What triggers a "must be explicitly imported" error?

General Usage
1

Let’s say you have a couple example files:

# woof.jl

function println(foo::Module, bar::AbstractString)
  "cool. cool. alright"
end

println() = println(Main, "huh?")

# bark.jl

println(a::Float64, b::Integer, c::String) = println(Main, c)

You now want to load them into a module, Moo.jl:

# moo.jl

module Moo
  cur_array = []

  cur_files = ["woof.jl", "bark.jl"]

  # for cur_file in cur_files
  #   include(cur_file)
  # end

  for cur_file in cur_files
    open_file = open(cur_file)
    cur_expr = parse(open_file)
    while cur_expr != nothing
      push!(cur_array, cur_expr)
      cur_expr = parse(open_file)
    end
    close(open_file)
  end

  for cur_expr in cur_array
    Moo.eval(cur_expr)
  end
end

Why does this method of loading the files trigger an “explicitly define” error:

ERROR: LoadError: error in method definition: function Base.println must be explicitly imported to be extended

Whereas using the standard include(cur_file) work perfectly?

// edit: println could have just as well been print, read, countlines, etc.

2

If I copy the content into those filenames and do include("moo.jl") it works fine for me on 0.6.0 (and fails for unrelated reasons on trunk – 24455).