cat > main.d << EOF
void main() @safe
{
    struct ThrowingElement
    {
        int i;
        ~this()
        {
            assert(1);
        }
    }
    
    ThrowingElement[int] aa;
    aa[0] = ThrowingElement(0);
}
EOF


The problem is that the generated opAssign is @system:

ref return @system ThrowingElement opAssign(ThrowingElement p)
{
	(ThrowingElement __swap2 = void;) , __swap2 = this , this = p , __swap2.~this();
	return this;
}