<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/62136>62136</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            [Clang] Cannot use static constexpr member variable in constant expression
        </td>
    </tr>

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

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

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

<pre>
    ## Clang Version:
```
clang version 17.0.0 (https://github.com/llvm/llvm-project.git 27f27d15f6c90b026eca23b8ee238fdbf772fd80)
Target: x86_64-unknown-linux-gnu
Thread model: posix
InstalledDir: /opt/compiler-explorer/clang-trunk/bin
```

## Steps to reproduce
Try to compile the following simple program with `clang++-16 -std=c++20 source.cc` ( see also [godbolt](https://godbolt.org/z/1bsazYvK7):
```
struct Mesh {
 static constexpr int kDimMesh = 3;
};

void checkMesh(const Mesh& mesh) {
  constexpr auto kDimMesh = mesh.kDimMesh;
}

int main() {
  Mesh mesh;
  checkMesh(mesh);
}
```

Error Message:
```
<source>:6:18: error: constexpr variable 'kDimMesh' must be initialized by a constant expression
  constexpr auto kDimMesh = mesh.kDimMesh;
                 ^          ~~~~~~~~~~~~~
<source>:6:29: note: function parameter 'mesh' with unknown value cannot be used in a constant expression
  constexpr auto kDimMesh = mesh.kDimMesh;
 ^
<source>:5:28: note: declared here
void checkMesh(const Mesh& mesh) {
                           ^
1 error generated.
```

As far as I can tell, this is perfectly valid c++ and it compiles fine with gcc and MSVC.
</pre>
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