Update array length checks

DAFFODIL-2711

Expression.scala: Update login to check prior steps for indexing predicates, require path expressions for arrays and optionals used in fn:count function.

Functions.tdml: Add tests for count function. Note that using a group reference for an optional when combined with an array will throw a ClassCastException, this may be a bug.

TestDFDLExpressions.scala: Add tests for count expressions.

daffodil-core/src/main/scala/org/apache/daffodil/core/dpath/Expression.scala

@@ -780,17 +780,10 @@ abstract class PathExpression() extends Expression {
   private lazy val priorStepsHaveIndexingPredicates: Boolean = {
     steps.last.pred.isEmpty && // last cannot have a [N] pred
     steps.dropRight(1).forall {
-      // for all the prior steps
-      // if they mention an array element, there
-      // must be a [N] predicate.
-      // neither Up (..) nor Self (.) steps should have indexing
-      step =>
-        if (step.isInstanceOf[Up]) true
-        else if (step.isInstanceOf[Up2]) SDE("Up2 step must not have indexing")
-        else if (step.isInstanceOf[Self]) SDE("Self step must not have indexing")
-        else if (step.isInstanceOf[Self2]) SDE("Self2 step must not have indexing")
-        else if (step.isArray) step.pred.isDefined // no special case to check for optional
-        else true
+      case _: Up => true
+      case _: Up2 => SDE("Up2 step must not have indexing")
+      case _: SelfStepExpression => SDE("Self step must not have indexing")
+      case step => !step.isArray || step.pred.isDefined
     }
   }
 
@@ -2342,17 +2335,15 @@ abstract class FunctionCallBase(
   protected def checkArgArrayOrOptional(): Unit = {
     lazy val isArrayOrOptional = expressions.head match {
       case pe: PathExpression => pe.isPathToOneWholeArrayOrOptional
-      case ie: IfExpression => {
-        val tp = ie.thenPart
-        val ep = ie.elsePart
-        tp.isPathToOneWholeArrayOrOptional || ep.isPathToOneWholeArrayOrOptional
-      }
-      case _ => true
+      case _ => false
     }
     if (!isArrayOrOptional) argArrayOrOptionalErr()
   }
   protected def argArrayOrOptionalErr() = {
-    SDE("The %s function requires an array or optional.", functionQName.toPrettyString)
+    SDE(
+      "The %s function requires an array or optional path expression.",
+      functionQName.toPrettyString,
+    )
   }
 
   final def checkArgCount(n: Int): Unit = {