`copy!(::AbstractSet, ::AbstractSet)` behaves incorrectly under aliasing (also AbstractDict)

[yurivish]

Mutating copy on abstract sets is implemented as union!(empty!(dst), src):

julia/base/abstractset.jl

Line 6 in 6dfa690

copy!(dst::AbstractSet, src::AbstractSet) = union!(empty!(dst), src)

This fails when copying a Set to itself — arguments are evaluated from left to right and the set is emptied and returned.

julia> s = Set([1, 2, 3])
Set{Int64} with 3 elements:
  2
  3
  1
julia> copy!(s, s)
Set{Int64}()

The same behavior exists for Dicts:

julia/base/abstractdict.jl

Line 158 in 61ae86d

copy!(dst::AbstractDict, src::AbstractDict) = merge!(empty!(dst), src)

julia> d = Dict(1 => 2, 3 => 4)
Dict{Int64, Int64} with 2 entries:
  3 => 4
  1 => 2

julia> copy!(d, d)
Dict{Int64, Int64}()