Weaken failing assertion in Scanner

Fixes #12605

Author: odersky

compiler/src/dotty/tools/dotc/parsing/Scanners.scala

@@ -601,7 +601,9 @@ object Scanners {
       case r: Indented
       if !r.isOutermost
          && closingRegionTokens.contains(token)
-         && !(token == CASE && r.prefix == MATCH) =>
+         && !(token == CASE && r.prefix == MATCH)
+         && next.token == EMPTY  // can be violated for ill-formed programs, e.g. neg/i12605.sala
+      =>
         currentRegion = r.enclosing
         insert(OUTDENT, offset)
       case _ =>

tests/neg/i12605.scala

@@ -0,0 +1,4 @@
+object Foo:
+  def joe(): List[(Int, Int)] =
+    List((2, 3), (3, 4)).filter case (a, b) => b > a // error // error
+// error