could `firstindex` be changed to return isbits for the `AbstractArray` subtypes defined in `Base`

[nsajko]

I often assume the arr[begin] indexing syntax is a zero-cost abstraction, assuming arr is known to use one-based indexing at compile time. However, the begin is actually firstindex(arr), which returns heavy types like BigInt, which relies on ccall a lot, in some cases:

julia> typeof(firstindex(big(1000):big(3000)))
BigInt

julia> typeof(firstindex(Base.OneTo(big(3000))))
BigInt

AFAIK all the concrete AbstractArray subtypes defined in Base use one-based indexing, so it seems like it should be OK for firstindex(arr) to just return 1 (of type Int) for any concrete type that comes by default with Julia.

[mikmoore]

I don't think that firstindex should return a different type than lastindex or eltype ∘ eachindex. Sooner or later that may create a problem. Besides, if you're dealing with a BigInt-keyed array then you've probably got other BigInt ops and a couple extra don't seem likely to devastate performance.