Terminal colon for templates with empty bodies should be permitted

[propensive]

Compiler version

Scala 3.0.0-RC2

Minimized example

trait Foo:

Output

Error: indented definitions expected, eof found

Expectation

Template definitions which end in a : but have empty bodies turn out to be quite a common occurrence during development, usually because the non-compiling body gets temporarily commented out. But the presence of the : combined with the empty body is considered erroneous by the parser.

This creates a small but disproportionate amount of friction. Seeing as commenting-out a template's body is as simple as pressing Ctrl+/, it's very easy to do, but I find that I make the same mistake of forgetting to also delete the : over and over again. And then I also have to add the : back when I reinstate the template body.

I would prefer the : to be optional for templates with empty bodies. A compromise possibility would be to reduce it to a warning.

[prolativ]

In my understanding the colon is actually the key difference between a class/trait/object with a body and without it. Theoretically we could live without colon here at all and rely only on indentation in braceless syntax e.g.

trait Foo

trait Foo
  def bar = 1

but here it would be quite easy to miss the difference to

trait Foo
def bar = 1

A colon makes the difference more explicit

trait Foo:
  def bar = 1

trait Foo
def bar = 1

Allowing

trait Foo:
def bar = 1

with the meaning of the def outside of the trait would be even more confusing than getting rid of the colon at all.

[propensive]

I wasn't proposing to remove the colon in all cases (and I think that was discussed at length a few months ago). But I'd agree that,

trait Foo:
def bar = 1

seems like it should really be at least a warning in all cases. In the absence of braces, the indentation shouldn't be optional. Too much flexibility around indentation is a bad thing.

[som-snytt]

You can use end. Not sure that is helpful for the use case.

The PSA slogan would go something like: "end isn't just for long things."

trait T:
// still not sure here
end T

Edit: "Not sure when pandemic will end?"

trait Pandemic:
end Pandemic

[odersky]

I think we do not have a clear understanding how to improve this.