Small number of invalidations when defining new `AbstractUnitRange{Int}` type

[Tokazama]

I consistently (across multiple Julia versions) have found a couple of invalidations whenever I define a new AbstractUnitRange{Int} type. It usually starts with a method like last(::OrdinalRange) but usually I can trace the issue back to last(::Slice). It's gotten a lot better in more recent releases (on v8.2 it only causes 2 invalidations), but I figured enough code interacts with Slice it was worth being proactive about it becoming a bigger issue later on.

I think it could be fixed if we changed its definition from Slice{T<:AbstractUnitRange} <: AbstractUnitRange{Int} to Slice{T<:AbstractUnitRange{Int} <: AbstractUnitRange{Int}. Is there any reason we don't enforce the indices have this subtype?

[jishnub]

Wouldn't this impact something like this:

julia> view(big(1):2, :, 1)[:]
2-element Vector{BigInt}:
 1
 2

which involves constructing a Base.Slice{Base.OneTo{BigInt}}} at an intermediate step?

[Tokazama]

The type of the indices shouldn't change the resulting vector's elements

julia> view(big(1):2, 1:2, 1)[1:2]
2-element Vector{BigInt}:
 1
 2

Maybe it's an attempt to support indices bigger than typemax(Int)? If so, I'm not sure it's actually accomplishing that. Base.Slice still subtypes AbstractUnitRange{Int} so the element types should (and are Int) anyway

julia> s = Base.Slice(big(1):2)
Base.Slice(1:2)

julia> typeof(s[1])
Int64

julia> typeof(s[big(1)])
Int64

[jishnub]

It's not fully consistent currently, but I agree that indexing should be equivalent to indexing into an AbstractVector{Int}.

julia> typeof(s[big(1)])
Int64

julia> typeof(s[big(1):big(1)])
UnitRange{BigInt}

Perhaps I didn't provide the best example, it's not the indexing that's the issue, rather constructing the view:

julia> @code_warntype view(big(1):2, :, 1)
MethodInstance for view(::UnitRange{BigInt}, ::Colon, ::Int64)
  from view(A::AbstractArray, I::Vararg{Any, N}) where N in Base at subarray.jl:174
Static Parameters
  N = 2
Arguments
  #self#::Core.Const(view)
  A::UnitRange{BigInt}
  I::Tuple{Colon, Int64}
Locals
  #185::Base.var"#185#186"{UnitRange{BigInt}}
  J::Tuple{Base.Slice{Base.OneTo{BigInt}}, Int64}
Body::SubArray{BigInt, 1, Base.ReshapedArray{BigInt, 2, UnitRange{BigInt}, Tuple{}}, Tuple{Base.Slice{Base.OneTo{BigInt}}, Int64}, true}
1 ─       nothing
│   %2  = Base.:(var"#185#186")::Core.Const(Base.var"#185#186")
│   %3  = Core.typeof(A)::Core.Const(UnitRange{BigInt})
│   %4  = Core.apply_type(%2, %3)::Core.Const(Base.var"#185#186"{UnitRange{BigInt}})
│         (#185 = %new(%4, A))
│   %6  = #185::Base.var"#185#186"{UnitRange{BigInt}}
│   %7  = Base.to_indices(A, I)::Tuple{Base.Slice{Base.OneTo{BigInt}}, Int64}
│         (J = Base.map(%6, %7))
└──       goto #3 if not $(Expr(:boundscheck))
2 ─ %10 = Core.tuple(A)::Tuple{UnitRange{BigInt}}
└──       Core._apply_iterate(Base.iterate, Base.checkbounds, %10, J)
3 ┄ %12 = Core._apply_iterate(Base.iterate, Base.index_ndims, J)::Core.Const((true, true))
│   %13 = Base._maybe_reshape_parent(A, %12)::Base.ReshapedArray{BigInt, 2, UnitRange{BigInt}, Tuple{}}
│   %14 = Core.tuple(%13)::Tuple{Base.ReshapedArray{BigInt, 2, UnitRange{BigInt}, Tuple{}}}
│   %15 = Core._apply_iterate(Base.iterate, Base.unsafe_view, %14, J)::Core.PartialStruct(SubArray{BigInt, 1, Base.ReshapedArray{BigInt, 2, UnitRange{BigInt}, Tuple{}}, Tuple{Base.Slice{Base.OneTo{BigInt}}, Int64}, true}, Any[Base.ReshapedArray{BigInt, 2, UnitRange{BigInt}, Tuple{}}, Tuple{Base.Slice{Base.OneTo{BigInt}}, Int64}, Int64, Core.Const(1)])
└──       return %15

lists Base.to_indices(A, I)::Tuple{Base.Slice{Base.OneTo{BigInt}}, Int64}. However, on further testing, I'm uncertain if this is necessary, as the following doesn't work:

julia> view(1:big(2)^64, :, 1)
ERROR: InexactError: Int64(18446744073709551616)
Stacktrace:
 [1] Type
   @ ./gmp.jl:365 [inlined]
 [2] to_shape
   @ ./abstractarray.jl:814 [inlined]
 [3] map
   @ ./tuple.jl:222 [inlined]
 [4] to_shape
   @ ./abstractarray.jl:810 [inlined]
 [5] reshape
   @ ./reshapedarray.jl:111 [inlined]
 [6] reshape
   @ ./reshapedarray.jl:142 [inlined]
 [7] _maybe_reshape_parent
   @ ./subarray.jl:127 [inlined]
 [8] view(::UnitRange{BigInt}, ::Function, ::Int64)
   @ Base ./subarray.jl:178
 [9] top-level scope
   @ REPL[21]:1

So perhaps this is broken currently after all. In this example, the issue seems to be the same as #40076, which is labelled as a bug, so it might be fixed in the future.

[Tokazama]

It seems like we either need to have Slice use the same subtype as it's parent range or just force use of Int