<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/87969>87969</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            [libc++] `num_get::get` extracts too many characters when the input is invalid
        </td>
    </tr>

    <tr>
      <th>Labels</th>
      <td>
            libc++
      </td>
    </tr>

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

    <tr>
      <th>Reporter</th>
      <td>
          frederick-vs-ja
      </td>
    </tr>
</table>

<pre>
    Currently, the following program prints 2 when using libc++, which means that the call to `num_get::get` accumulates the whole "XX" when parsing a `double` value ([Godbolt link](https://godbolt.org/z/v9Web7f6G)).

```C++
#include <iostream>
#include <sstream>
#include <locale>

int main()
{
    struct num_get_helper : std::num_get<char, char*> {};
 num_get_helper ng;

    char str[]{"XX"};
 std::ios_base::iostate err{};
    double x = -1.0;
 std::istringstream is;
    auto it = ng.get(str, str + sizeof(str) - 1, is, err, x);

    std::cout << it - str << '\n';
}
```

Per [[facet.num.get.virtuals]](https://eel.is/c++draft/facet.num.get.virtuals#3):

> If it is not discarded, then a check is made to determine if `c` is allowed as the next character of an input field of the conversion specifier returned by Stage 1.

The leading 'X' is already a disallowed character for specifier `l` (for `double`), so it shouldn't be accumulated and thus the printed value should be 0.

Likewise, if `"XX"` is replaced with `"0XX"`, the printed value should be 2 ('0' and the first 'X' are accumulated), but libc++ currently reports 3 ([Godbolt link](https://godbolt.org/z/6d69dscha)).

It seems that some checking for being "allowed as the next character of an input field of the conversion specifier" is missing.
</pre>
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