Disallow opaque as a local modifier

Author: odersky

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

@@ -201,11 +201,11 @@ object Parsers {
       canStartExpressionTokens.contains(in.token) &&
       !in.isSoftModifierInModifierPosition
 
-    def isDefIntro(allowedMods: BitSet): Boolean =
+    def isDefIntro(allowedMods: BitSet, excludedSoftModifiers: Set[TermName] = Set.empty): Boolean =
       in.token == AT ||
       (defIntroTokens `contains` in.token) ||
       (allowedMods `contains` in.token) ||
-      in.isSoftModifierInModifierPosition
+      in.isSoftModifierInModifierPosition && !excludedSoftModifiers.contains(in.name)
 
     def isStatSep: Boolean =
       in.token == NEWLINE || in.token == NEWLINES || in.token == SEMI
@@ -3014,7 +3014,7 @@ object Parsers {
           stats += implicitClosure(in.offset, Location.InBlock, modifiers(closureMods))
         else if (isExprIntro)
           stats += expr(Location.InBlock)
-        else if (isDefIntro(localModifierTokens))
+        else if (isDefIntro(localModifierTokens, excludedSoftModifiers = Set(nme.`opaque`)))
           if (closureMods.contains(in.token)) {
             val start = in.offset
             var imods = modifiers(closureMods)

docs/docs/internals/syntax.md

@@ -320,12 +320,12 @@ Binding           ::=  (id | ‘_’) [‘:’ Type]
 Modifier          ::=  LocalModifier
                     |  AccessModifier
                     |  ‘override’
+                    |  ‘opaque’
 LocalModifier     ::=  ‘abstract’
                     |  ‘final’
                     |  ‘sealed’
                     |  ‘implicit’
                     |  ‘lazy’
-                    |  ‘opaque’
                     |  ‘inline’
                     |  ‘erased’
 AccessModifier    ::=  (‘private’ | ‘protected’) [AccessQualifier]

tests/neg/opaque.scala

@@ -18,6 +18,10 @@ object opaquetypes {
     val s: O = "" // should be OK
   }
 
+  def foo() = {
+    opaque type X = Int   // error
+  }
+
 }
 
 object logs {