Tell us about your environment

• ESLint Version:
  v4.6.1
• Node Version:
  v8.3.0
• npm Version:
  v5.3.0

What parser (default, Babel-ESLint, etc.) are you using?
babel-eslint

Please show your full configuration:

// relevant section:
"function-paren-newline" : ["error", "multiline"],

What did you do? Please include the actual source code causing the issue.

function foo(
  param1
)

// or

foo(
  "some very long argument that I'd love to have on it's own line"
);

What did you expect to happen?

No errors

What actually happened? Please include the actual, raw output from ESLint.

    1:13  error  Unexpected newline after '('                                 function-paren-newline
    3:1   error  Unexpected newline before ')'                                function-paren-newline
    7:4   error  Unexpected newline after '('                                 function-paren-newline
    9:1   error  Unexpected newline before ')'                                function-paren-newline

Some elaboration: with the multiline setting it is only allowed to have newlines in function parenthesis if there are at least 2 arguments. I would expect it to also accept newlines in function parenthesis for single arguments as well (as long as it's done consistent).

I'm not sure if this is actually a bug or intended behaviour, if it's the latter then an option to allow newlines for single-argument functions would be very welcome.