Fix ProductIterator regression by using custom MaybeValue wrapper

MaybeValue is just a simplified version of Nullable reserved for internal
use which wraps a value or no value in a type-stable way. Information
about whether a value is present or not is carried separately.

base/iterators.jl

@@ -5,7 +5,7 @@ Methods for working with Iterators.
 """
 module Iterators
 
-import Base: start, done, next, isempty, length, size, eltype, iteratorsize, iteratoreltype, axes, ndims, pairs, last, first
+import Base: start, done, next, isempty, length, size, eltype, iteratorsize, iteratoreltype, axes, ndims, pairs, last, first, get
 
 using Base: tail, tuple_type_head, tuple_type_tail, tuple_type_cons, SizeUnknown, HasLength, HasShape,
  IsInfinite, EltypeUnknown, HasEltype, OneTo, @propagate_inbounds, Generator, AbstractRange
@@ -746,17 +746,25 @@ function next(P::ProductIterator, state)
 end
 done(P::ProductIterator, state) = state[1]
 
+struct MaybeValue{T}
+ x::T
+ MaybeValue{T}() where {T} = new{T}()
+ MaybeValue{T}(x::T) where {T} = new{T}(x)
+end
+
+get(v::MaybeValue) = v.x
+
 _prod_start(iterators::Tuple{}) = false, (), ()
 function _prod_start(iterators)
  iter1 = first(iterators)
  state1 = start(iter1)
  d, tailstates, tailnvalues = _prod_start(tail(iterators))
  if done(iter1, state1)
  d = true
- nvalue1 = nothing
+ nvalue1 = MaybeValue{eltype(iter1)}()
  else
  value1, state1 = next(iter1, state1)
- nvalue1 = Some{eltype(iter1)}(value1)
+ nvalue1 = MaybeValue{eltype(iter1)}(value1)
  end
  return (d, (state1, tailstates...), (nvalue1, tailnvalues...))
 end
@@ -767,15 +775,15 @@ function _prod_next(iterators, states, nvalues)
  state1 = first(states)
  if !done(iter1, state1)
  value1, state1 = next(iter1, state1)
- nvalue1 = Some{eltype(iter1)}(value1)
+ nvalue1 = MaybeValue{eltype(iter1)}(value1)
  return false, (state1, tail(states)...), (nvalue1, tail(nvalues)...)
  else
  d, tailstates, tailnvalues = _prod_next(tail(iterators), tail(states), tail(nvalues))
  if d # all iterators are done
- nvalue1 = nothing
+ nvalue1 = MaybeValue{eltype(iter1)}()
  else
  value1, state1 = next(iter1, start(iter1)) # iter cannot be done immediately
- nvalue1 = Some{eltype(iter1)}(value1)
+ nvalue1 = MaybeValue{eltype(iter1)}(value1)
  end
  return d, (state1, tailstates...), (nvalue1, tailnvalues...)
  end