Make IteratorSize(::Iterators.Cycle) not always IsInfinite add to docstring as example

base/generator.jl

@@ -89,6 +89,12 @@ Base.HasShape{1}()
 
 julia> Base.IteratorSize((2,3))
 Base.HasLength()
+
+julia> Base.IteratorSize(Iterators.cycle(1:5))
+Base.SizeUnknown()
+
+julia> Base.IteratorSize(Iterators.cycle(1:0))
+Base.SizeUnknown()
 ```
 """
 IteratorSize(x) = IteratorSize(typeof(x))

base/iterators.jl

@@ -1017,7 +1017,10 @@ cycle(xs, n::Integer) = flatten(repeated(xs, n))
 
 eltype(::Type{Cycle{I}}) where {I} = eltype(I)
 IteratorEltype(::Type{Cycle{I}}) where {I} = IteratorEltype(I)
-IteratorSize(::Type{Cycle{I}}) where {I} = IsInfinite() # XXX: this is false if iterator ever becomes empty
+function IteratorSize(::Type{Cycle{I}}) where {I}
+    IS = IteratorSize(I)
+    (IS === IsInfinite() || IS == HasShape{0}()) ? IsInfinite() : SizeUnknown()
+end
 
 iterate(it::Cycle) = iterate(it.xs)
 isdone(it::Cycle) = isdone(it.xs)

test/iterators.jl

@@ -1221,6 +1221,18 @@ end
     @test isempty(Docs.undocumented_names(Iterators))
 end
 
+@testset "cycle IteratorSize (#53169)" begin
+    @test Base.IteratorSize(Iterators.cycle(1:3)) == Base.SizeUnknown()
+    @test Base.IteratorSize(Iterators.cycle(1:0)) == Base.SizeUnknown()
+    @test Base.IteratorSize(Iterators.cycle(7)) == Base.IsInfinite()
+    @test collect(Iterators.cycle(1:0))::Vector{Int} == Int[]
+    @test Base.IteratorSize(Iterators.cycle(Iterators.Repeated(6))) == Base.IsInfinite()
+
+    @test Base.IteratorSize(typeof(Iterators.cycle(1:3))) == Base.SizeUnknown()
+    @test Base.IteratorSize(typeof(Iterators.cycle(1:0))) == Base.SizeUnknown()
+    @test Base.IteratorSize(typeof(Iterators.cycle(Iterators.Repeated(6)))) == Base.IsInfinite()
+end
+
 # Filtered list comprehension (`Filter` construct) type inference
 @test Base.infer_return_type((Vector{Any},)) do xs
     [x for x in xs if x isa Int]