Weaken failing assertion in Scanner

[odersky]

Fixes #12605

[som-snytt]

Just to document, REPL is OK with the fix.

scala> def f =
     |   List((2, 3)).filter case (a, b) => b > a
2 |  List((2, 3)).filter case (a, b) => b > a
  |                      ^^^^
  |                      unindent expected, but 'case' found

scala> def f =
     | List((2, 3)).filter case (a, b) => b > a
2 |List((2, 3)).filter case (a, b) => b > a
  |                    ^^^^
  |                    eof expected, but 'case' found

I was curious that scalac follows up a syntax error with a type error, instead of halting earlier.

-- [E040] Syntax Error: i12605.scala:4:24 ----------------------------------------------------------------------------
4 |    List((2, 3)).filter case (a, b) => b > a
  |                        ^^^^
  |                        unindent expected, but 'case' found
-- [E040] Syntax Error: i12605.scala:10:0 ----------------------------------------------------------------------------
10 |
   |^
   |'}' expected, but eof found
-- [E007] Type Mismatch Error: i12605.scala:4:4 ----------------------------------------------------------------------
4 |    List((2, 3)).filter case (a, b) => b > a
  |    ^^^^^^^^^^^^^^^^^^^
  |    Found:    (((Int, Int)) => Boolean) => List[(Int, Int)]
  |    Required: List[(Int, Int)]

More opinionated messaging might help the user trying to remember if braceless syntax like for one-line catch applies here; I'm still not sure why there is no one-line match. Possibly "Big problem" never happens; it happens with case because of lookahead for case class, interacting with case in block.

    def observeOutdented(): Unit = currentRegion match
      case r: Indented if !r.isOutermost && closingRegionTokens.contains(token) =>
        if !(token == CASE && r.prefix == MATCH) then
          if next.token == EMPTY then
            currentRegion = r.enclosing
            insert(OUTDENT, offset)
          else if token == CASE then error("That case is messed up")
          else error("Big problem with lookahead trying to insert outdent")
      case _ =>

[bishabosha]

LGTM