Naming conflicts

Differents namespaces could end up having the same class name which would conflict in some contexts.

For example, in the context of a module, the built-in class Socket could conflicts with the custom class MyNetwork::Socket.

require "socket"

module MyNetwork
  def self.test(addr : String)
    Socket::IPAddress.new(addr, 8080)
  end

  class Socket
    # something else
  end
end

MyNetwork.test("127.0.0.1")

Hence, in this context inside MyNetwork.test, the programm would think you are trying to call the local MyNetwork::Socket class while you are in fact trying to call the top-level Socket class. Since MyNetwork::Socket::IPAddress doesn't exist, you'll face this error:

Showing last frame. Use --error-trace for full trace.

In test.cr:5:5

 5 | Socket::IPAddress.new(addr, 8080)
     ^----------------
Error: undefined constant Socket::IPAddress

Because, in a local context both MyNetwork::Socket and Socket would call the custom class, you have to call ::Socket in order to explicitely use the built-in class. The :: prefix denotes it should use the top level type and not the local scope one.