<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/67046>67046</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            C++20 std::midpoint is non-conformant
        </td>
    </tr>

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

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

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

<pre>
    The second (pointer) overload of `std::midpoint()` fails to compile when used explicitly. This is a failure to accept valid code according to ISO/IEC 14882:2020 25.10.16/4 [numeric.ops.midpoint].

Example:
```
#include <numeric>

template<typename T>
  T* mp(T* b, T*e) {
    return std::midpoint<T>(b, e);
  }

int main() {
    char a[2];
 char* p = mp(a, a+2);
}
```
gives the following compilation errors.
```
<source>:5:12: error: no matching function for call to 'midpoint'
    5 |     return std::midpoint<T>(b, e);
      | ^~~~~~~~~~~~~~~~
<source>:10:15: note: in instantiation of function template specialization 'mp<char>' requested here
   10 |     char* p = mp(a, a+2);
      | ^
/opt/compiler-explorer/clang-17.0.1/bin/../include/c++/v1/__numeric/midpoint.h:40:1: note: candidate function template not viable: no known conversion from 'char *' to 'char' for 1st argument; dereference the argument with *
   40 | midpoint(_Tp __a, _Tp __b) noexcept
      | ^ ~~~~~~~
/opt/compiler-explorer/clang-17.0.1/bin/../include/c++/v1/__numeric/midpoint.h:61:1: note: candidate template ignored: requirement 'is_pointer_v<char>' was not satisfied [with _TPtr = char]
   61 | midpoint(_TPtr __a, _TPtr __b) noexcept
      | ^
/opt/compiler-explorer/clang-17.0.1/bin/../include/c++/v1/__numeric/midpoint.h:78:1: note: candidate template ignored: requirement 'is_floating_point_v<char>' was not satisfied [with _Fp = char]
   78 | midpoint(_Fp __a, _Fp __b) noexcept
      | ^
1 error generated.
Compiler returned: 1
```
The following patch resolves the error.
```
57c57,58
< enable_if_t<is_object_v<remove_pointer_t<_TPtr>>
---
> enable_if_t<is_pointer_v<_TPtr>
>              && is_object_v<remove_pointer_t<_TPtr>>
59,60c60,61
<              && (sizeof(remove_pointer_t<_TPtr>) > 0), _TPtr*>
< midpoint(_TPtr* __a, _TPtr* __b) noexcept
---
>              && (sizeof(remove_pointer_t<_TPtr>) > 0), _TPtr>
> midpoint(_TPtr __a, _TPtr __b) noexcept
```
</pre>
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