[Bug c++/102603] New: Compile-time evaluation of indirection via dangling pointer is not rejected

[fchelnokov at gmail dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102603

            Bug ID: 102603
           Summary: Compile-time evaluation of indirection via dangling
                    pointer is not rejected
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: fchelnokov at gmail dot com
  Target Milestone: ---

The program as follows is ill-formed:
```
constexpr bool g() {
    int * p = nullptr;
    {
        int c = 0;
        p = &c;
    }
    return *p == 0; //indirection via dangling pointer
};

int main() {
    static_assert( g() );
}
```
but GCC erroneously accepts it, demo: https://gcc.godbolt.org/z/8xE7csGKo

The compiler must fail the program, see stackoverflow explanation:
https://stackoverflow.com/a/69446660/7325599