederick-vs-ja
      </td>
    </tr>
</table>

<pre>
    The following code snippet is rejected with libc++ ([Godbolt link](https://godbolt.org/z/q1x1E8Tqq)):

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

// note that *+[]{} dereferences a function pointer converted from the lambda expression
static_assert(std::is_same_v<decltype(std::pair{0, *+[]{}}), std::pair<int, void(*)()>>);
```

It seems that the related implicit deduction guide synthesized from the constructor (`pair(const T1&, const T2&) -> pair<T1, T2>`) should be excluded by SFINAE mechanisms before instantiating invalid `pair<int, void()>`, and the decaying explicit deduction guide (`pair(T1, T2) -> pair<T1, T2>`) should work.

But libc++ currently makes the instantiation a bit too eager, and then causes hard error. I've roughly verified that this can (and perhaps should) be fixed within the library implementation.
</pre>
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