ranges: fix empty length for smallints (JuliaLang#43042)

Fixes JuliaLang#29801

base/range.jl

@@ -798,7 +798,12 @@ let smallints = (Int === Int64 ?
                 Union{Int8, UInt8, Int16, UInt16})
     global length, checked_length
     # n.b. !(step isa T)
-    length(r::OrdinalRange{<:smallints}) = div(Int(last(r)) - Int(first(r)), step(r)) + 1
+    function length(r::OrdinalRange{<:smallints})
+        s = step(r)
+        s == zero(s) && return 0 # unreachable, by construction, but avoids the error case here later
+        isempty(r) && return 0
+        return div(Int(last(r)) - Int(first(r)), s) + 1
+    end
     length(r::AbstractUnitRange{<:smallints}) = Int(last(r)) - Int(first(r)) + 1
     length(r::OneTo{<:smallints}) = Int(r.stop)
     checked_length(r::OrdinalRange{<:smallints}) = length(r)

test/ranges.jl

@@ -1049,13 +1049,19 @@ end
     @test length(map(identity, UInt64(1):UInt64(5))) == 5
     @test length(map(identity, UInt128(1):UInt128(5))) == 5
 end
-@testset "issue #8531" begin
+@testset "issue #8531, issue #29801" begin
     smallint = (Int === Int64 ?
-                (Int8,UInt8,Int16,UInt16,Int32,UInt32) :
-                (Int8,UInt8,Int16,UInt16))
+                (Int8, UInt8, Int16, UInt16, Int32, UInt32) :
+                (Int8, UInt8, Int16, UInt16))
     for T in smallint
         s = typemin(T):typemax(T)
-        @test length(s) == checked_length(s) == 2^(8*sizeof(T))
+        @test length(s) === checked_length(s) === 2^(8*sizeof(T))
+        s = T(10):typemax(T):T(10)
+        @test length(s) === checked_length(s) === 1
+        s = T(10):typemax(T):T(0)
+        @test length(s) === checked_length(s) === 0
+        s = T(10):typemax(T):typemin(T)
+        @test length(s) === checked_length(s) === 0
     end
 end