Method ambiguity in _promote when promoting x::T to T if T <: PooledVector

[nickrobinson251]

julia> using CSV

julia> aa = IOBuffer("""
       a,b
       a,2
       a,3
       """)
IOBuffer(data=UInt8[...], readable=true, writable=false, seekable=true, append=false, size=12, maxsize=Inf, ptr=1, mark=-1)

julia> bb = IOBuffer("""
       a,b
       ab,22
       ab,33
       """)
IOBuffer(data=UInt8[...], readable=true, writable=false, seekable=true, append=false, size=16, maxsize=Inf, ptr=1, mark=-1)

julia> CSV.File([aa, bb])
ERROR: MethodError: _promote(::Type{PooledArrays.PooledVector{String3, UInt32, Vector{UInt32}}}, ::PooledArrays.PooledVector{String3, UInt32, Vector{UInt32}}) is ambiguous. Candidates:
  _promote(::Type{A}, x::A) where A in CSV at /Users/nick/.julia/packages/CSV/owrEo/src/utils.jl:171
  _promote(::Type{PooledArrays.PooledVector{T, R, RA}}, x) where {T, R, RA} in CSV at /Users/nick/.julia/packages/CSV/owrEo/src/utils.jl:182
Possible fix, define
  _promote(::Type{PooledArrays.PooledVector{T, R, RA}}, ::PooledArrays.PooledVector{T, R, RA}) where {T, R, RA}
Stacktrace:
 [1] chaincolumns!(a::Any, b::Any)
   @ CSV ~/.julia/packages/CSV/owrEo/src/utils.jl:206
 [2] CSV.File(sources::Vector{IOBuffer}; source::Nothing, kw::Base.Iterators.Pairs{Union{}, Union{}, Tuple{}, NamedTuple{(), Tuple{}}})
   @ CSV ~/.julia/packages/CSV/owrEo/src/file.jl:1010
 [3] CSV.File(sources::Vector{IOBuffer})
   @ CSV ~/.julia/packages/CSV/owrEo/src/file.jl:985
 [4] top-level scope
   @ REPL[19]:1

(jl_weYOTi) pkg> st
      Status `/private/var/folders/hx/1h0bbkfd18d4n1qrnwmrl4j00000gn/T/jl_weYOTi/Project.toml`
  [336ed68f] CSV v0.9.6

[nickrobinson251]

little reproducer

julia> b_col = PooledArray{String3}(["bb", "bb"]);

julia> CSV._promote(typeof(b_col), b_col)
ERROR: MethodError: _promote(::Type{PooledVector{String3, UInt32, Vector{UInt32}}}, ::PooledVector{String3, UInt32, Vector{UInt32}}) is ambiguous. Candidates:
...

we're hitting this branch:

CSV.jl/src/utils.jl

Lines 202 to 206 in 1f7a552

	# both arrays are non-missing, but not same eltype, just need to promote
	P = pooledtype(promote_types(eltype(a), eltype(b)))
	end
	px = _promote(P, a)
	py = _promote(P, b)

i'll open a PR adding the missing method