<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/62282>62282</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            [libc++] `long double` function call relies on `double` function
        </td>
    </tr>

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

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

    <tr>
      <th>Reporter</th>
      <td>
          JoseLuis-SolidSands
      </td>
    </tr>
</table>

<pre>
    In line 745 of file _libcxx/include/cmath_ the following function template is defined:
```c++
template <class _Tp> _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 _Tp __constexpr_scalbn(_Tp __x, int __exp)
```
This function can be called with a value of type `long double` as the first argument (e.g., line 710 of file _libcxx/include/complex_). However, its return statement is:
```c++
return __builtin_scalbn(__x, __exp);
```
This other built-in function takes a `double` as the first argument (instead of a `long double`) and also returns a `double` (instead of a `long double`). This accuracy loss yields undesired results, as it can be seen in the following example ([Compiler Explorer](https://cpp.godbolt.org/z/19MqbWhMc)):

```c++
#include <complex>

int main( ){
    bool res = true;

    const std::complex<long double> &a = {0.L, 0.5L};
    const std::complex<long double> &b = {0.L, 0.549306144334054845697622618461262852323745278911374725867L};

 std::complex<long double> x = std::atan<>( a );

    res &= ( std::real<long double>(x) == std::real<long double>(b) );
    res &= ( std::imag<long double>(x) == std::imag<long double>(b) );

    return !res;
}
``` 
This bug was found in `clang 16.0.0` using the SuperTest suite, `clang 15.0.0` still works fine.
Compilation flags: `-m64 -std=c++20 -stdlib=libc++ -O0 -funsigned-char`
Replacing the call to `__builtin_scalbn()` with a call to `__builtin_scalbnl()` for this case should fix it (or creating a specialization).
</pre>
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