[GROOVY-7691] Type checking error on generic property with covariant type in subclass

• Type: Bug
• Status: Closed
• Priority: Major
• Resolution: Fixed
• Affects Version/s: 2.4.5
• Fix Version/s: 2.6.0-alpha-4, 3.0.0-alpha-3, 2.5.0-rc-3
• Component/s: Static Type Checker
• Labels: test
• Assignee: Daniel Sun
• Reporter: Frank Pavageau
• Created: 26/Nov/15 22:27
• Updated: 09/Jun/20 10:07
• Resolved: 13/May/18 13:18

Description

A property declared with a generic type in a superclass is not seen with the correct covariant type in a subclass narrowing the type:

abstract class AbstractNumberWrapper<S extends Number> {
    protected final S number;

    AbstractNumberWrapper(S number) {
        this.number = number
    }
}
class LongWrapper<S extends Long> extends AbstractNumberWrapper<S> {
    LongWrapper(S longNumber) {
        super(longNumber)
    }

    S getValue() {
        return number;
    }
}
assert new LongWrapper<Long>(42L).value == 42L

raises the following error when type checked:

[Static type checking] - Cannot return value of type java.lang.Number on method returning type S
 @ line 15, column 28.
                       return number;
                              ^

1 error