Empty `DefDef: Unit` with end marker

[dos65]

3.0.0-RC2

Should the compiler allow the following empty def?
Currently, it fails with an error from the parser.

 def foo: Unit = //<- [error] expression expected but end found
 end foo 

[som-snytt]

Related use case at #12185 (comment) which was about (transiently) empty class template during development. (Where empty template body after colon is allowed with end marker.)

What if the answer here is yes, but it expands to

def f: Unit =
  ???
end f

corresponding to the intention to fill in the blank later.

I don't think Unit is special here. If only "value discard" covered other singleton types, especially this.type.

def add(x: X): this.type =
end add

would I want a default implementation or NotImplemented? Also it looks like stack trace printing in REPL is broken.

[odersky]

Should the compiler allow the following empty def?

I think logically it should not. end markers an optional addition to a program. The program needs to be syntactically correct even if they are missing.

[SethTisue]

recent discussion thread https://contributors.scala-lang.org/t/pre-sip-improve-explicit-end-terminator-syntax-for-unit-return/7142