<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/61007>61007</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            std::default_initializable in libc++ accepts an std::unordered_set of a type with no hash
        </td>
    </tr>

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

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

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

<pre>
    Godbolt: https://clang.godbolt.org/z/aqahhrdvK

Code to reproduce:
```cpp
#include <concepts>
#include <unordered_set>

struct foo {};

static_assert(!std::default_initializable<std::unordered_set<foo>>);
```

This compiles correctly in gcc as far back as 10, and indeed clang also accepts it when using `libstdc++` as far back as clang-13.

When using libc++ however, it fails (on clang 13, 14, 15, and trunk):
```
<source>:6:1: error: static assertion failed due to requirement '!std::default_initializable<std::unordered_set<foo, std::hash<foo>, std::equal_to<foo>, std::allocator<foo>>>'
static_assert(!std::default_initializable<std::unordered_set<foo>>);
^ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1 error generated.
Compiler returned: 1
```

Each component of `default_initializable` (`constructible_from` and a default constructor itself) also pass the `static_assert`.  But in all cases, clang does correctly fail to actually construct an `std::unordered_set<foo>`.

It does correctly conclude that `std::default_initializable<std::hash<foo>>` is false.
</pre>
<img width="1px" height="1px" alt="" src="http://email.email.llvm.org/o/eJy8VUFv8zYM_TXKhWhgy4mdHHxI02YYdh2wY0FLdKx9iuRKVIt-h_32QXbaJEXRARuwwHCUUOIj-R4pjNEcHVEr1vdi_bDAxIMPrcKR0TgVEquB4qLz-q39xevOWxbVDgbmMYpqJ-RByIOy6I7L42xe-nAU8vBTyAM-4zAE_fKbKB5EsZvfe68J2EOgMXidFGU3s70u5keN4_kfWRmnbNIEotor7xSNHEX1-JU5OR80BdJPkfiyZ3pHDkkx9N6DaO5F8yCq-1s7slFPGCMFFnIjZBlZ58iqnaYek-Un4wwbtOYndpZEtf_Y8Al433uf4atHIbcXnPfkrmF_H0wE5U-jsZQXIZBi-wbGwVEpwAg9BuhQ_cjrshByD-g0GKeJNEx1B7TRA6qpNGAYXgdykKJxRxB1YU0XWSsh7_NTF5-dTj7uymp5HdcfFxfWdOfDMPhXeqGQgzAMPRobQciNd-dAyiqbytX0Xr_HyiG5H1MlPtN8_lnto08h6-BRVLtaVLsyS4xC8CEvZm5g5sZ4NyGTBp3OOnpOJtCJHIOQzX9mTu7hwz5gHC6EXlvoOaF9Yv-1Fa31CjnHf6WGvKn53_S2foS__sVnPl3O5YcjOQrIpJfvzTtpNUAgTsFRjgjKbxT-iGqYFO5dJsj3WZNfZ1gXWU25_b2b-9V0lp764E-Tbp0GhPNR-NjjAxiOZHsht3MrjBgj8EAZ6bbOdbEEuE-c-wutBYWRYiZu1q_2N02YZZYFhooTWvt2wQR0s_PvCamLm6b6lT8j5IE2TS8ekG88_qMGbpU5gYHJnW0jLRe6rfS22uKC2rJumlW1KlflYmg3WPR93ZTlqug3fVcXzUZWfbku191KFbRemFYWsiqkrMtmvV6Vy1Xfa2o2UtbrLW2pFKuCTmjs0tqXUx71CxNjorYui6JZWOzIxukukdLRK0xGIWW-WkKbz9x16RjFqrAmcrx4YcOW2u_zz7RdjaP3kYcOvmYiqw2B30aCV8MDOA-5bIsUbHt7gR0ND6lbKn8S8pBjOn_djcH_SYqFPEyZRCEPU6Z_BwAA__8UPk_9">