<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/96820>96820</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            Cannot format a duration with unsigned (or user-defined) representation
        </td>
    </tr>

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

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

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

<pre>
    Testcase: https://godbolt.org/z/EM7WxfEfT

```
#include <chrono>
#include <format>

int main()
{
    (void)
        std::format("{}", std::chrono::duration<unsigned>{});
}
```

Does not build:

```
In file included from /opt/compiler-explorer/clang-trunk-20240626/bin/../include/c++/v1/chrono:952:
/opt/compiler-explorer/clang-trunk-20240626/bin/../include/c++/v1/__chrono/formatter.h:594:63: error: no matching function for call to 'abs'
  594 | __formatter::__format_chrono_using_chrono_specs(__sstr, chrono::abs(__value), __chrono_specs);
      | ^~~~~~~~~~~
```

`chrono::abs` is constrained for durations whose representation type is signed. This excludes unsigned types as well as any user-defined representation type.

Matching bug in libstdc++: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115668
</pre>
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