<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/106635>106635</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            Enforce std::map and std::unordered_map value_type requirements
        </td>
    </tr>

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

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

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

<pre>
    Per [[associative.reqmts.general]/8](https://eel.is/c++draft/associative.reqmts.general#8), the requirements placed on a container's `value_type` instead apply to its `key_type` and `mapped_type`. This means that the following should all be rejected by the compiler.

```cpp
std::map<int const, int> K1; // same as volatile
std::map<int, int const> M1; // same as volatile
std::map<int, int&> M2; // (int& as a key is already rejected for overload ambiguity reasons)
std::map<int&&, int> K3;
std::map<int, int&&> M3;
std::map<int, int[]> M4;
std::map<int[n], int> K5;
std::map<int, int[n]> M5;
```
```cpp
std::unordered_map<S const, int> K1; // (int const as a key is already rejected because std::hash<int const> isn't allowed)
std::unordered_map<int, int const> M1; // same as volatile
std::unordered_map<int, int&> K2;
std::unordered_map<int, int&&> K3;
std::unordered_map<int, int[]> M4;
std::unordered_map<int, int[2]> M5;
```
</pre>
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