Fix #8256: Disallow silent indent for template bodies

[odersky]

The new syntax requires a :. The old syntax was still supported
under a Config option which was on by default. But the test is further
confirmation that silent identation without a specific leading token
is a bad idea.

[odersky]

The actual change are just two lines, one in Parser the other in Config. But a lot of tests and infrastructure code had already switched to the shorthand, even though it was "officially" supported for only one release window, which was some months back. I guess that's natural, there's a strong tendency to pick the shortest form available.

[smarter]

It still seems weird that the object was detected as more indented than the trait when they were indented the same amount, couldn't this happen in other situations ?

[odersky]

: It still seems weird that the object was detected as more indented than the trait when they were indented the same amount, couldn't this happen in other situations ?

I don't think so. We have for each statement sequence an indentation width, which is the indentation of the first statement in the sequence. Following statements can be on the right of that width. That's what happened here: The trait was indented too far to the right, and the folllowing statement was interpreted as indented as well, but now indented relative to the trait because it was to the right of the previous indentation width. With an explicit : this cannot happen. What can happen is this:

val a = ???
  trait T:
  val x = 1

Here, again the definition of x would be treated as part of trait T. But there is a clear indication (the :) that this is intended. Wr could make this an error by checking the indentation width of the previous statement but I'd like to observe real world usage a bit more to see whether this is really a problem.