Tweak observeOutdented condition

Some OUTDENTs were not inserted because next token was not EMPTY. But the
intent was that this should not be used for control flow. We now make
the conditions when not to insert OUTDENT more explicit: don't insert
OUTDENT in front of a `case` if the current region can accept `case`.

# Conflicts:
#	compiler/src/dotty/tools/dotc/parsing/Scanners.scala

Author: odersky

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

@@ -718,21 +718,18 @@ object Scanners {
 
     /** Insert an <outdent> token if next token closes an indentation region.
      *  or next token is a comma and we expect a comma in an outer region.
-     *  Exception: continue if indentation region belongs to a `match` or `catch1
-     *  and next token is `case`.
+     *  Exception: don't insert outdent in front of case if indentation region
+     *  accepts case
      */
     def observeOutdented(): Unit = currentRegion match
-      case r: Indented if !r.isOutermost =>
-        if closingRegionTokens.contains(token)
-            && !(token == CASE && r.prefix == MATCH)
-            && next.token == EMPTY
-          ||
-            token == COMMA
-            && r.outer.commasExpectedInEnclosing
-            && next.token == EMPTY
-        then
-          //if next.token != EMPTY then
-          //  println(i"NOT inserting outdent in front of $this, ${next.token}, ${r.prefix}")
+      case r: Indented
+      if !r.isOutermost
+          && (closingRegionTokens.contains(token)
+              && !(token == CASE && regionPrefixesAcceptingCase.contains(r.prefix))
+              ||
+              token == COMMA && r.outer.commasExpectedInEnclosing)
+          && next.token == EMPTY
+        =>
           insert(OUTDENT, offset)
       case _ =>
 

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

@@ -301,6 +301,8 @@ object Tokens extends TokensCommon {
 
   final val closingParens = BitSet(RPAREN, RBRACKET, RBRACE)
 
+  final val regionPrefixesAcceptingCase = BitSet(MATCH, CATCH, COLONeol)
+
   final val softModifierNames = Set(nme.inline, nme.into, nme.opaque, nme.open, nme.transparent, nme.infix)
     // Note: update, consume and erased are missing here since they are only modifiers under some import