Strange Proc return type behavior

[Blacksmoke16]

https://play.crystal-lang.org/#/r/6bxj

require "json"

class Base
end

module Foo
  def to_json(val : Int32)
    1
  end
end

class Model1 < Base
  include Foo  
end

class Model2 < Base
  include Foo  
end

proc = ->(key : String) do
  if key == "1"
    Model1.new
  else
    Model2.new
  end
end

r = proc.call "1"

pp r.to_json 8

Error in line 30: no overload matches 'Base#to_json' with type Int32
Overloads are:
 - Object#to_json(io : IO)
 - Object#to_json()

Few thing going on here.

1. Making class Base abstract fixes the problem.
2. Removing the require "json" changes the error, but should still work.
3. Having the proc only return one type fixes the problem.

[asterite]

Yes, this is expected behavior. Please read this.

[Blacksmoke16]

Oh fair enough.