From eef0324b146d791aa3ac5258b40bbad4d6875919 Mon Sep 17 00:00:00 2001
From: Andy Dienes <andydienes@gmail.com>
Date: Wed, 5 Jul 2023 22:33:23 -0400
Subject: [PATCH] add length asserts to reverse(::Zip)

---
 base/iterators.jl | 17 ++++++++++++++++-
 test/iterators.jl |  4 ++++
 2 files changed, 20 insertions(+), 1 deletion(-)

diff --git a/base/iterators.jl b/base/iterators.jl
index 11e94d3384de80..9f6f8ae40c5dc9 100644
--- a/base/iterators.jl
+++ b/base/iterators.jl
@@ -387,6 +387,16 @@ function _zip_min_length(is)
     end
 end
 _zip_min_length(is::Tuple{}) = nothing
+function _zip_lengths_finite_equal(is)
+    i = is[1]
+    b, n = _zip_lengths_finite_equal(tail(is))
+    if IteratorSize(i) isa Union{IsInfinite, SizeUnknown}
+        return (false, nothing)
+    else
+        return (b && (n === nothing || n == length(i)), length(i))
+    end
+end
+_zip_lengths_finite_equal(is::Tuple{}) = (true, nothing)
 size(z::Zip) = _promote_tuple_shape(Base.map(size, z.is)...)
 axes(z::Zip) = _promote_tuple_shape(Base.map(axes, z.is)...)
 _promote_tuple_shape((a,)::Tuple{OneTo}, (b,)::Tuple{OneTo}) = (intersect(a, b),)
@@ -468,8 +478,13 @@ zip_iteratoreltype() = HasEltype()
 zip_iteratoreltype(a) = a
 zip_iteratoreltype(a, tail...) = and_iteratoreltype(a, zip_iteratoreltype(tail...))
 
-reverse(z::Zip) = Zip(Base.map(reverse, z.is)) # n.b. we assume all iterators are the same length
 last(z::Zip) = getindex.(z.is, minimum(Base.map(lastindex, z.is)))
+function reverse(z::Zip)
+    if !first(_zip_lengths_finite_equal(z.is))
+        throw(ArgumentError("Zipped iterators of unknown, infinite, or unequal lengths must be collected before reversing"))
+    end
+    Zip(Base.map(reverse, z.is))
+end
 
 # filter
 
diff --git a/test/iterators.jl b/test/iterators.jl
index 59588bdac96840..5f74f47a749fc6 100644
--- a/test/iterators.jl
+++ b/test/iterators.jl
@@ -11,6 +11,10 @@ using Dates: Date, Day
 # issue #4718
 @test collect(Iterators.filter(x->x[1], zip([true, false, true, false],"abcd"))) == [(true,'a'),(true,'c')]
 
+# issue #45085
+@test_throws ArgumentError Iterators.reverse(zip("abc", "abcd"))
+@test_throws ArgumentError Iterators.reverse(zip("abc", Iterators.cycle("ab")))
+
 let z = zip(1:2)
     @test size(z) == (2,)
     @test collect(z) == [(1,), (2,)]