[Bug sanitizer/100878] New: enabling UBSAN leads to false positive `'this' pointer is null` when casting lambda to function pointer

[mail at milianw dot de via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100878

            Bug ID: 100878
           Summary: enabling UBSAN leads to false positive `'this' pointer
                    is null` when casting lambda to function pointer
           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: ---

```
const auto lambda = [](int i) -> int { return i + 1; };

using Callback = int (*)(int);

int run(Callback callback, int i)
{
    return callback(i);
}

int main()
{
    return run(lambda, 52);
}
```

compile with `-g -Og -fsanitize=undefined -Wall -Werror -Wpedantic -Wextra`
leads to:

```
<source>: In static member function 'static constexpr
int<lambda(int)>::_FUN(int)':
<source>:1:54: error: 'this' pointer is null [-Werror=nonnull]
    1 | const auto lambda = [](int i) -> int { return i + 1; };
      |                                                      ^
<source>:1:21: note: in a call to non-static member function '<lambda(int)>'
    1 | const auto lambda = [](int i) -> int { return i + 1; };
      |                     ^
cc1plus: all warnings being treated as errors
Compiler returned: 1
```

see also: https://godbolt.org/z/jTYYWbMsz