Honestly, I'm not sure if it deserves effort to be solved, but I've had been asked why is this disallowed. I believe that specific line is here: https://github.com/lampepfl/dotty/pull/47/files#diff-2fe35c59082b97d7128120593f9ec2e7R129 see also https://stackoverflow.com/questions/4652095/why-does-the-scala-compiler-disallow-overloaded-methods-with-default-arguments