From 03d9390b5fb29ebd17131b14cb251c7676c72869 Mon Sep 17 00:00:00 2001
From: Jameson Nash <vtjnash@gmail.com>
Date: Fri, 12 Nov 2021 10:42:42 -0500
Subject: [PATCH] ranges: fix empty length for smallints (#43042)

Fixes #29801

(cherry picked from commit b71330de9847e46ea7033a8267c5a56025ffefdb)
---
 base/range.jl  |  7 ++++++-
 test/ranges.jl | 14 ++++++++++----
 2 files changed, 16 insertions(+), 5 deletions(-)

diff --git a/base/range.jl b/base/range.jl
index 07d9f598d4339..bb5a17f4d4a5a 100644
--- a/base/range.jl
+++ b/base/range.jl
@@ -754,7 +754,12 @@ 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)
diff --git a/test/ranges.jl b/test/ranges.jl
index e0558ff75b0b7..b39dff7506061 100644
--- a/test/ranges.jl
+++ b/test/ranges.jl
@@ -1014,13 +1014,19 @@ 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