Permit the construction of a 1D StepRangeLen of CartesianIndexes (#…

…50457)

base/multidimensional.jl

@@ -83,6 +83,7 @@ module IteratorsMD
     CartesianIndex{N}(index::Integer...) where {N} = CartesianIndex{N}(index)
     CartesianIndex{N}() where {N} = CartesianIndex{N}(())
     # Un-nest passed CartesianIndexes
+    CartesianIndex{N}(index::CartesianIndex{N}) where {N} = index
     CartesianIndex(index::Union{Integer, CartesianIndex}...) = CartesianIndex(flatten(index))
     flatten(::Tuple{}) = ()
     flatten(I::Tuple{Any}) = Tuple(I[1])
@@ -166,6 +167,19 @@ module IteratorsMD
     Base.iterate(::CartesianIndex) =
         error("iteration is deliberately unsupported for CartesianIndex. Use `I` rather than `I...`, or use `Tuple(I)...`")
 
+    # ranges are deliberately disabled to prevent ambiguities with the colon constructor
+    Base.range_start_step_length(start::CartesianIndex, step::CartesianIndex, len::Integer) =
+        error("range with a specified length is deliberately unsupported for CartesianIndex arguments."*
+            " Use StepRangeLen($start, $step, $len) to construct this range")
+
+    # show is special-cased to avoid the start:stop:step display,
+    # which constructs a CartesianIndices
+    # See #50784
+    function show(io::IO, r::StepRangeLen{<:CartesianIndex})
+        print(io, "StepRangeLen(", first(r), ", ",
+                    step(r), ", ", length(r), ")")
+    end
+
     # Iteration
     const OrdinalRangeInt = OrdinalRange{Int, Int}
     """

test/ranges.jl

@@ -2558,3 +2558,24 @@ end
     a = StepRangeLen(1,2,3,2)
     @test a[UInt(1)] == -1
 end
+
+@testset "StepRangeLen of CartesianIndex-es" begin
+    CIstart = CartesianIndex(2,3)
+    CIstep = CartesianIndex(1,1)
+    r = StepRangeLen(CIstart, CIstep, 4)
+    @test length(r) == 4
+    @test first(r) == CIstart
+    @test step(r) == CIstep
+    @test last(r) == CartesianIndex(5,6)
+    @test r[2] == CartesianIndex(3,4)
+
+    @test repr(r) == "StepRangeLen($CIstart, $CIstep, 4)"
+
+    r = StepRangeLen(CartesianIndex(), CartesianIndex(), 3)
+    @test all(==(CartesianIndex()), r)
+    @test length(r) == 3
+    @test repr(r) == "StepRangeLen(CartesianIndex(), CartesianIndex(), 3)"
+
+    errmsg = ("deliberately unsupported for CartesianIndex", "StepRangeLen")
+    @test_throws errmsg range(CartesianIndex(1), step=CartesianIndex(1), length=3)
+end