<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/106460>106460</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            [libc++] std::regex_constants::ECMAScript mask not set
        </td>
    </tr>

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

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

    <tr>
      <th>Reporter</th>
      <td>
          androm3da
      </td>
    </tr>
</table>

<pre>
    This program fails with libc++ and passes for libstdc++.  Is this the expected behavior?

```
#include <stdlib.h>
#include <regex>
#include <iostream>

using namespace std::regex_constants;

bool check(syntax_option_type a) {
  auto q = basic & (~basic);
  auto orig = a;
  if (a == q) {
      return false;
  }
  a &= ~q;
  if (a != orig) {
      return false;
  }
  return true;
}

int main() {
  if (!check(std::regex_constants::collate)) {
      std::cerr << "fail 'collate'\n";
      exit(1);
  }
  if (std::regex_constants::ECMAScript == (icase & (~icase))) {
      std::cerr << "Hmmm....\n";
  }
  if (!check(std::regex_constants::ECMAScript)) {
      std::cerr << "fail 'ECMAScript'\n";
      exit(1);
 }
}

```

Context:
```
$ clang++ ~/tmp/reg_const_ecma.cpp -stdlib=libc++ -std=c++20 && ./a.out ; echo $?
Hmmm....
fail 'ECMAScript'
1
$ clang++ ~/tmp/reg_const_ecma.cpp -std=c++20 && ./a.out ; echo $?
0
$ clang++ --version
clang version 18.1.8
Target: x86_64-unknown-linux-gnu
Thread model: posix
InstalledDir: /local/mnt/workspace/install/clang-latest/bin
```
</pre>
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