fix #30643, correctly propagate iterator traits through Stateful (#30644

)

base/iterators.jl

@@ -1094,10 +1094,9 @@ end
 
 @inline peek(s::Stateful, sentinel=nothing) = s.nextvalstate !== nothing ? s.nextvalstate[1] : sentinel
 @inline iterate(s::Stateful, state=nothing) = s.nextvalstate === nothing ? nothing : (popfirst!(s), nothing)
-IteratorSize(::Type{Stateful{VS,T}} where VS) where {T} =
-    isa(IteratorSize(T), SizeUnknown) ? SizeUnknown() : HasLength()
+IteratorSize(::Type{Stateful{T,VS}}) where {T,VS} = IteratorSize(T) isa HasShape ? HasLength() : IteratorSize(T)
 eltype(::Type{Stateful{T, VS}} where VS) where {T} = eltype(T)
-IteratorEltype(::Type{Stateful{VS,T}} where VS) where {T} = IteratorEltype(T)
+IteratorEltype(::Type{Stateful{T,VS}}) where {T,VS} = IteratorEltype(T)
 length(s::Stateful) = length(s.itr) - s.taken
 
 end

test/iterators.jl

@@ -549,3 +549,27 @@ end
     @test ps isa Iterators.Pairs
     @test collect(ps) == [1 => :a, 2 => :b]
 end
+
+@testset "Stateful fix #30643" begin
+    @test Base.IteratorSize(1:10) isa Base.HasShape
+    a = Iterators.Stateful(1:10)
+    @test Base.IteratorSize(a) isa Base.HasLength
+    @test length(a) == 10
+    @test length(collect(a)) == 10
+    @test length(a) == 0
+    b = Iterators.Stateful(Iterators.take(1:10,3))
+    @test Base.IteratorSize(b) isa Base.HasLength
+    @test length(b) == 3
+    @test length(collect(b)) == 3
+    @test length(b) == 0
+    c = Iterators.Stateful(Iterators.countfrom(1))
+    @test Base.IteratorSize(c) isa Base.IsInfinite
+    @test length(Iterators.take(c,3)) == 3
+    @test length(collect(Iterators.take(c,3))) == 3
+    d = Iterators.Stateful(Iterators.filter(isodd,1:10))
+    @test Base.IteratorSize(d) isa Base.SizeUnknown
+    @test length(collect(Iterators.take(d,3))) == 3
+    @test length(collect(d)) == 2
+    @test length(collect(d)) == 0
+end
+