make vector cmp match axes (#59500)

closes #48916 as suggested by
@oscardssmith
> I believe the more consistent behavior here would be for isless to
error when comparing two Arrays a and b where
eachindex(a,b)∉(eachindex(a), eachindex(b)) (similarly to how it works
with Complex numbers or other collections like Dict that don't have an
ordering.

Author: adienes

base/abstractarray.jl

@@ -3020,6 +3020,8 @@ function isequal(A::AbstractArray, B::AbstractArray)
 end
 
 function cmp(A::AbstractVector, B::AbstractVector)
+    ai1, bi1 = firstindex(A), firstindex(B)
+    isequal(ai1, bi1) || return cmp(ai1, bi1)
     for (a, b) in zip(A, B)
         if !isequal(a, b)
             return isless(a, b) ? -1 : 1
@@ -3040,7 +3042,8 @@ end
 """
     isless(A::AbstractVector, B::AbstractVector)
 
-Return `true` when `A` is less than `B` in lexicographic order.
+Return `true` when `A` is less than `B`. Vectors are first compared by
+their starting indices, and then lexicographically by their elements.
 """
 isless(A::AbstractVector, B::AbstractVector) = cmp(A, B) < 0
 

test/arrayops.jl

@@ -1465,6 +1465,18 @@ end
     @test cmp([UInt8(1), UInt8(0)], [UInt8(0), UInt8(0)]) == 1
     @test cmp([UInt8(1), UInt8(0)], [UInt8(1), UInt8(0)]) == 0
     @test cmp([UInt8(0), UInt8(0)], [UInt8(1), UInt8(1)]) == -1
+
+    x = [1, 2, 3]
+    y = OffsetVector(x, -1)
+    @test cmp(x, y) == 1
+    @test cmp(y, x) == -1
+    @test !isless(x, y)
+    @test isless(y, x)
+
+    y2 = OffsetVector([1, 2, 3], 0)
+    @test cmp(x, y2) == 0
+    @test !isless(x, y2)
+    @test !isless(y2, x)
 end
 
 @testset "sort on arrays" begin