[GROOVY-9998] Wildcards with a lower bound fail static type checking

• Type: Bug
• Status: Closed
• Priority: Major
• Resolution: Fixed
• Affects Version/s: 3.0.7
• Fix Version/s: 4.0.0-alpha-3, 3.0.20
• Component/s: Static compilation, Static Type Checker
• Assignee: Eric Milles
• Reporter: Lyuben Atanasov
• Created: 23/Mar/21 10:08
• Updated: 11/Dec/23 18:15
• Resolved: 24/Mar/21 12:48

Description

The following code fails to compile when using @CompileStatic:

class A {
	public final int order;
	
	public A(int order) {
		this.order = order;
	}
}

class B extends A {
	public B(int order) {
		super(order);
	}
}

class Test {
	public void test() {
		Comparator<? super A> comparator = (a1, a2) -> Integer.compare(a1.order, a2.order);
		List<B> list = [new B(2), new B(3), new B(1), new B(0)];
		list.stream().sorted(comparator);
	}
}

The error is:

Exception in thread "main" org.codehaus.groovy.control.MultipleCompilationErrorsException: startup failed:
Script_49a5c0348548765c6a0257437aa3b031.groovy: 19: [Static type checking] - Cannot call java.util.stream.Stream <B>#sorted(java.util.Comparator <? super B>) with arguments [java.util.Comparator <A>] 
 @ line 19, column 3.
   		list.stream().sorted(comparator);
     ^

For some reason the type checker is confused when we have a method with a generic parameter that has a wildcard with a lower bound. The same code compiles with javac if I move the A and B classes to separate files and create the list java-style.

Curiously, if I change the comparator's type from Comparator<? super A> to Comparator<? extends A>, the code compiles successfully in Groovy (and it shouldn't!), but causes compilation failure in Java.

Issue Links

• causes: GROOVY-10765 STC: Closure implementation of Java @FunctionalInterface loses type information
• is related to: GROOVY-7992 Sort methods that accept a comparator should accept Comparator<? super T> (fix type checker ability to cope with super in DGM methods)
• relates to: GROOVY-10959 Type checking fails when casting method references