Dictionary-like containers would define get(::ContainerType, key) while we need get(obj, ::Lens). This would introduce ambiguities.

I think I just ran into this ambiguity, but for a different reason (because of method delegation)

using Setfield: @set!

d = Dict(:k => (x=3,))
@set! d[:k].x += 4 # works

using DataStructures: DefaultDict

dd = DefaultDict((x=0,), :k=>(x=3,))
@set! dd[:k].x += 4 # does not:

ERROR: MethodError: get(::DefaultDict{Symbol,NamedTuple{(:x,),Tuple{Int64}},NamedTuple{(:x,),Tuple{Int64}}}, ::Setfield.ComposedLens{Setfield.IndexLens{Tuple{Symbol}},Setfield.PropertyLens{:x}}) is ambiguous. Candidates:
  get(a::DefaultDict, args...) in DataStructures at /Users/goretkin/.julia/packages/DataStructures/iWRBf/src/delegate.jl:21
  get(obj, l::Setfield.ComposedLens) in Setfield at /Users/goretkin/.julia/packages/Setfield/XM37G/src/lens.jl:163
Possible fix, define
  get(::DefaultDict, ::Setfield.ComposedLens)

1.0 release #146

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