Define eltype(::Type{Union{}}) = Union{}?

[TotalVerb]

Currently, eltype(Union{}) throws a method ambiguity error. Since there is nothing of type Union{}, it is natural that are no objects that are elements of objects of type Union{}, so the definition eltype(::Type{Union{}}) = Union{} makes sense.

[Keno]

The problem you run into is that your inferred result becomes Union{Union{},Foo}, whereas an error collapses the union.

[TotalVerb]

This seems to be a change from 0.5. I came across it while trying to get Lint to work on 0.6.

0.5:

julia> eltype(Union{})
Union{}

0.6:

julia> eltype(Union{})
ERROR: MethodError: eltype(::Type{Union{}}) is ambiguous. Candidates:
  eltype{E,T<:Tuple{Vararg{E,N} where N}}(::Type{T}) in Base at tuple.jl:52
  eltype{E,A<:(AbstractArray{E,N} where N)}(::Type{A}) in Base
  eltype{T<:AbstractString}(::Type{T}) in Base at strings/basic.jl:54
  eltype{T<:Base.Dates.Period}(::Type{T}) in Base.Dates at dates/types.jl:238
  eltype{T<:Number}(::Type{T}) in Base at number.jl:28

@Keno, I don't understand what you mean.

[Keno]

julia> Base.return_types(eltype, Tuple{(Type{T} where T<:(Array{Int64,N} where N))})
1-element Array{Any,1}:
 Type{Int64}

julia> Base.eltype(::Type{Union{}}) = Union{}

julia> Base.return_types(eltype, Tuple{(Type{T} where T<:(Array{Int64,N} where N))})
2-element Array{Any,1}:
 Type{Union{}}
 Type{Int64}

[TotalVerb]

OK, so this change would pessimize type inference in certain situations, and was therefore intentional?

[JeffBezanson]

It's a koan: "if you have no object at all, is it iterable?"

I would say no.

[timholy]

We should get into the habit of implementing these policy decisions via informative error messages. As it is now, this is an ambiguity error which leaves the intentionality...ambiguous. See #20568.