Fix handling of arrays in java annotation arguments

Previously, the array literal argument `{ "foo", "bar" }` was typed as
an `Array["foo" | "bar"]`, which is not a subtype of the corresponding
parameter type (`Array[String]`). This lead to a crash when forcing
the annotation. This is a serious problem because in Java 9+, JFrame
contains such an annotation, therefore this affects basically any code
using Swing/AWT.

Similary an empty array `{}` was always typed as `Array[Object]` which
does not always match the parameter type, this one is trickier to fix
since the serialized annotation does not keep track of the element type
of an empty array and we don't want to force the serialization at this
point, so we use a WildcardType.

Author: smarter

compiler/src/dotty/tools/dotc/core/classfile/ClassfileParser.scala

@@ -518,8 +518,8 @@ class ClassfileParser(
         else {
           val elems = arr.toList
           val elemType =
-            if (elems.isEmpty) defn.ObjectType
-            else ctx.typeComparer.lub(elems.tpes).widen
+            if (elems.isEmpty) WildcardType // No way to figure out the element type without forcing the annotation constructor
+            else elems.head.tpe.widen
           Some(JavaSeqLiteral(elems, TypeTree(elemType)))
         }
       case ANNOTATION_TAG =>

tests/pos-java-interop-separate/array-annot/Annot_1.java

@@ -0,0 +1,3 @@
+public @interface Annot_1 {
+  String[] values() default {};
+}

tests/pos-java-interop-separate/array-annot/Class_1.java

@@ -0,0 +1,8 @@
+public class Class_1 {
+  @Annot_1(values = { "foo", "bar" })
+  void foo() {}
+
+  @Annot_1(values = {})
+  void foo2() {}
+}
+

tests/pos-java-interop-separate/array-annot/Test_2.scala

@@ -0,0 +1 @@
+class Test extends Class_1