Fix function list params with single default values and improve safety of loading functions

src/main/java/ch/njol/skript/lang/function/Function.java

@@ -90,7 +90,19 @@ public boolean isSingle() {
 		for (int i = 0; i < parameters.length; i++) {
 			Parameter<?> parameter = parameters[i];
 			Object[] parameterValue = parameter.keyed ? convertToKeyed(parameterValues[i]) : parameterValues[i];
-			if (parameterValue == null) { // Go for default value
+
+			// see https://github.com/SkriptLang/Skript/pull/8135
+			if ((parameterValues[i] == null || parameterValues[i].length == 0)
+				&& parameter.keyed
+				&& parameter.def != null
+			) {
+				Object[] defaultValue = parameter.def.getArray(event);
+				if (defaultValue.length == 1) {
+					parameterValue = KeyedValue.zip(defaultValue, null);
+				} else {
+					parameterValue = defaultValue;
+				}
+			} else if (parameterValue == null) { // Go for default value
 				assert parameter.def != null; // Should've been parse error
 				Object[] defaultValue = parameter.def.getArray(event);
 				if (parameter.keyed && KeyProviderExpression.areKeysRecommended(parameter.def)) {

src/main/java/ch/njol/skript/lang/function/Functions.java

@@ -98,7 +98,17 @@ public static JavaFunction<?> registerFunction(JavaFunction<?> function) {
 			Skript.debug((signature.local ? "local " : "") + "function " + name + "(" + StringUtils.join(params, ", ") + ")"
 				+ (c != null ? " :: " + (signature.isSingle() ? c.getName().getSingular() : c.getName().getPlural()) : "") + ":");
 
-		Function<?> function = new ScriptFunction<>(signature, node);
+		Function<?> function;
+		try {
+			function = new ScriptFunction<>(signature, node);
+		} catch (SkriptAPIException ex) {
+			//noinspection ThrowableNotThrown
+			Skript.exception(ex, "Error while trying to load a function");
+
+			// avoid getting a "function is already registered" error when the function implementation is not known yet
+			Functions.unregisterFunction(signature);
+			return null;
+		}
 
 		if (namespace.getFunction(signature.name) == null) {
 			namespace.addFunction(function);

src/test/skript/tests/syntaxes/structures/StructFunction.sk

@@ -47,6 +47,12 @@ test "function structure type hints":
 		type_hint_argument_test({_number})
 	assert the first element of the last parse logs is set # contains "The function 'type_hint_argument_test(boolean)' does not exist." (NEEDS FIXING)
 
+local function list_argument_single_default(x: texts="hey") :: texts:
+    return {_x::*}
+
+test "function list argument with a single default":
+	assert list_argument_single_default() = "hey"
+
 local function literal_test(x: entity type):
 	stop
 local function literal_test_projectile(x: entity):