Make sealed an annotation

# Conflicts:
#	tests/pos-special/stdlib/collection/ArrayOps.scala

Author: odersky

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

@@ -1042,7 +1042,14 @@ class Namer { typer: Typer =>
           tp
 
       val rhs1 = typedAheadType(rhs)
-      val rhsBodyType: TypeBounds = addVariances(rhs1.tpe).toBounds
+      val rhsBodyType: TypeBounds =
+        val bounds = addVariances(rhs1.tpe).toBounds
+        if sym.is(Sealed) then
+          sym.resetFlag(Sealed)
+          bounds.derivedTypeBounds(bounds.lo,
+            AnnotatedType(bounds.hi, Annotation(defn.Caps_SealedAnnot, rhs1.span)))
+        else bounds
+
       val unsafeInfo = if (isDerived) rhsBodyType else abstracted(rhsBodyType)
 
       def opaqueToBounds(info: Type): Type =

tests/neg/class-mods.scala

@@ -2,7 +2,7 @@ open final class Foo1       // error
 sealed open class Foo2      // error
 
 open type T1     // error
-sealed type T2   // error
+type T2   // ok
 abstract type T3 // error
 abstract open type T4 // error
 

tests/pos-custom-args/captures/sealed-value-class.scala

@@ -0,0 +1,3 @@
+class Ops[sealed A](xs: Array[A]) extends AnyVal:
+
+  def f(p: A => Boolean): Array[A] = xs