linspace with 1 or 0 elements is sorted (#15245)

* ranges with 1 or 0 elements are sorted regardless of step sign

* dates ranges of length <= 1 are sorted

base/range.jl

@@ -883,7 +883,7 @@ reverse(r::LinSpace) = LinSpace(r.stop, r.start, r.len, r.divisor)
 ## sorting ##
 
 issorted(r::AbstractUnitRange) = true
-issorted(r::Range) = step(r) >= zero(step(r))
+issorted(r::Range) = length(r) <= 1 || step(r) >= zero(step(r))
 
 sort(r::AbstractUnitRange) = r
 sort!(r::AbstractUnitRange) = r

test/dates/ranges.jl

@@ -77,8 +77,8 @@ function test_all_combos()
  @test length(reverse(dr)) == 0
  @test first(reverse(dr)) > l1
  @test last(reverse(dr)) <= l1
- @test !issorted(dr)
- @test sortperm(dr) == 0:-1:1
+ @test issorted(dr)
+ @test sortperm(dr) == 1:1:0
  @test !(l1 in dr)
  @test !(l1 in dr)
  @test !(l1-neg_step in dr)
@@ -106,7 +106,7 @@ function test_all_combos()
  end
  @test !isempty(reverse(dr))
  @test length(reverse(dr)) == len
- @test !issorted(dr)
+ @test issorted(dr) == (len <= 1)
  @test l in dr
  end
  end
@@ -182,8 +182,8 @@ function test_all_combos()
  @test length(reverse(dr)) == 0
  @test first(reverse(dr)) > l1
  @test last(reverse(dr)) <= l1
- @test !issorted(dr)
- @test sortperm(dr) == 0:-1:1
+ @test issorted(dr)
+ @test sortperm(dr) == 1:1:0
  @test !(l1 in dr)
  @test !(l1 in dr)
  @test !(l1-neg_step in dr)
@@ -212,7 +212,7 @@ function test_all_combos()
  @test !isempty(reverse(dr))
  @test length(reverse(dr)) == len
  @test last(reverse(dr)) >= l
- @test !issorted(dr)
+ @test issorted(dr) == (len <= 1)
  @test l in dr
 
  end

test/ranges.jl

@@ -391,6 +391,10 @@ for T = (Float32, Float64)
  end
 end
 
+# linspace with 1 or 0 elements (whose step length is NaN)
+@test issorted(linspace(1,1,0))
+@test issorted(linspace(1,1,1))
+
 # near-equal ranges
 @test 0.0:0.1:1.0 != 0.0f0:0.1f0:1.0f0