Report forbidden modifiers from modifiers

Missing: tests.

Had to add to `modifiers` a new `parsedSeparately` parameter, and since its
interface is tricky I wrote some documentation.

Author: Blaisorblade

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

@@ -1650,6 +1650,7 @@ object Parsers {
       case PROTECTED => Mod.Protected()
       case SEALED    => Mod.Sealed()
     }
+    private def flagsOfModToken(tok: Int): FlagSet = modOfToken(tok).flags
 
     /** Drop `private' modifier when followed by a qualifier.
      *  Contract `abstract' and `override' to ABSOVERRIDE
@@ -1705,7 +1706,14 @@ object Parsers {
         }
       } else mods
 
-    /** {Annotation} {Modifier}
+    /**
+     * Extend the start Modifiers by parsing modifier tokens in the allowed BitSet, appearing in any order.
+     * Ignore modifier tokens in the parsedSeparately BitSet, so that the caller can parse them after modifier returns
+     * (suitable to parse `AllowedModifiers* lazy`).
+     * Report modifier tokens that appear neither in allowed nor in parsedSeparately.
+     *
+     * Grammar:
+     * {Annotation} {Modifier}
      *  Modifiers      ::= {Modifier}
      *  LocalModifiers ::= {LocalModifier}
      *  AccessModifier ::= (private | protected) [AccessQualifier]
@@ -1714,7 +1722,7 @@ object Parsers {
      *                  |  override
      *  LocalModifier  ::= abstract | final | sealed | implicit | lazy
      */
-    def modifiers(allowed: BitSet = modifierTokens, start: Modifiers = Modifiers()): Modifiers = {
+    def modifiers(allowed: BitSet = modifierTokens, start: Modifiers = Modifiers(), parsedSeparately: BitSet = BitSet.empty): Modifiers = {
       @tailrec
       def loop(mods: Modifiers): Modifiers = {
         if (allowed contains in.token) {
@@ -1725,6 +1733,8 @@ object Parsers {
           in.nextToken()
           loop(mods)
         } else {
+          if ((modifierTokens contains in.token) && !(parsedSeparately contains in.token))
+            syntaxError(hl"Modifier `${flagsOfModToken(in.token)}' not allowed at this position")
           mods
         }
       }
@@ -2471,7 +2481,7 @@ object Parsers {
         else if (isDefIntro(localModifierTokens))
           if (in.token == IMPLICIT || in.token == ERASED) {
             val start = in.offset
-            var imods = modifiers(funArgMods)
+            var imods = modifiers(funArgMods, parsedSeparately = BitSet(LAZY))
             if (isBindingIntro) stats += implicitClosure(start, Location.InBlock, imods)
             else stats +++= localDef(start, imods)
           } else {