`hcat(A, B)` is SArray, but `[ A ;; B]` is not

Hi,

consider this MWE:

using LinearAlgebra, StaticArrays

A = @SMatrix randn(3, 4)
B = @SMatrix randn(3, 2)

as expected,

hcat(A, B)  #returns 3×6 SMatrix{3, 6, Float64, 18}

but

[ A ;; B]  # returns 3×6 Matrix{Float64}

If we compare this to vcat,

A = @SMatrix randn(3, 4)
B = @SMatrix randn(2, 4)
vcat(A, B) # returns 5×4 SMatrix{5, 4, Float64, 20} (as expected)
[ A ; B] # returns 5×4 SMatrix{5, 4, Float64, 20} (as expected)

So somehow, the array concatenation works for vcat but not for hcat.

Is this expected behavior?

No source file or test is named. Reproduce the MWE with StaticArrays and compare the types returned by hcat(A, B), [A ;; B], vcat(A, B), and [A ; B]. Determine whether horizontal concatenation should preserve the SMatrix type like vertical concatenation, then add or update coverage for the chosen behavior.