Introduce SubTypeAnnotations

The idea is that such annotations create some proper subtype of the type
they annotate. Hence, they cannot be stripped away like normal annotations are.

Author: odersky

compiler/src/dotty/tools/dotc/ast/Trees.scala

@@ -213,6 +213,31 @@ object Trees {
 
     override def toText(printer: Printer) = printer.toText(this)
 
+    def sameTree(that: Tree[_]): Boolean = {
+      def isSame(x: Any, y: Any): Boolean =
+        x.asInstanceOf[AnyRef].eq(y.asInstanceOf[AnyRef]) || {
+          x match {
+            case x: Tree[_] =>
+              y match {
+                case y: Tree[_] => x.sameTree(y)
+                case _ => false
+              }
+            case x: List[_] =>
+              y match {
+                case y: List[_] => x.corresponds(y)(isSame)
+                case _ => false
+              }
+            case _ =>
+              false
+          }
+        }
+      this.getClass == that.getClass && {
+        val it1 = this.productIterator
+        val it2 = that.productIterator
+        it1.corresponds(it2)(isSame)
+      }
+    }
+
     override def hashCode(): Int = uniqueId // for debugging; was: System.identityHashCode(this)
     override def equals(that: Any) = this eq that.asInstanceOf[AnyRef]
 

compiler/src/dotty/tools/dotc/core/Annotations.scala

@@ -34,6 +34,9 @@ object Annotations {
     def isEvaluated: Boolean = true
 
     def ensureCompleted(implicit ctx: Context): Unit = tree
+
+    def sameAnnotation(that: Annotation)(implicit ctx: Context) =
+      symbol == that.symbol && tree.sameTree(that.tree)
   }
 
   case class ConcreteAnnotation(t: Tree) extends Annotation {

compiler/src/dotty/tools/dotc/core/Definitions.scala

@@ -694,6 +694,8 @@ class Definitions {
   def ClassfileAnnotationClass(implicit ctx: Context) = ClassfileAnnotationType.symbol.asClass
   lazy val StaticAnnotationType        = ctx.requiredClassRef("scala.annotation.StaticAnnotation")
   def StaticAnnotationClass(implicit ctx: Context) = StaticAnnotationType.symbol.asClass
+  lazy val SubTypeAnnotationType        = ctx.requiredClassRef("scala.annotation.SubTypeAnnotation")
+  def SubTypeAnnotationClass(implicit ctx: Context) = SubTypeAnnotationType.symbol.asClass
 
   // Annotation classes
   lazy val AliasAnnotType = ctx.requiredClassRef("scala.annotation.internal.Alias")

compiler/src/dotty/tools/dotc/core/TypeComparer.scala

@@ -236,8 +236,8 @@ class TypeComparer(initctx: Context) extends DotClass with ConstraintHandling {
         compareWild
       case tp2: LazyRef =>
         !tp2.evaluating && recur(tp1, tp2.ref)
-      case tp2: AnnotatedType =>
-        recur(tp1, tp2.tpe) // todo: refine?
+      case tp2: AnnotatedType if !tp2.isSubTypeAnnotated =>
+        recur(tp1, tp2.tpe)
       case tp2: ThisType =>
         def compareThis = {
           val cls2 = tp2.cls
@@ -345,7 +345,7 @@ class TypeComparer(initctx: Context) extends DotClass with ConstraintHandling {
         // because that would cause an assertionError. Return false instead.
         // See i859.scala for an example where we hit this case.
         !tp1.evaluating && recur(tp1.ref, tp2)
-      case tp1: AnnotatedType =>
+      case tp1: AnnotatedType if !tp1.isSubTypeAnnotated =>
         recur(tp1.tpe, tp2)
       case AndType(tp11, tp12) =>
         if (tp11.stripTypeVar eq tp12.stripTypeVar) recur(tp11, tp2)
@@ -567,6 +567,9 @@ class TypeComparer(initctx: Context) extends DotClass with ConstraintHandling {
             false
         }
         compareTypeBounds
+      case tp2: AnnotatedType if tp2.isSubTypeAnnotated =>
+        (tp1.derivesAnnotWith(tp2.annot.sameAnnotation) || defn.isBottomType(tp1)) &&
+        recur(tp1, tp2.tpe)
       case ClassInfo(pre2, cls2, _, _, _) =>
         def compareClassInfo = tp1 match {
           case ClassInfo(pre1, cls1, _, _, _) =>
@@ -661,6 +664,8 @@ class TypeComparer(initctx: Context) extends DotClass with ConstraintHandling {
           case _ =>
         }
         either(recur(tp11, tp2), recur(tp12, tp2))
+      case tp1: AnnotatedType if tp1.isSubTypeAnnotated =>
+        isNewSubType(tp1.tpe)
       case JavaArrayType(elem1) =>
         def compareJavaArray = tp2 match {
           case JavaArrayType(elem2) => isSubType(elem1, elem2)
@@ -700,7 +705,7 @@ class TypeComparer(initctx: Context) extends DotClass with ConstraintHandling {
               }
             case tycon1: TypeVar =>
               isMatchingApply(tycon1.underlying)
-            case tycon1: AnnotatedType =>
+            case tycon1: AnnotatedType if !tycon1.isSubTypeAnnotated =>
               isMatchingApply(tycon1.underlying)
             case _ =>
               false
@@ -811,7 +816,9 @@ class TypeComparer(initctx: Context) extends DotClass with ConstraintHandling {
                 fourthTry
             }
           }
-        case _: TypeVar | _: AnnotatedType =>
+        case _: TypeVar =>
+          recur(tp1, tp2.superType)
+        case tycon2: AnnotatedType if !tycon2.isSubTypeAnnotated =>
           recur(tp1, tp2.superType)
         case tycon2: AppliedType =>
           fallback(tycon2.lowerBound)
@@ -1546,7 +1553,7 @@ class TypeComparer(initctx: Context) extends DotClass with ConstraintHandling {
       }
     case tp1: TypeVar if tp1.isInstantiated =>
       tp1.underlying & tp2
-    case tp1: AnnotatedType =>
+    case tp1: AnnotatedType if !tp1.isSubTypeAnnotated =>
       tp1.underlying & tp2
     case _ =>
       NoType
@@ -1565,7 +1572,7 @@ class TypeComparer(initctx: Context) extends DotClass with ConstraintHandling {
       ExprType(rt1 | tp2.widenExpr)
     case tp1: TypeVar if tp1.isInstantiated =>
       tp1.underlying | tp2
-    case tp1: AnnotatedType =>
+    case tp1: AnnotatedType if !tp1.isSubTypeAnnotated =>
       tp1.underlying | tp2
     case _ =>
       NoType

