[Bug c++/53596] g++-4.7 -Wall shouldn't complain for non-virtual protected dtor

[redi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53596

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> 2012-06-06 22:43:23 UTC ---
The warning is valid, the fact Base is protected is irrelevant, you're not
using delete with a Base* operand.

Consider:

class MoreDerived : public Derived {
public:
  ~Derived()         { std::cout << "MoreDerived::~MoreDerived()" << std::endl;
}
};

int main() {
  Derived* morederived = new MoreDerived();
  morederived->foo();
  delete morederived; // undefined behaviour
}

Now the warning is entirely correct, you are deleting a Derived* which is a
polymorphic type without a virtual destructor, so the program has undefined
behaviour.  That's substantially the same as your example, on the line:

  delete derived;

the compiler doesn't know that the Derived* points to a Derived or a
MoreDerived, so it warns you it might cause undefined behaviour.

N.B. Clang (known for much better diagnostics than MSVC or Sun CC) also warns
for your example:

w.cc:19:3: warning: delete called on 'Derived' that has virtual functions but
non-virtual destructor [-Wdelete-non-virtual-dtor]
  delete derived; // legal, there must be no warning!
  ^
1 warning generated.