[fix] cardinality of CastExpression#toFunction

Author: line-o

exist-core/src/main/java/org/exist/xquery/CastExpression.java

@@ -190,7 +190,7 @@ public Function toFunction() throws XPathException {
 	    try {
             final QName qname = QName.parse(context, typeName);
             final FunctionSignature signature = new FunctionSignature(qname);
-            final SequenceType argType = new SequenceType(Type.ITEM, Cardinality.EXACTLY_ONE);
+            final SequenceType argType = new SequenceType(Type.ITEM, Cardinality.ZERO_OR_ONE);
             signature.setArgumentTypes(new SequenceType[]{argType});
             signature.setReturnType(new SequenceType(CastExpression.this.requiredType, CastExpression.this.cardinality));
             return new FunctionWrapper(this, signature);

exist-core/src/test/xquery/xquery3/arrowop.xql

@@ -278,3 +278,24 @@ function ao:for-each-pair-with-contextitem () {
         => string-join()
 };
 
+(:~
+  check if CastExpression#toFuntion has correct cardinality check
+  see https://github.com/eXist-db/exist/issues/4971
+:)
+declare
+    %test:assertEmpty
+function ao:type-constructor-after-arrow-empty-sequence () {
+    () => xs:string()
+};
+
+declare
+    %test:assertEquals("1")
+function ao:type-constructor-after-arrow-integer () {
+    1 => xs:string()
+};
+
+declare
+    %test:assertError("XPTY0004")
+function ao:type-constructor-after-arrow-integer-sequence () {
+    (1,2) => xs:string()
+};