Consider a move assign to a vector<int> from src to this. The first step is for this to free its resources, the second step is to assign src resources to this, the third step is to set src resources to null.

If src and this are equal and you don't check for it, then you end up destroying src/this resources and the end result would be an empty vector (since the last step is to clear everything out).

The expected behavior is a no-op.