compiler/src/dotty/tools/dotc/core/Types.scala

@@ -283,6 +283,15 @@ object Types {
       case _ => false
     }
 
+    /** Does this type have a supertype with an annotation satisfying given predicate `p`? */
+    def derivesAnnotWith(p: Annotation => Boolean)(implicit ctx: Context): Boolean = this match {
+      case tp: AnnotatedType => p(tp.annot) || tp.tpe.derivesAnnotWith(p)
+      case tp: TypeProxy => tp.superType.derivesAnnotWith(p)
+      case AndType(l, r) => l.derivesAnnotWith(p) || r.derivesAnnotWith(p)
+      case OrType(l, r) => l.derivesAnnotWith(p) && r.derivesAnnotWith(p)
+      case _ => false
+    }
+
     /** Does this type occur as a part of type `that`? */
     final def occursIn(that: Type)(implicit ctx: Context): Boolean =
       that existsPart (this == _)
@@ -3693,6 +3702,17 @@ object Types {
 
     override def stripAnnots(implicit ctx: Context): Type = tpe.stripAnnots
 
+    private[this] var isSubTypeAnnotatedKnown = false
+    private[this] var isSubTypeAnnotatedCache: Boolean = _
+
+    def isSubTypeAnnotated(implicit ctx: Context) = {
+      if (!isSubTypeAnnotatedKnown) {
+        isSubTypeAnnotatedCache = annot.symbol.derivesFrom(defn.SubTypeAnnotationClass)
+        isSubTypeAnnotatedKnown = true
+      }
+      isSubTypeAnnotatedCache
+    }
+
     override def iso(that: Any, bs: BinderPairs): Boolean = that match {
       case that: AnnotatedType => tpe.equals(that.tpe, bs) && (annot `eq` that.annot)
       case _ => false

compiler/src/dotty/tools/dotc/typer/Checking.scala

@@ -485,9 +485,10 @@ object Checking {
             prefix = apply(tp.prefix),
             classParents =
               tp.parents.map { p =>
-                apply(p).stripAnnots match {
-                  case ref: TypeRef => ref
-                  case ref: AppliedType => ref
+                val p1 = apply(p)
+                p1.stripAnnots match {
+                  case ref: TypeRef => p1
+                  case ref: AppliedType => p1
                   case _ => defn.ObjectType // can happen if class files are missing
                 }
               }

library/src/scala/annotation/SubTypeAnnotation.scala

@@ -0,0 +1,3 @@
+package scala.annotation
+
+trait SubTypeAnnotation extends StaticAnnotation

tests/pos/immutableAnnot.scala

@@ -0,0 +1,26 @@
+import scala.annotation._
+class Immutable extends Annotation with SubTypeAnnotation
+
+object Test extends App {
+
+  def assertImmutable[T](n: T): T @ Immutable = ???
+
+  def g[S](x: S, y: S) = if (???) x else y
+
+  def f(n: Int @ Immutable): Int @ Immutable =
+    if n == 0 then assertImmutable(0)
+    else {
+      val n1 = f(f(n))
+      val x = g(n1, n1)
+      val y = g(n1, assertImmutable(3))
+      val z = g(x, y)
+      val r =
+        if (???) x
+        else if (???) y
+        else if (???) z
+        else assertImmutable(n1 - 1)
+      val s = g(assertImmutable(List(1, 2, 3)), assertImmutable("a"))
+      val scheck: java.io.Serializable = s
+      r
+    }
+}