foldr(-,2:2) throws an error while foldl(-,2:2) does not

[dlfivefifty]

I assume this change was intentional:

julia> foldr(-,2:2)  # 0.5
2

julia> foldr(-,2:2) # 0.6
ERROR: BoundsError: attempt to access 1-element UnitRange{Int64} at index [0]
Stacktrace:
 [1] throw_boundserror(::UnitRange{Int64}, ::Int64) at ./abstractarray.jl:426
 [2] getindex at ./range.jl:464 [inlined]
 [3] mapfoldr_impl(::Base.#identity, ::Base.#-, ::Int64, ::UnitRange{Int64}, ::Int64) at ./reduce.jl:110
 [4] mapfoldr(::Base.#identity, ::Function, ::UnitRange{Int64}) at ./reduce.jl:139
 [5] foldr(::Function, ::UnitRange{Int64}) at ./reduce.jl:166

The reason behind this change makes sense, but the error message was confusing.

[dlfivefifty]

Actually, I think it's a bug:

               _
   _       _ _(_)_     |  A fresh approach to technical computing
  (_)     | (_) (_)    |  Documentation: http://docs.julialang.org
   _ _   _| |_  __ _   |  Type "?help" for help.
  | | | | | | |/ _` |  |
  | | |_| | | | (_| |  |  Version 0.6.0-pre.alpha.193 (2017-03-20 02:33 UTC)
 _/ |\__'_|_|_|\__'_|  |  Commit 28ea9d4 (0 days old master)
|__/                   |  x86_64-apple-darwin16.4.0

julia> reduce(-,2:2)
2

julia> foldl(-,2:2)
2

julia> foldr(-,2:2)
ERROR: BoundsError: attempt to access 1-element UnitRange{Int64} at index [0]
Stacktrace:
 [1] throw_boundserror(::UnitRange{Int64}, ::Int64) at ./abstractarray.jl:426
 [2] getindex at ./range.jl:464 [inlined]
 [3] mapfoldr_impl(::Base.#identity, ::Base.#-, ::Int64, ::UnitRange{Int64}, ::Int64) at ./reduce.jl:110
 [4] mapfoldr(::Base.#identity, ::Function, ::UnitRange{Int64}) at ./reduce.jl:139
 [5] foldr(::Function, ::UnitRange{Int64}) at ./reduce.jl:166

[dlfivefifty]

Note this is related to #20144