Created attachment 30076 [details]
Test case

http://en.cppreference.com/w/cpp/memory/shared_ptr/shared_ptr says:
"When constructing a shared_ptr from a raw pointer to an object of a
type derived from std::enable_shared_from_this, the constructors of
shared_ptr detect other owners of the object through the weak
reference stored in that base class. This allows ownership of the
object to be shared with existing shared_ptrs. Otherwise, it is
undefined behavior to construct a shared_ptr for an object that is
already managed by another shared_ptr."

Therefore, I gather the attached program should share ownership,
but it doesn't: It says "gp2.use_count() = 1;" and aborts due to
double deletion.

Using shared_from_this() works, though, so it's probably not a
serious problem.