Fix extracting the elemType of a union of arrays (#16569)

Author: smarter

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

@@ -533,6 +533,9 @@ class TypeApplications(val self: Type) extends AnyVal {
     case JavaArrayType(elemtp) => elemtp
     case tp: OrType if tp.tp1.isBottomType => tp.tp2.elemType
     case tp: OrType if tp.tp2.isBottomType => tp.tp1.elemType
-    case _ => self.baseType(defn.SeqClass).argInfos.headOption.getOrElse(NoType)
+    case _ =>
+      self.baseType(defn.SeqClass)
+      .orElse(self.baseType(defn.ArrayClass))
+      .argInfos.headOption.getOrElse(NoType)
   }
 }

tests/pos/i16562.scala

@@ -0,0 +1,8 @@
+class Test:
+  val a: Array[Any] = Array[Any]()
+  val b: Array[Any] = Array[Any]()
+
+  def ko(p: Boolean): Unit = foo((if p then a else b): _*)
+  def ok(p: Boolean): Unit = foo({ val x = if p then a else b; x }: _*)
+
+  def foo(in: Any*): Unit = ()