Inconsistent behaviour of argless Proc pointers from methods

[HertzDevil]

The ->foo literal means two different things depending on where it is applied:

• As an argument to a lib fun, the resulting Proc takes the same arguments as that of the corresponding fun parameter:

  lib L
    fun foo(x : Int32 ->)
  end
  
  def bar(x = 0)
  end
  
  L.foo(->bar) # expands to `->(arg0 : Int32) { bar(arg0) }`

• As an argument to regular defs, the resulting Proc takes no arguments:

  module L
    def self.foo(x : ->)
    end
  
    def self.foo(x : Int32 ->)
    end
  end
  
  def bar(x = 0)
  end
  
  L.foo(->bar) # expands to `-> { bar }`, calls the first overload

This situation is somewhat confusing, so I tend to always specify the argument types to those pointers. But it turns out explicitly providing parentheses to argless pointers is a syntax error:

def bar(x = 0)
end

->bar() # Error: unexpected token: )

The above should be allowed, similar to how super and previous_def accept empty parentheses. (See also #10204.)