[Bug sanitizer/101576] New: -fsaniitize=undefined silences clear nullptr dereference warning at compile time

[mail at milianw dot de via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101576

            Bug ID: 101576
           Summary: -fsaniitize=undefined silences clear nullptr
                    dereference warning at compile time
           Product: gcc
           Version: 11.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: sanitizer
          Assignee: unassigned at gcc dot gnu.org
          Reporter: mail at milianw dot de
                CC: dodji at gcc dot gnu.org, dvyukov at gcc dot gnu.org,
                    jakub at gcc dot gnu.org, kcc at gcc dot gnu.org, marxin at 
gcc dot gnu.org
  Target Milestone: ---

take this code:
```
struct Foo
{
    virtual ~Foo() {}
};

struct Bar : Foo
{
    ~Bar() override {};

    int bar() { return _bar; }

    int _bar = 42;
};

int doStuff(Foo *foo)
{
    if (auto *bar = dynamic_cast<Bar *>(foo))
    {
        return bar->bar();
    }
    else
    {
        // this is obviously wrong, but why is there no compiler warning?!
        return bar->bar();
    }
}

int main()
{
    Foo foo;
    return doStuff(&foo);
}
```

when you compile it with `-O -Wall -Wextra -Wpedantic -Werror` you'll get...
Nothing? Why?

But now also add `-Wnull-dereference` and you'll get:

https://godbolt.org/z/EKn8q85nT
```
<source>: In function 'int doStuff(Foo*)':
<source>:10:24: error: null pointer dereference [-Werror=null-dereference]
   10 |     int bar() { return _bar; }
      |                        ^~~~
cc1plus: all warnings being treated as errors
Compiler returned: 1
```

(Side note: It's unfortunate that this is not visible with -O0...)

Now add `-fsanitize=undefined` and the compile error is gone again which is
quite unfortunate...
https://godbolt.org/z/ebeaY1vP8