let f = function
    | [] -> 0
    | (_ :: _) & _ -> 0

Script1.fsx(1,9): warning FS0025: Incomplete pattern matches on this expression. For example, the value '[]' may indicate a case not covered by the pattern(s).

In VS2015 this gives me the above warning. As would be expected running the above with [] returns 0. It also gives the same warning when the last case is written out as _ :: (_ :: _). When & is replaced by as in the above code fragment the warning disappears.