Make == for associative arrays test for equality, not identity. _aaEq was added to runtime/internal/aaA.d which forwards to TypeInfo_AssociativeArray.equals in genobj.d. On the codegen side, DtoAAEquals was added to gen/aa.cpp and is called from EqualExp::toElem in gen/toir.cpp. I assume that the frontend will produce an error if == is used on associative arrays of different type. This fixes DMD bug 1429.

module union6;

struct S
{
    byte a;
    byte b;
}
union U
{
    byte a;
    byte b;
    S c;
}

void main()
{
    U u;
    auto a = u.c.b;
    //auto c = u.s.l;
}