Make IteratorSize(::Iterators.Cycle) not always IsInfinite and document why

base/generator.jl

@@ -83,12 +83,24 @@ This means that most iterators are assumed to implement [`length`](@ref).
 This trait is generally used to select between algorithms that pre-allocate space for their
 result, and algorithms that resize their result incrementally.
 
+Most iterators' `IteratorSize` category can be determined from their type. In this case
+the generic `IteratorSize(iterator) = IteratorSize(typeof(iterator))` fallback applies.
+However, some iterators (e.g. [`Iterators.cycle`](@ref)) have a size category that can only
+be determined at runtime. In this case the `IteratorSize(itr)` method will not be type
+stable and the `IteratorSize(::Type)` method will return `SizeUnknown`.
+
 ```jldoctest
 julia> Base.IteratorSize(1:5)
 Base.HasShape{1}()
 
 julia> Base.IteratorSize((2,3))
 Base.HasLength()
+
+julia> Base.IteratorSize(Iterators.cycle(1:5))
+Base.IsInfinite()
+
+julia> Base.IteratorSize(Iterators.cycle(1:0))
+Base.HasLength()
 ```
 """
 IteratorSize(x) = IteratorSize(typeof(x))

base/iterators.jl

@@ -988,7 +988,13 @@ 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
+    # TODO: find a better way of communicating the size of a cycle
+    # IsInfinite() would be false if iterator ever becomes empty
+    IteratorSize(I) === IsInfinite() ? IsInfinite() : SizeUnknown()
+end
+IteratorSize(it::Cycle) = isempty(it.xs) ? HasLength() : IsInfinite()
+length(it::Cycle) = isempty(it.xs) ? 0 : throw(ArgumentError("Cannot compute length of infinite iterator"))
 
 iterate(it::Cycle) = iterate(it.xs)
 isdone(it::Cycle) = isdone(it.xs)

test/iterators.jl

@@ -1049,3 +1049,15 @@ end
 @testset "Iterators docstrings" begin
     @test isempty(Docs.undocumented_names(Iterators))
 end
+
+@testset "cycle IteratorSize (#53169)" begin
+    @test Base.IteratorSize(Iterators.cycle(1:3)) == Base.IsInfinite()
+    @test Base.IteratorSize(Iterators.cycle(1:0)) == Base.HasLength()
+    @test length(Iterators.cycle(1:0)) == 0
+    @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