Product iterator is not zipped properly

[aquohn]

When zipping with a product iterator with no arguments (which produces only one output, the empty tuple), it seems the check for zip completion does not handle this case:

julia> for (z,) in zip(Iterators.product())
       @show z
       end
z = ()
ERROR: ArgumentError: tuple must be non-empty
Stacktrace:
 [1] first(#unused#::Tuple{})
   @ Base ./tuple.jl:140
 [2] _pisdone
   @ ./iterators.jl:1005 [inlined]
 [3] isdone
   @ ./iterators.jl:1012 [inlined]
 [4] _zip_isdone(is::Tuple{Base.Iterators.ProductIterator{Tuple{}}}, ss::Tuple{Tuple{Bool}})
   @ Base.Iterators ./iterators.jl:379
 [5] _zip_iterate_all
   @ ./iterators.jl:348 [inlined]
 [6] iterate
   @ ./iterators.jl:341 [inlined]
 [7] top-level scope
   @ ./REPL[79]:2

versioninfo() output:

Julia Version 1.7.1
Commit ac5cc99908 (2021-12-22 19:35 UTC)
Platform Info:
  OS: Linux (x86_64-pc-linux-gnu)
  CPU: Intel(R) Core(TM) i7-8565U CPU @ 1.80GHz
  WORD_SIZE: 64
  LIBM: libopenlibm
  LLVM: libLLVM-12.0.1 (ORCJIT, skylake)
Environment:
  JULIA_NUM_THREADS = 4
  JULIA_EDITOR = vim

EDIT: zip([()]) works fine though:

julia> for (z,) in zip([()])
       @show z
       end
z = ()

julia> 

[JeffBezanson]

I think we just need this:

isdone(::Iterators.ProductIterator{Tuple{}}, states) = true