Add test

Author: liufengyun

tests/neg-custom-args/isInstanceOf/html.scala

@@ -0,0 +1,18 @@
+object HTML:
+  type AttrArg = AppliedAttr | Seq[AppliedAttr]
+  opaque type AppliedAttr = String
+  opaque type AppliedTag = StringBuilder
+
+  case class Tag(name: String):
+    def apply(attrs: AttrArg*): AppliedTag = {
+      val sb = StringBuilder()
+      sb.append(s"<$name")
+      attrs.filter(_ != Nil).foreach{
+        case s: Seq[AppliedAttr] =>
+          s.foreach(sb.append(" ").append)
+        case s: Seq[Int] => // error
+        case e: AppliedAttr =>
+          sb.append(" ").append(e)
+      }
+      sb
+    }