Merge pull request JuliaLang#30215 from blegat/ReverseEmptyStepRangeLen

Fix reverse with empty StepRangeLen

base/range.jl

@@ -941,7 +941,13 @@ Array{T,1}(r::AbstractRange{T}) where {T} = vcat(r)
 collect(r::AbstractRange) = vcat(r)
 
 reverse(r::OrdinalRange) = (:)(last(r), -step(r), first(r))
-reverse(r::StepRangeLen) = StepRangeLen(r.ref, -r.step, length(r), length(r)-r.offset+1)
+function reverse(r::StepRangeLen)
+ # If `r` is empty, `length(r) - r.offset + 1 will be nonpositive hence
+ # invalid. As `reverse(r)` is also empty, any offset would work so we keep
+ # `r.offset`
+ offset = isempty(r) ? r.offset : length(r)-r.offset+1
+ StepRangeLen(r.ref, -r.step, length(r), offset)
+end
 reverse(r::LinRange) = LinRange(r.stop, r.start, length(r))
 
 ## sorting ##

test/ranges.jl

@@ -1435,3 +1435,13 @@ end
  # require a keyword arg
  @test_throws ArgumentError range(1, 100)
 end
+
+@testset "Reverse empty ranges" begin
+ @test reverse(1:0) === 0:-1:1
+ @test reverse(Base.OneTo(0)) === 0:-1:1
+ # Almost `1.0:-1.0:2.0`, only different is the step which is
+ # `Base.TwicePrecision(-1.0, 0.0)`
+ @test reverse(1.0:0.0) === StepRangeLen(Base.TwicePrecision(1.0, 0.0),
+ Base.TwicePrecision(-1.0, -0.0), 0)
+ @test reverse(reverse(1.0:0.0)) === 1.0:0.0
+end