Allow Iterators.flatten for empty tuple

[KlausC]

What happened:

$ julia

               _
   _       _ _(_)_     |  Documentation: https://docs.julialang.org
  (_)     | (_) (_)    |
   _ _   _| |_  __ _   |  Type "?" for help, "]?" for Pkg help.
  | | | | | | |/ _` |  |
  | | |_| | | | (_| |  |  Version 1.1.0-DEV.222 (2018-09-09)
 _/ |\__'_|_|_|\__'_|  |  Commit 97864d39ea (0 days old master)
|__/                   |

julia> collect(Iterators.flatten(()))
ERROR: MethodError: Base.IteratorEltype(::Type{Union{}}) is ambiguous. Candidates:
  Base.IteratorEltype(::Type{#s57} where #s57<:Base.Iterators.Repeated) in Base.Iterators at iterators.jl:725
  Base.IteratorEltype(::Type{#s57} where #s57<:Base.Iterators.PartitionIterator{T}) where T in Base.Iterators at iterators.jl:939
  Base.IteratorEltype(::Type{#s560} where #s560<:Base.Broadcast.Broadcasted) in Base.Broadcast at broadcast.jl:250
  Base.IteratorEltype(::Type{#s57} where #s57<:(Base.Iterators.Rest{I,S} where S)) where I in Base.Iterators at iterators.jl:501
Possible fix, define
  Base.IteratorEltype(::Type{Union{}})
Stacktrace:
 [1] _flatteneltype(::Type, ::Base.HasEltype) at ./iterators.jl:877
 [2] Base.IteratorEltype(::Type{Base.Iterators.Flatten{Tuple{}}}) at ./iterators.jl:876
 [3] Base.IteratorEltype(::Base.Iterators.Flatten{Tuple{}}) at ./generator.jl:123
 [4] collect(::Base.Iterators.Flatten{Tuple{}}) at ./array.jl:557
 [5] top-level scope at none:0

I propose, it should be:

$ julia

               _
   _       _ _(_)_     |  Documentation: https://docs.julialang.org
  (_)     | (_) (_)    |
   _ _   _| |_  __ _   |  Type "?" for help, "]?" for Pkg help.
  | | | | | | |/ _` |  |
  | | |_| | | | (_| |  |  Version 1.1.0-DEV.223 (2018-09-10)
 _/ |\__'_|_|_|\__'_|  |  krc/flatten_empty_tuple/046be325bb (fork: 1 commits, 0 days)
|__/                   |

julia> collect(Iterators.flatten(()))
0-element Array{Union{},1}

[JeffBezanson]

This can go in generator.jl along with the other standard methods for IteratorEltype; it's not specific to flatten.

[KlausC]

ok - I will move the line to generator.jl

[JeffBezanson]

Thanks!

[StefanKarpinski]

Should this perhaps have known element type Union{}? I could imagine this pessimizing something.

[JeffBezanson]

eltype(Union{}}) throws an error, so I think this is fine.

[martinholters]

This is unfortunate in combination with

julia/base/generator.jl

Line 123 in 1968b23

IteratorEltype(x) = IteratorEltype(typeof(x))

If x cannot be inferred exactly, we get typeof(x)::Type{<:Something}, which includes Type{Union{}}, although x will of course never be ::Union{}. E.g.

julia> code_warntype(Base.IteratorEltype, Tuple{Array{Int}})
Body::Union{EltypeUnknown, HasEltype}
123 1 ─ %1 = (Base.typeof)(x)::Type{#s55} where #s55<:Array{Int64,N} where N                                    │
    │   %2 = (isa)(%1, Type{Union{}})::Bool                                                                     │
    └──      goto #3 if not %2                                                                                  │
    2 ─      goto #4                                                                                            │
    3 ─ %5 = (Base.IteratorEltype)(%1)::Union{EltypeUnknown, HasEltype}                                         │
    └──      goto #4                                                                                            │
    4 ┄ %7 = φ (#2 => $(QuoteNode(Base.EltypeUnknown())), #3 => %5)::Union{EltypeUnknown, HasEltype}            │
    └──      return %7   

Compare with this method removed:

julia> code_warntype(Base.IteratorEltype, Tuple{Array{Int}})
Body::Base.HasEltype
123 1 ─     return $(QuoteNode(Base.HasEltype()))  

This causes the regression noted in #29464.

[JeffreySarnoff]

fwiw

julia> code_warntype(Base.IteratorEltype, Tuple{Array{Int}})
Body::Base.HasEltype
123 1 ─     return $(QuoteNode(Base.HasEltype()))

julia> versioninfo()
Julia Version 1.0.0
Commit 5d4eaca0c9 (2018-08-08 20:58 UTC)
OS: Linux (x86_64-pc-linux-gnu)
LLVM: libLLVM-6.0.0 (ORCJIT, skylake)

[KristofferC]

Yes, the point here is that there is a regression in 1.0.1 vs 1.0.0.

[JeffBezanson]

This method should be changed to throw an error instead, like eltype.

[JeffBezanson]

Ah, that won't quite work. Instead we can add a special case for flatten when eltype(I) is Union{}.