<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/78841>78841</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            Clang crashes when generating a reference of auto type from a nullptr casted to a pointer to an integer array of unknown size
        </td>
    </tr>

    <tr>
      <th>Labels</th>
      <td>
            clang
      </td>
    </tr>

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

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

<pre>
    Extension from https://github.com/llvm/llvm-project/issues/76634, since below the surface of that issue's crash problem there is another issue causing a crash of clang.

Consider this code, simplified from 76634:
```
int main() {
    int m = 1;
    auto& x = *(int(*)[m])(nullptr);
 return x[0];
}
```

Compiling this with the latest version of clang will trigger the following assertion:

> clang-18: /home/drives/fastSSD/llvm-project/clang/include/clang/AST/Type.h:753: const clang::ExtQualsTypeCommonBase* clang::QualType::getCommonPtr() const: Assertion `!isNull() && "Cannot retrieve a NULL type pointer"' failed.

Because in ExprConstant.cpp.9275, the dynamic cast to a ConstantArrayType fails and clang tries to add an unsized array to the result.

If we add a check for a nullptr at this position (and return false in that case), several test cases in test/clang/AST/Interp/arrays.cpp (line 500) fail since the diagnostic errors for incomplete arrays are not emitted. For example, the _must be initialized by a constant expression_ diagnostic.

At which point in the clang AST parsing can we add the check for _must be initialized by a constant expression_, which should trigger when we try to perform the cast to a pointer to an integer array of unknown size?
</pre>
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