Version used: Visual Studio 15.6 Preview 6 (same behavior in latest master bits)

Sample code:

class C
{
    void M()
    {
        var condition = true;

        switch (true)
        {
            case true
        }
    }
}

After case true, start typing when.
Expected behavior: when is offered and selected
Actual behavior: it's not offered at all

After when, start typing condition. No completion whatsoever: