[JEP-409] Add support for non-sealed classes and interfaces in Java

[Cherry-picked b734eb0]

Author: hamzaremmal

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

@@ -486,6 +486,15 @@ object JavaParsers {
           case SEALED =>
             flags |= Flags.Sealed
             in.nextToken()
+          // JEP-409: Special trick for the 'non-sealed' java keyword
+          case IDENTIFIER if in.name.toString == "non" =>
+            val lookahead = in.LookaheadScanner()
+            ({lookahead.nextToken(); lookahead.token}, {lookahead.nextToken(); lookahead.name.toString}) match
+              case (MINUS, "sealed") =>
+                in.nextToken(); in.nextToken() // skip '-' and 'sealed'. Nothing more to do
+              case _ =>
+                syntaxError(em"Identifier '${in.name}' is not allowed here")
+            in.nextToken()
           case _ =>
             val privateWithin: TypeName =
               if (isPackageAccess && !inInterface) thisPackageName

tests/neg/i18533.check

@@ -2,3 +2,7 @@
 3 |class Pet permits Cat { // error
   |          ^^^^^^^
   |          A type declaration that has a permits clause should have a sealed modifier
+-- Error: tests/neg/i18533/non-SCALA_ONLY.java:4:7 ---------------------------------------------------------------------
+4 |public non class Test { // error
+  |       ^^^
+  |       Identifier 'non' is not allowed here

tests/neg/i18533/Cat_SCALA_ONLY.java

@@ -1,5 +1,5 @@
 package i18533;
 
-public class Cat extends Pet {
+public final class Cat extends Pet {
 
 }

tests/neg/i18533/non-SCALA_ONLY.java

@@ -0,0 +1,6 @@
+package i18533;
+
+// Special test for the non-sealed trick (See JavaParsers.scala::modifiers)
+public non class Test { // error
+
+}

tests/pos/i18533/Dog.java

@@ -0,0 +1,5 @@
+package i18533;
+
+public non-sealed class Dog extends Pet {
+
+}

tests/pos/i18533/Pet.java

@@ -1,5 +1,5 @@
 package i18533;
 
-public sealed class Pet permits Cat {
+public sealed class Pet permits Cat, Dog {
 
 }