Union types turn into Any when splatted

[bramtayl]

t = Tuple{Int,String,Union{Missing, Float64}}[(1, "a", missing), (2, "b", 2.0)]
test(t) = Base.tail(first(t))
@code_warntype test(t)
# Tuple{String,Any}

[bramtayl]

Further reduction:

t = Tuple{Int, Union{Missing, Float64}}[(1, 2.0)]
argtail(x, rest...) = rest
test(t) = argtail(first(t)...)
@code_warntype test(t)

[bramtayl]

This solves it:

macro tail(is::Int...)
    esc(:(begin 
        $(map(is) do i
            Ts = map(j -> Symbol("T$j"), 1:i)
            :(tail(t::Tuple{$(Ts...)}) where {$(Ts...)} =
                ($(map(j -> :(t[$j]), 2:i)...),))
        end...)
    end))
end

@tail 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

But I think it's only a band-aid over the loss of type information caused by splatting.

[bramtayl]

Is this something that's on purpose? If Union{Missing, T} types are unsplattable, it makes them almost a no-go.