Corner case of skipmissing

[bkamins]

This is a minor thing, but I guess we do not want to have dispatch ambiguities in Base:

julia> sum(skipmissing([missing]))
ERROR: MethodError: reduce_empty(::typeof(Base.add_sum), ::Type{Union{}}) is ambiguous. Candidates:
  reduce_empty(::typeof(Base.add_sum), ::Type{T}) where T<:Union{Int16, Int32, Int8} in Base at reduce.jl:314
  reduce_empty(::typeof(Base.add_sum), ::Type{T}) where T<:Union{UInt16, UInt32, UInt8} in Base at reduce.jl:315
Possible fix, define
  reduce_empty(::typeof(Base.add_sum), ::Type{Union{}})

CC @nalimilan

[tkf]

#35843 will fix this

[bkamins]

It is interesting how things go in parallel in the same time. We are cleaning up reduce code for GroupedDataFrame in DataFrames.jl and the same issues pop-up 😄.

[tkf]

closed by #35843