Check indentation also at TopLevel

Issue "too far to the right" errors also at toplevel. Previously
this was done only inside braces.

Author: odersky

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

@@ -618,7 +618,7 @@ object Parsers {
      *  statement that's indented relative to the current region.
      */
     def checkNextNotIndented(): Unit = in.currentRegion match
-      case r: InBraces if in.isNewLine =>
+      case r: IndentSignificantRegion if in.isNewLine =>
         val nextIndentWidth = in.indentWidth(in.next.offset)
         if r.indentWidth < nextIndentWidth then
           warning(i"Line is indented too far to the right, or a `{' is missing", in.next.offset)

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

@@ -546,8 +546,7 @@ object Scanners {
       if indentSyntax && isNewLine then
         val nextWidth = indentWidth(next.offset)
         val lastWidth = currentRegion match
-          case r: Indented => r.width
-          case r: InBraces => r.width
+          case r: IndentSignificantRegion => r.indentWidth
           case _ => nextWidth
 
         if lastWidth < nextWidth then
@@ -1331,12 +1330,16 @@ object Scanners {
     def enclosing: Region = outer.asInstanceOf[Region]
 
     /** If this is an InBraces or Indented region, its indentation width, or Zero otherwise */
-    def indentWidth = IndentWidth.Zero
+    def indentWidth: IndentWidth = IndentWidth.Zero
   }
 
   case class InString(multiLine: Boolean, outer: Region) extends Region
   case class InParens(prefix: Token, outer: Region) extends Region
-  case class InBraces(var width: IndentWidth | Null, outer: Region) extends Region {
+
+  abstract class IndentSignificantRegion extends Region
+
+  case class InBraces(var width: IndentWidth | Null, outer: Region)
+  extends IndentSignificantRegion {
     override def indentWidth = width
   }
 
@@ -1345,7 +1348,8 @@ object Scanners {
    *  @param others  Other indendation widths > width of lines in the same region
    *  @param prefix  The token before the initial <indent> of the region
    */
-  case class Indented(width: IndentWidth, others: Set[IndentWidth], prefix: Token, outer: Region | Null) extends Region {
+  case class Indented(width: IndentWidth, others: Set[IndentWidth], prefix: Token, outer: Region | Null)
+  extends IndentSignificantRegion {
     override def indentWidth = width
   }
 

tests/neg-custom-args/indentRight.scala

@@ -1,3 +1,7 @@
+trait A
+    case class B() extends A  // error: Line is indented too far to the right
+    case object C extends A   // error: Line is indented too far to the right
+
 object Test {
 
   if (true)
@@ -27,3 +31,4 @@ object Test {
 
   println("!")  // error: expected a toplevel definition
 }
+