Disallow phantom var

Author: nicolasstucki

compiler/src/dotty/tools/dotc/transform/FirstTransform.scala

@@ -162,8 +162,10 @@ class FirstTransform extends MiniPhaseTransform with InfoTransformer with Annota
   }
 
   override def transformValDef(vdef: tpd.ValDef)(implicit ctx: Context, info: TransformerInfo): tpd.Tree = {
-    if (vdef.symbol.hasAnnotation(defn.VolatileAnnot) && vdef.tpt.tpe.isPhantom)
-      ctx.error("Phantom fields can not be @volatile", vdef.pos)
+    if (vdef.tpt.tpe.isPhantom) {
+      if (vdef.symbol.is(Mutable)) ctx.error("var fields can not have Phantom types", vdef.pos)
+      else if (vdef.symbol.hasAnnotation(defn.VolatileAnnot)) ctx.error("Phantom fields can not be @volatile", vdef.pos)
+    }
     vdef
   }
 

tests/neg/phantom-var.scala

@@ -0,0 +1,8 @@
+
+class Foo {
+  var foo = Boo.boo // error: var fields can not have Phantom types
+}
+
+object Boo extends Phantom {
+  def boo = assume
+}

tests/neg/phantom-volitile.scala

@@ -1,6 +1,7 @@
 
 class Foo {
-  @volatile var foo = Boo.boo // error
+  @volatile var foo1 = Boo.boo // error: var fields can not have Phantom types
+  @volatile val foo2 = Boo.boo // error: Phantom fields can not be @volatile
 }
 
 object Boo extends Phantom {