[GROOVY-7495] Diamond inheritance of interfaces makes method return type incompatible

• Type: Bug
• Status: Closed
• Priority: Major
• Resolution: Fixed
• Affects Version/s: 2.3.10, 2.4.3
• Fix Version/s: 2.4.4
• Component/s: Compiler
• Assignee: Jochen Theodorou
• Reporter: Lóránt Pintér
• Created: 07/Jul/15 10:28
• Updated: 19/Oct/15 16:11
• Resolved: 09/Jul/15 16:43

Description

While working on Gradle, I bumped into a failure with the Groovy compiler that works fine in Java.

Example code that fails with both Groovy compiler 2.3.10 and 2.4.3:

Groovy

interface Item {}
interface DerivedItem extends Item {}

interface Base {
  Item getItem()
}
class BaseImpl implements Base {
  Item getItem() { null }
}

interface First extends Base {
  DerivedItem getItem()
}

class FirstImpl extends BaseImpl implements First {
  DerivedItem getItem() { null }
}

interface Second extends First {}
class SecondImpl extends FirstImpl implements Second {}

The error message is:

Script1.groovy: 8: The return type of Item getItem() in BaseImpl is incompatible with DerivedItem in First. At [8:3]  @ line 8, column 3.
     Item getItem() { null }
     ^

However, changing the implementation of SecondImpl like this fixes the compile error:

Fixed Groovy

class SecondImpl extends FirstImpl implements Second {
  DerivedItem getItem() { super.item }
}

The equivalent code compiles fine in Java:

Java

public class CompileJava {
    interface Item {}
    interface DerivedItem extends Item {}
    
    interface Base {
      Item getItem();
    }
    class BaseImpl implements Base {
      public Item getItem() { return null; }
    }
    
    interface First extends Base {
      DerivedItem getItem();
    }
    
    class FirstImpl extends BaseImpl implements First {
      public DerivedItem getItem() { return null; }
    }
    
    interface Second extends First {}
    class SecondImpl extends FirstImpl implements Second {}
}