<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/87368>87368</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            Meaningless constraints (enable_if-s) on basic_string constructors
        </td>
    </tr>

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

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

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

<pre>
    Since the commit 76b26852b6be6e54c86741c7c80ca6b5d74eab2e some constructors of `basic_string` are "constrained" by `enable_if` which depends only on template parameters of the class `basic_string` itself. It's meaningless, and I'd say even harmful because it just slows down compilation but doesn't bring any value. For instance,
- https://github.com/llvm/llvm-project/blob/6b7b18a1a7fd37cea5f5d51569309e7ec954cefb/libcxx/include/string#L970
- https://github.com/llvm/llvm-project/blob/6b7b18a1a7fd37cea5f5d51569309e7ec954cefb/libcxx/include/string#L977
- https://github.com/llvm/llvm-project/blob/6b7b18a1a7fd37cea5f5d51569309e7ec954cefb/libcxx/include/string#L1027
(note that `_Allocator` is the template parameter of the class `basic_string`, but not the constructor).

If `_Allocator` doesn't satisfy the requirement `__is_allocator`, then compilation fails during instantiation of `basic_string` (https://gcc.godbolt.org/z/7jbdrheov), it's not SFINAE error, because substitution of the type of NTTP parameter is not invoked lazily (https://gcc.godbolt.org/z/oaWbhbve9).
</pre>
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