2 files changed, 22 insertions, 2 deletions
diff --git a/src/dotty/tools/dotc/core/Types.scala b/src/dotty/tools/dotc/core/Types.scala
index 26886ed1d..af2b694f5 100644
--- a/src/dotty/tools/dotc/core/Types.scala
+++ b/src/dotty/tools/dotc/core/Types.scala
@@ -648,6 +648,10 @@ object Types {
         Set((Set(), Set()))
     }
 
+    /** The element type of a sequence or array */
+    def elemType(implicit ctx: Context): Type =
+      firstBaseTypeArg(defn.SeqClass) orElse firstBaseTypeArg(defn.ArrayClass)
+
 // ----- Substitutions -----------------------------------------------------
 
     /** Substitute all types that refer in their symbol attribute to
@@ -742,6 +746,21 @@ object Types {
       else TypeAlias(this)
     }
 
+    /** The type arguments of the base type instance wrt `base` of this type */
+    final def baseTypeArgs(base: Symbol)(implicit ctx: Context): List[Type] =
+      if (this <:< base.symRef)
+        base.typeParams map (param => member(param.name).info.argType(param))
+      else
+        Nil
+
+    /** The first type argument of the base type instance wrt `base` of this type */
+    final def firstBaseTypeArg(base: Symbol)(implicit ctx: Context): Type = base.typeParams match {
+      case param :: _ if this <:< base.symRef =>
+        member(param.name).info.argType(param)
+      case _ =>
+        NoType
+    }
+
     /** If this is an encoding of a (partially) applied type, return its arguments,
      *  otherwise return Nil
      */
diff --git a/src/dotty/tools/dotc/core/pickling/UnPickler.scala b/src/dotty/tools/dotc/core/pickling/UnPickler.scala
index 3b9ca7ba5..70798ad8a 100644
--- a/src/dotty/tools/dotc/core/pickling/UnPickler.scala
+++ b/src/dotty/tools/dotc/core/pickling/UnPickler.scala
@@ -780,7 +780,7 @@ class UnPickler(bytes: Array[Byte], classRoot: ClassDenotation, moduleClassRoot:
     val end = readNat() + readIndex
     // array elements are trees representing instances of scala.annotation.Annotation
     SeqLiteral(
-      TypeTree(defn.AnnotationClass.typeConstructor),
+      defn.SeqType.appliedTo(defn.AnnotationClass.typeConstructor :: Nil),
       until(end, () => readClassfileAnnotArg(readNat())))
   }
 
@@ -983,7 +983,8 @@ class UnPickler(bytes: Array[Byte], classRoot: ClassDenotation, moduleClassRoot:
       case ARRAYVALUEtree =>
         val elemtpt = readTreeRef()
         val trees = until(end, readTreeRef)
-        SeqLiteral(elemtpt, trees)
+        SeqLiteral(defn.SeqType.appliedTo(elemtpt.tpe :: Nil), trees)
+          // note can't deal with trees passed to Java methods as arrays here
 
       case FUNCTIONtree =>
         setSym()