Refine hashing of AnnotatedTypes

Since we are always comparing annotations in AnnotatedTypes with `eq`,
they should always be hashed with identityHashCode.

Author: odersky

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

@@ -90,6 +90,9 @@ trait Hashable {
   protected final def doHash(bs: Binders, x1: Any, tp2: Type): Int =
     finishHash(bs, hashing.mix(hashSeed, x1.hashCode), 1, tp2)
 
+  protected final def doHash(bs: Binders, x1: Int, tp2: Type): Int =
+    finishHash(bs, hashing.mix(hashSeed, x1), 1, tp2)
+
   protected final def doHash(bs: Binders, tp1: Type, tp2: Type): Int =
     finishHash(bs, hashSeed, 0, tp1, tp2)
 

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

@@ -4698,8 +4698,10 @@ object Types {
 
     // equals comes from case class; no matching override is needed
 
-    override def computeHash(bs: Binders): Int = doHash(bs, annot, parent)
-    override def hashIsStable: Boolean = parent.hashIsStable
+    override def computeHash(bs: Binders): Int =
+      doHash(bs, System.identityHashCode(annot), parent)
+    override def hashIsStable: Boolean =
+      parent.hashIsStable
 
     override def eql(that: Type): Boolean = that match
       case that: AnnotatedType => (parent eq that.parent) && (annot eq that.annot)