Fix ExprFunctionCall conversion (#4986)

* Change default return type and implement conversion

src/main/java/ch/njol/skript/expressions/ExprVelocity.java

@@ -51,6 +51,7 @@ public Vector convert(Entity e) {
 	}
 
 	@Override
+	@Nullable
 	@SuppressWarnings("null")
 	public Class<?>[] acceptChange(ChangeMode mode) {
 		if ((mode == ChangeMode.ADD || mode == ChangeMode.REMOVE || mode == ChangeMode.SET || mode == ChangeMode.DELETE || mode == ChangeMode.RESET))
@@ -61,7 +62,7 @@ public Class<?>[] acceptChange(ChangeMode mode) {
 	@Override
 	@SuppressWarnings("null")
 	public void change(Event e, @Nullable Object[] delta, ChangeMode mode) {
-		assert delta != null;
+		assert mode == ChangeMode.DELETE || mode == ChangeMode.RESET || delta != null;
 		for (final Entity entity : getExpr().getArray(e)) {
 			if (entity == null)
 				return;

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

@@ -24,6 +24,7 @@
 import ch.njol.skript.registrations.Converters;
 import ch.njol.skript.util.Utils;
 import ch.njol.util.Kleenean;
+import ch.njol.util.coll.CollectionUtils;
 import org.bukkit.event.Event;
 import org.eclipse.jdt.annotation.Nullable;
 
@@ -33,11 +34,24 @@ public class ExprFunctionCall<T> extends SimpleExpression<T> {
 	private final Class<? extends T>[] returnTypes;
 	private final Class<T> returnType;
 
-	@SuppressWarnings("unchecked")
 	public ExprFunctionCall(FunctionReference<T> function) {
+		this(function, function.returnTypes);
+	}
+
+	@SuppressWarnings("unchecked")
+	public ExprFunctionCall(FunctionReference<?> function, Class<? extends T>[] expectedReturnTypes) {
 		this.function = function;
-		this.returnTypes = function.returnTypes;
-		this.returnType = (Class<T>) Utils.getSuperType(returnTypes);
+		Class<?> functionReturnType = function.getReturnType();
+		assert  functionReturnType != null;
+		if (CollectionUtils.containsSuperclass(expectedReturnTypes, functionReturnType)) {
+			// Function returns expected type already
+			this.returnTypes = new Class[] {functionReturnType};
+			this.returnType = (Class<T>) functionReturnType;
+		} else {
+			// Return value needs to be converted
+			this.returnTypes = expectedReturnTypes;
+			this.returnType = (Class<T>) Utils.getSuperType(expectedReturnTypes);
+		}
 	}
 
 	@Override
@@ -48,6 +62,19 @@ protected T[] get(Event e) {
 		return Converters.convertArray(returnValue, returnTypes, returnType);
 	}
 
+	@Override
+	@Nullable
+	@SuppressWarnings("unchecked")
+	public <R> Expression<? extends R> getConvertedExpression(Class<R>... to) {
+		if (CollectionUtils.containsSuperclass(to, getReturnType()))
+			return (Expression<? extends R>) this;
+		assert function.getReturnType() != null;
+		if (Converters.converterExists(function.getReturnType(), to)) {
+			return new ExprFunctionCall<>(function, to);
+		}
+		return null;
+	}
+
 	@Override
 	public boolean isSingle() {
 		return function.isSingle();

src/test/skript/tests/regressions/4524-function call conversion.sk

@@ -3,3 +3,5 @@ function functionCallConversion() :: item:
 
 test "function call conversion":
 	assert name of functionCallConversion() is "abc" with "Name of item from function call failed"
+
+	set {_p}'s velocity to vector({_x}, {_y}, {_z})