<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/77186>77186</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            [Clang] Insufficient diagnostics for dereferencing a null pointer in constant evaluation
        </td>
    </tr>

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

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

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

<pre>
    Not sure whether there should be a separated issue, as it may be desired to track this as [CWG2823](https://cplusplus.github.io/CWG/issues/2823.html).

Currently Clang accepts this program with warnings ([Godbolt link](https://godbolt.org/z/9xhxezh8d)):
```C++
static_assert([](int&) { return true; }(*static_cast<int*>(nullptr)));
static_assert(([]() -> int& { return *static_cast<int*>(nullptr); }(), true));

int main() {}
```
```
<source>:2:38: warning: binding dereferenced null pointer to reference has undefined behavior [-Wnull-dereference]
    2 | static_assert(([]() -> int& { return *static_cast<int*>(nullptr); }(), true));
      | ^~~~~~~~~~~~~~~~~~~~~~~~~~~
```

The first line should also be diagnosed.

Also, it seems that dereferencing a null pointer should be an error in constant evaluation instead of a warning. In the following program, Clang doesn't make the constant evaluation fail due to this kind of UB and thus selects the first overload, which is wrong. [Godbolt link](https://godbolt.org/z/s8x95hrq7).
```C++
#include <type_traits>

template<class T, bool = ([]() -> T& { return *static_cast<T*>(nullptr); }(), true)>
std::true_type fun(int);
template<class T>
std::false_type fun(...);

static_assert(!decltype(fun<int>(0))::value);
```
</pre>
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