Indices of indices aren't preserved in vector setindex! with offset arrays

[jishnub]

MWE

julia> a = [1:7;];

julia> a[Base.IdentityUnitRange(3:4)] = 2:3
2:3

julia> a[Base.IdentityUnitRange(3:4)] .= 2:3
ERROR: DimensionMismatch: array could not be broadcast to match destination
Stacktrace:
 [1] check_broadcast_shape
   @ ./broadcast.jl:540 [inlined]
 [2] check_broadcast_axes
   @ ./broadcast.jl:543 [inlined]
 [3] instantiate
   @ ./broadcast.jl:284 [inlined]
 [4] materialize!
   @ ./broadcast.jl:871 [inlined]
 [5] materialize!(dest::SubArray{Int64, 1, Vector{Int64}, Tuple{Base.IdentityUnitRange{UnitRange{Int64}}}, true}, bc::Base.Broadcast.Broadcasted{Base.Broadcast.DefaultArrayStyle{1}, Nothing, typeof(identity), Tuple{UnitRange{Int64}}})
   @ Base.Broadcast ./broadcast.jl:868
 [6] top-level scope
   @ REPL[82]:1

Ideally, these two should be equivalent, and the setindex! should really only work if the RHS has the same indices.