Fix type NamedArg of annotations when pt is nullable

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

@@ -905,8 +905,9 @@ class Typer extends Namer
      * of annotation defined as `@interface Annot { int[] value() }`
      * We assume that calling `typedNamedArg` in context of Java implies that we are dealing
      * with annotation contructor, as named arguments are not allowed anywhere else in Java.
+     * Under explicit nulls, the pt could be nullable. We need to strip `Null` type first.
      */
-    val arg1 = pt match {
+    val arg1 = pt.stripNull match {
       case AppliedType(a, typ :: Nil) if ctx.isJava && a.isRef(defn.ArrayClass) =>
         tryAlternatively { typed(tree.arg, pt) } {
             val elemTp = untpd.TypedSplice(TypeTree(typ))

tests/explicit-nulls/pos/annotaions-args/JAnnots.java

@@ -0,0 +1,7 @@
+public class JAnnots {
+  @SuppressWarnings("unused")
+  public void f1() {}
+
+  @SuppressWarnings({"unused"})
+  public void f2() {}
+}

tests/explicit-nulls/pos/annotaions-args/SAnnots.scala

@@ -0,0 +1,4 @@
+class SAnnots {
+  @SuppressWarnings(Array("unused"))
+  def f() = {}
+}