Not sure if this should be considered a bug but Assert.Equal compares IEnumerable<T> and IReadOnlyDictionary<TKey, TValue> correctly, so one could assume ILookup<TKey, TValue> are covered too.

It is not the case, Assert.Equal sees lookups as collections of IGrouping<TKey, TElement> which implements IEnumerable<TElement> and therefore only compares values regardless of keys.

I don't think the IReadOnlyDictionary<TKey, TValue> case is explicitly covered, but comparing collections of KeyValuePair<TKey, TValue> compares both the key and the value which is not the case with IGrouping<TKey, TElement>.

As an example:

ILookup<bool, int> evens =
    Enumerable.Range(0, 10).ToLookup(
        i => ((i % 2) == 0),
        i => i);

ILookup<bool, int> odds =
    Enumerable.Range(0, 10).ToLookup(
        i => ((i % 2) == 1),
        i => i);

Assert.Equal(evens, odds);

succeeds because both lookups have the same values but the keys associated to these values are different.