Changing `_setindex(v, i::Integer, first, tail...)` implementation to help constant propagation

[OlivierHnt]

Follow-up of this discourse thread.

Thanks to @mbauman's investigation, it seems that we loose constant propagation in the definition of _setindex:

julia/base/tuple.jl

Line 57 in c708ca2

return (ifelse(i == 1, v, first), _setindex(v, i - 1, tail...)...)

To illustrate, the following MWE (tested on v1.5.3 and v1.6) shows that the compiler cannot predict the dimension of selectdim(A, 1, 1) if A is an AbstractArray of dimension strictly greater than 2:

julia> f(A) = selectdim(A, 1, 1);

julia> A = rand(2,2);

julia> @code_warntype f(A)
Variables
  #self#::Core.Compiler.Const(f, false)
  A::Array{Float64,2}

Body::SubArray{Float64,1,Array{Float64,2},Tuple{Int64,Base.Slice{Base.OneTo{Int64}}},true}
1 ─ %1 = Main.selectdim(A, 1, 1)::SubArray{Float64,1,Array{Float64,2},Tuple{Int64,Base.Slice{Base.OneTo{Int64}}},true}
└──      return %1

julia> A = rand(2,2,2);

julia> @code_warntype f(A)
Variables
  #self#::Core.Compiler.Const(f, false)
  A::Array{Float64,3}

Body::Union{SubArray{Float64,2,Array{Float64,3},Tuple{Int64,Base.Slice{Base.OneTo{Int64}},Base.Slice{Base.OneTo{Int64}}},true}, SubArray{Float64,1,Array{Float64,3},Tuple{Int64,Base.Slice{Base.OneTo{Int64}},Int64},true}}
1 ─ %1 = Main.selectdim(A, 1, 1)::Union{SubArray{Float64,2,Array{Float64,3},Tuple{Int64,Base.Slice{Base.OneTo{Int64}},Base.Slice{Base.OneTo{Int64}}},true}, SubArray{Float64,1,Array{Float64,3},Tuple{Int64,Base.Slice{Base.OneTo{Int64}},Int64},true}}
└──      return %1

In the linked discourse discussion, @mbauman proposed the following solution:

julia> @inline function Base._setindex(v, i::Int, args...)
           ntuple(dim->ifelse(i==dim, v, args[dim]), length(args))
       end

julia> f(A)
2×2 view(::Array{Float64, 3}, 1, :, :) with eltype Float64:
 0.901717  0.427261
 0.130018  0.669248

julia> @code_warntype f(A)
Variables
  #self#::Core.Const(f)
  A::Array{Float64, 3}

Body::SubArray{Float64, 2, Array{Float64, 3}, Tuple{Int64, Base.Slice{Base.OneTo{Int64}}, Base.Slice{Base.OneTo{Int64}}}, true}