<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/101614>101614</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            [Clang] libstdc++’s `std::forward_like` doesn’t compile
        </td>
    </tr>

    <tr>
      <th>Labels</th>
      <td>
            clang:frontend,
            c++23,
            libstdc++
      </td>
    </tr>

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

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

<pre>
    Attempting to use libstdc++’s implementation of `std::forward_like` (https://godbolt.org/z/cGK1bsjzM)
```c++
#include <utility>

void f() {
    (void) std::forward_like<int&&>(1);
}
```
results in this error
```
<source>:4:17: error: function 'forward_like<int &&, int>' with deduced return type cannot be used before it is defined
    4 |     (void) std::forward_like<int&&>(1);
      |                 ^
/opt/compiler-explorer/gcc-snapshot/lib/gcc/x86_64-linux-gnu/15.0.0/../../../../include/c++/15.0.0/bits/move.h:93:3: note: 'forward_like<int &&, int>' declared here
   93 |   forward_like(_Up&& __x) noexcept
      |   ^
```
whereas the same code compiles just fine if `libc++` is used. GCC of course compiles this just fine with `libstdc++`. 

I haven’t looked into this too much, but it might have something to do with the fact that we treat `std::forward_like` as a builtin, and libstdc++’s declaration uses `decltype(auto)` as the return type of `std::forward_like`, which might be confusing us somehow.
</pre>
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