Broadcast on nested Arrays gives suboptimal type information

[Keno]

a = Array(Array{Float64,1},2)
a[1] = [1.;2.;3.]
a[2] = [3.;4.;5.]
broadcast(*,[2.;3.],a)

gives

2-element Array{Any,1}:
 [2.0,4.0,6.0]
 [9.0,12.0,15.0]

I would have expected an Array{Array{Float64,1},1}.

[simonster]

Right now the return type is determined using promote_eltype or by a set of ad-hoc functions in broadcast.jl. To get this right generally, it seems like we need to either (ab)use type inference or use a trick similar to map.

[stevengj]

map(*, [2.,3.], a) gives an Array{Array{Float64,1},1}, so this is another instance that would apparently be fixed by adapting map's algorithm to broadcast (#16164). Duplicate of #4883.