support isless for zero-dimensional AbstractArrays (#55772)

Fixes #55771

NEWS.md

@@ -117,6 +117,7 @@ New library features
 * `RegexMatch` objects can now be used to construct `NamedTuple`s and `Dict`s ([#50988])
 * `Lockable` is now exported ([#54595])
 * New `ltruncate`, `rtruncate` and `ctruncate` functions for truncating strings to text width, accounting for char widths ([#55351])
+* `isless` (and thus `cmp`, sorting, etc.) is now supported for zero-dimensional `AbstractArray`s ([#55772])
 
 Standard library changes
 ------------------------

base/abstractarray.jl

@@ -3044,6 +3044,15 @@ function cmp(A::AbstractVector, B::AbstractVector)
     return cmp(length(A), length(B))
 end
 
+"""
+    isless(A::AbstractArray{<:Any,0}, B::AbstractArray{<:Any,0})
+
+Return `true` when the only element of `A` is less than the only element of `B`.
+"""
+function isless(A::AbstractArray{<:Any,0}, B::AbstractArray{<:Any,0})
+    isless(only(A), only(B))
+end
+
 """
     isless(A::AbstractVector, B::AbstractVector)
 

test/arrayops.jl

@@ -1409,6 +1409,14 @@ end
 end
 
 @testset "lexicographic comparison" begin
+    @testset "zero-dimensional" begin
+        vals = (0, 0.0, 1, 1.0)
+        for l ∈ vals
+            for r ∈ vals
+                @test cmp(fill(l), fill(r)) == cmp(l, r)
+            end
+        end
+    end
     @test cmp([1.0], [1]) == 0
     @test cmp([1], [1.0]) == 0
     @test cmp([1, 1], [1, 1]) == 0