I see this sort of code sometimes and it makes me cringe
similar(a::Array{T,1}) where {T} = Vector{T}(undef, size(a,1))
similar(a::Array{T,2}) where {T} = Matrix{T}(undef, size(a,1), size(a,2))
similar(a::Array{T,1}, S::Type) where {T} = Vector{S}(undef, size(a,1))
similar(a::Array{T,2}, S::Type) where {T} = Matrix{S}(undef, size(a,1), size(a,2))
https://github.com/JuliaLang/julia/blob/0ea61fa153c69e052097f1307dafbedd00049aae/base/array.jl#L373
Because we already have this
similar(a::AbstractArray{T}) where {T} = similar(a, T)
similar(a::AbstractArray, ::Type{T}) where {T} = similar(a, T, to_shape(axes(a)))
similar(a::AbstractArray{T}, dims::Tuple) where {T} = similar(a, T, to_shape(dims))
similar(a::AbstractArray{T}, dims::DimOrInd...) where {T} = similar(a, T, to_shape(dims))
similar(a::AbstractArray, ::Type{T}, dims::DimOrInd...) where {T} = similar(a, T, to_shape(dims))
https://github.com/JuliaLang/julia/blob/0ea61fa153c69e052097f1307dafbedd00049aae/base/abstractarray.jl#L781
I believe that in this case it is written to special case low low numbers of arguments to avoid packing and unpacking into an NTuple, but I’m not sure. Is this necessary for maximum performance? Is it possible to delegate those special casings to the compiler whenever the NTuple’s N is known at compile time (almost always)? Does the compiler already do this?