Fix method ambiguities introduced by DataStructures.jl (when combined with Setfield.jl)

[tkf]

(This PR is more like an issue report but I wanted to attach a possible solution. I am open to alternative fixes if any.)

It seems that the combination of DataStructures and Setfield introduces some method ambiguities to Base.get. They are between get(::DictType_from_DataStructures.jl, ...) and get(obj, ::Setfield.Lens). I'm posting the issue here because I think it's easier to fix in DataStructures.jl. (cc @jw3126)

You can invoke the ambiguity by, for example

julia> using Setfield, DataStructures

julia> get(MultiDict(:a=>[1]), @lens _[:a])
ERROR: MethodError: get(::MultiDict{Symbol,Int64}, ::Setfield.IndexLens{Tuple{Symbol}}) is ambiguous. Candidates:
  get(obj, l::Setfield.IndexLens) in Setfield at /home/takafumi/.julia/dev/Setfield/src/lens.jl:224
  get(a::MultiDict, args...) in DataStructures at /home/takafumi/.julia/packages/DataStructures/6r6kb/src/delegate.jl:21
Possible fix, define
  get(::MultiDict, ::Setfield.IndexLens)

I think the problem is that the @delegate macro declares the ownership of too many method signatures. It can be fixed by only delegating the 3-arg get method. In this PR, I propose to do this by adding a mapping between the method name and the desired arity like this

DataStructures.jl/src/delegate.jl

Lines 12 to 15 in 3f5c280

	const _arities = Dict(
	:get => 3,
	:get! => 3,
	)

It's not the most elegant solution but it works and extensible (i.e., supporting a new method can be done by just adding a new entry to _arities).

[codecov]

Codecov Report

Merging #511 into master will increase coverage by 3.74%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #511      +/-   ##
==========================================
+ Coverage   89.11%   92.86%   +3.74%     
==========================================
  Files          31       31              
  Lines        2408     2314      -94     
==========================================
+ Hits         2146     2149       +3     
+ Misses        262      165      -97

Impacted Files	Coverage Δ
src/delegate.jl	92.59% <100%> (+0.92%)	⬆️
src/int_set.jl	100% <0%> (+0.79%)	⬆️
src/heaps/minmax_heap.jl	100% <0%> (+0.95%)	⬆️
src/multi_dict.jl	75% <0%> (+1.08%)	⬆️
src/mutable_list.jl	99.32% <0%> (+1.32%)	⬆️
src/list.jl	100% <0%> (+1.53%)	⬆️
src/disjoint_set.jl	98.07% <0%> (+1.85%)	⬆️
src/priorityqueue.jl	98.64% <0%> (+1.95%)	⬆️
src/deque.jl	98.55% <0%> (+2.09%)	⬆️
src/sorted_multi_dict.jl	83.33% <0%> (+3.68%)	⬆️
... and 8 more

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[jw3126]

Solutions on the Setfield side I can think of would be:

• Rename get. But I fear there is no name, which is as good as get.
• Switch the argument order of get (and set and modify for consistency) urghs