<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/63421>63421</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            [libc++] <format> has uninitialized variables when formatting a string with zero arguments. 
        </td>
    </tr>

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

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

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

<pre>
    When formatting a string with 0 arguments, `std::format("Hello World!");` , GCC 13.1 complains there are maybe uninitialized variables. This causes spurious warnings/errors with appropriate build flags enabled. 

```
[build] /workspaces/include/c++/v1/__format/format_functions.h:71:65: error: '<anonymous>' may be used uninitialized [-Werror=maybe-uninitialized]
[build]    71 |   return _VSTD::__format_arg_store<_Context, _Args...>(__args...);
```

The culprit is in the constructor of  `__format_arg_store`. When constructing an argument store of 0 size, it never writes a value to `__packed_format_arg_store::__types_`. A quick fix is to give `__packed_format_arg_store::__types_` a default value of 0.

```
template <class _Context, size_t N>
struct __packed_format_arg_store {
  __basic_format_arg_value<_Context> __values_[N];
  uint64_t __types_ = 0;
};
```
</pre>
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