To fix issue 15579, PR 5361 was merged.
https://github.com/D-Programming-Language/dmd/pull/5361.

But I noticed that it has changed extern (D) class layout, not only for extern (C++).
https://github.com/D-Programming-Language/dmd/pull/5361/files#r51126555

Test case:

extern (D) //extern (C++)
{
    class Base
    {
        ~this() {}
        size_t x = 4;
    }

    interface Interface
    {
        int Method();
    }

    class Derived : Base, Interface
    {
        size_t y = 5;
        int Method() { return 3; }
    }
}

void main()
{
    Derived d;                  // dmd 2.069             master
    pragma(msg, d.x.offsetof);  // (void*).sizeof * 2
    pragma(msg, d.y.offsetof);  // (void*).sizeof * 3 -> 4
}

I'm not sure it was expected, so I open this regression issue. If it's an intentional change, we should update http://dlang.org/spec/abi ABI page, so there's an explanation for D class object layout.