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 tangotests.sync3;

void main()
{
    int i = foo();
}

int foo()
{
    int res;
    synchronized
    {
        scope(exit) res--;
        res++;
        return res;
    }
}