Unable to define an empty trait with indentation-based syntax

[julienrf]

Minimized code

trait Foo:
end Foo

Output

This code does not compile. The error message is:

end Foo
^^^
indented definitions expected

However, it is possible to create an empty trait with the brace-significant syntax:

trait Foo {
}

Expectation

The first definition of trait Foo should compile just like the last one.

[odersky]

In fact, you can define an empty class with an end marker:

trait Foo
end Foo

But it does not work anymore if you add a :.

[julienrf]

I think the following should be valid:

trait Foo

trait Foo:
// Equivalent to
trait Foo {}

trait Foo:
end Foo
// Equivalent to
trait Foo {
}

But the following should be rejected:

trait Foo
end Foo
// Equivalent to
trait Foo
}

[som-snytt]

I was about to contribute a reversion when I noticed the behavior was intentional.

This violates the principle that end marker is optional and can be removed without breaking compilation. End markers are not braces.

#12340 (comment)

end markers an optional addition to a program. The program needs to be syntactically correct even if they are missing.

#12918 (comment)

Related judgment

In my understanding the colon is actually the key difference between a class/trait/object with a body and without it.

#12185

Notice that there are a couple of disappointed expectations in the previous comment #10080 (comment)

trait T:   // not ok without body

trait T
end T   // perfectly fine

The misapprehension at #12340 is that there is something wrong with

trait T {
}
end T

where I will contribute my diabolical suggestion

trait T:
  def t = ???
end T
// unindents to
trait T {
  def t = ???
} // end T

because if it needs an end marker, then it also needs an end comment.

Currently, -rewrite -no-indent does not modify

class C:
end C