Step range subtraction step size issues

[m-wells]

This is a callback to #10391

julia> versioninfo()
Julia Version 1.1.0
Commit 80516ca202 (2019-01-21 21:24 UTC)
Platform Info:
  OS: Linux (x86_64-pc-linux-gnu)
  CPU: Intel(R) Core(TM) i7-6700HQ CPU @ 2.60GHz
  WORD_SIZE: 64
  LIBM: libopenlibm
  LLVM: libLLVM-6.0.1 (ORCJIT, skylake)

julia> a=1:1:5
1:1:5

julia> a[2:end] .- a[1:end-1]
ERROR: ArgumentError: step cannot be zero
Stacktrace:
 [1] steprange_last(::Int64, ::Int64, ::Int64) at ./range.jl:209
 [2] _rangestyle at ./range.jl:199 [inlined]
 [3] _range at ./range.jl:110 [inlined]
 [4] #range#31 at ./range.jl:87 [inlined]
 [5] #range at ./none:0 [inlined]
 [6] - at ./range.jl:1006 [inlined]
 [7] broadcasted at ./broadcast.jl:988 [inlined]
 [8] broadcasted(::Function, ::StepRange{Int64,Int64}, ::StepRange{Int64,Int64}) at ./broadcast.jl:1167
 [9] top-level scope at none:0

julia> a[2] - a[1]
1

Is there a reason we can't implement something like

import Base.-

function -( x :: UnitRange
          , y :: UnitRange
          )
    length(x) != length(y) && throw(DimensionMismatch)
    return StepRangeLen(x.start-y.start, 0, length(x))
end

function -( x :: StepRange
          , y :: StepRange
          )
    length(x) != length(y) && throw(DimensionMismatch)
    if x.step == y.step
        return StepRangeLen(x.start-y.start, 0, length(x))
    else
        return (x.start-y.start):(x.step-y.step):(x.stop-y.stop)
    end
end

Also this seems related to

julia> range(1,step=0,length=10)
ERROR: ArgumentError: step cannot be zero
Stacktrace:
 [1] steprange_last(::Int64, ::Int64, ::Int64) at ./range.jl:209
 [2] Type at ./range.jl:199 [inlined]
 [3] _rangestyle at ./range.jl:112 [inlined]
 [4] _range at ./range.jl:110 [inlined]
 [5] #range#31 at ./range.jl:87 [inlined]
 [6] (::getfield(Base, Symbol("#kw##range")))(::NamedTuple{(:step, :length),Tuple{Int64,Int64}}, ::typeof(range), ::Int64) at ./none:0
 [7] top-level scope at none:0

julia> range(1.0,step=0,length=10)
1.0:0.0:1.0

which could also be promoted to a StepRangeLen type.

[mbauman]

Let's keep all the discussion on this issue in #10391. Feel free to post this proposal there.