Flag offence for empty parentheses in single line method definitions

[samrjenkins]

Problem

Rubocop allows single line method definitions to use empty parentheses.
Eg.

def method_1() 'foo'; end
def method_2() = 'bar'

Though this behaviour has been part of Rubocop for many years, as far as I can see it is contrary to the style guide which expressly instructs against using parentheses in a method definition when there are no parameters:

Method Definition Parentheses

Use def with parentheses when there are parameters. Omit the parentheses when the method doesn’t accept any parameters.

Link: Method Definition Parentheses

I have been unable to find an exception to this rule for single line methods anywhere in the guide (though I might be missing it).

Solution

Style/DefWithParentheses flags offences for single line method definitions with parentheses which do not accept parameters

Alternatives

I have been unable to discover the rationale behind originally allowing empty parentheses in this commit. If it is behaviour we want to retain, perhaps there should be a mention of it in the style guide (assuming there isn't one already which I have been unable to find).

[koic]

def foo() do_something end will continue to allow because def foo do_something end is a syntax error.
On the other hand, be aware that the endless method definition's parentheses can be omitted. I've opened #10421.