Iterators.cycle, IteratorSize: infinite but empty iterator

[nsajko]

The IteratorSize doc string says that if its return value is IsInfinite(), "the iterator yields values forever". The iterator returned by Iterators.cycle is documented to be empty in some cases, even though it seems to always be an infinite iterator currently:

julia> it = Iterators.cycle(())
Base.Iterators.Cycle{Tuple{}}(())

julia> isempty(it)
true

julia> iterate(it)

julia> Base.IteratorSize(typeof(it))
Base.IsInfinite()

julia> versioninfo()
Julia Version 1.11.0-DEV.1451
Commit f117a500ca9 (2024-02-01 15:38 UTC)
Build Info:
  Official https://julialang.org/ release
Platform Info:
  OS: Linux (x86_64-linux-gnu)
  CPU: 8 × AMD Ryzen 3 5300U with Radeon Graphics
  WORD_SIZE: 64
  LLVM: libLLVM-16.0.6 (ORCJIT, znver2)
Threads: 1 default, 0 interactive, 1 GC (on 8 virtual cores)

Not sure what's a good fix, perhaps make Iterators.cycle return () when it's given an empty iterator as input. This wouldn't always be type stable, though.

[Seelengrab]

In a similar vein (though not a duplicate): #43235

There too the IteratorSize won't match the actual iterator length.