hcat not type-stable with union types

[cstjean]

On 0.6.2, hcat has quite interesting behaviour with Vector{Union{...}}. It's particularly problematic with Missing values (it returns Matrix{Any}).

julia> hcat([1,2], Union{Int, Float64}[2, 3])     # type-stable
2×2 Array{Union{Float64, Int64},2}:
 1  2
 2  3

julia> hcat(Union{Int, Float64}[1,2], Union{Int, Float64}[2, 3])  # not stable
2×2 Array{Int64,2}:
 1  2
 2  3

julia> hcat(Union{Float64, Int}[1,2])   # not stable either
2×1 Array{Int64,2}:
 1
 2

AFAICT, this was supposed to be fixed in #19387 . vcat seems to work fine.

[nalimilan]

Confirmed on master too. Cc: @pabloferz

[nalimilan]

It's particularly problematic with Missing values (it returns Matrix{Any}).

Note that this particular use case has been fixed on master (#25553):

julia> hcat([1,missing])
2×1 Array{Union{Missing, Int64},2}:
 1       
  missing

(But of course due to the problem described above hcat(Union{Int,Missing}[1,2]) creates an Array{Int}.)