Visual C++ has C4150 warning for cases when delete is applied to a pointer to incomplete type.
Such cases yield undefined behavior according to the Standard. AFAIK in Visual C++ they result in default operator delete() function and no destructor being called which allows for numerous bugs.
Now I could have used #prarma warning( error : 4150 ) in Visual C++ to treat that warning as error. I guess there're reasons why it is a warning and not an error by default in Visual C++.
In what real life code would I want to allow such cases? Why would I not switch that warning into a compiler error?
It's not always an UB.
If the object being deleted has incomplete class type at the point of deletion and the complete class has a non-trivial destructor or a deallocation function, the behavior is undefined.