Merge pull request scala#5209 from adriaanm/trait-no-native-meth

Prohibit @Native method in trait

Author: lrytz

src/compiler/scala/tools/nsc/typechecker/RefChecks.scala

@@ -1659,24 +1659,32 @@ abstract class RefChecks extends InfoTransform with scala.reflect.internal.trans
         // inside annotations.
         applyRefchecksToAnnotations(tree)
         var result: Tree = tree match {
-          case DefDef(_, _, _, _, _, EmptyTree) if sym hasAnnotation NativeAttr =>
-            sym resetFlag DEFERRED
-            transform(deriveDefDef(tree)(_ => typed(gen.mkSysErrorCall("native method stub"))))
-
-          case ValDef(_, _, _, _) | DefDef(_, _, _, _, _, _) =>
+          case vod: ValOrDefDef =>
             checkDeprecatedOvers(tree)
-            checkInfiniteLoop(tree.asInstanceOf[ValOrDefDef])
+            checkInfiniteLoop(vod)
             if (settings.warnNullaryUnit)
               checkNullaryMethodReturnType(sym)
             if (settings.warnInaccessible) {
               if (!sym.isConstructor && !sym.isEffectivelyFinalOrNotOverridden && !sym.isSynthetic)
                 checkAccessibilityOfReferencedTypes(tree)
             }
-            tree match {
-              case dd: DefDef => checkByNameRightAssociativeDef(dd)
-              case _          =>
+            vod match {
+              case dd: DefDef =>
+                checkByNameRightAssociativeDef(dd)
+
+                if (sym hasAnnotation NativeAttr) {
+                  if (sym.owner.isTrait) {
+                    reporter.error(tree.pos, "A trait cannot define a native method.")
+                    tree
+                  } else if (dd.rhs == EmptyTree) {
+                    // pretend it had a stub implementation
+                    sym resetFlag DEFERRED
+                    deriveDefDef(dd)(_ => typed(gen.mkSysErrorCall("native method stub")))
+                  } else tree
+                } else tree
+
+              case _ => tree
             }
-            tree
 
           case Template(parents, self, body) =>
             localTyper = localTyper.atOwner(tree, currentOwner)

src/library/scala/native.scala

@@ -16,8 +16,11 @@ package scala
   * @native def f(x: Int, y: List[Long]): String = ...
   * }}}
   *
-  * Method body is not generated if method is marked with `@native`,
-  * but it is type checked when present.
+  * A `@native` method is compiled to the platform's native method,
+  * while discarding the method's body (if any). The body will be type checked if present.
   *
-  * @since 2.6 */
+  * A method marked @native must be a member of a class, not a trait (since 2.12).
+  *
+  * @since 2.6
+  */
 class native extends scala.annotation.StaticAnnotation {}

test/files/neg/trait-no-native.check

@@ -0,0 +1,4 @@
+trait-no-native.scala:3: error: A trait cannot define a native method.
+  @native def foo = ???
+              ^
+one error found

test/files/neg/trait-no-native.scala

@@ -0,0 +1,4 @@
+trait T {
+  // should not compile, because it would result in a VerifyError
+  @native def foo = ???
+}