Why must a base class destructor be accessible only when a custom constructor is declared?

Question

Comeau, g++ (ideone) and EDG accept the following code without diagnostic. Visual C++ compiles successfully, albeit with warning C4624.

class indestructible_bas         


        

Answer 1

Well, if the automatically-generated constructor calls a possibly-throwing constructor, then it will give the same access error.

#include 

class indestructible_base
{
  ~indestructible_base();
  std::string s; // <------ this may throw
};

class T : indestructible_base
{
public:
  //T() {}
};

int main(void) { new T(); }

So I guess exceptions is the answer. In ANSI ISO IEC 14882 the only noexcept(true) string constructor is the move constructor. I believe this should compile but ideone says no.