[jira] [Created] (GROOVY-7495) Diamond inheritance of interfaces makes method return type incompatible

["Lóránt Pintér (JIRA)" <ji...@apache.org>]

Lóránt Pintér created GROOVY-7495:
-------------------------------------

             Summary: Diamond inheritance of interfaces makes method return type incompatible
                 Key: GROOVY-7495
                 URL: https://issues.apache.org/jira/browse/GROOVY-7495
             Project: Groovy
          Issue Type: Bug
          Components: Compiler
    Affects Versions: 2.4.3, 2.3.10
            Reporter: Lóránt Pintér


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:

{code:title=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 {}
{code}

The error message is:

{code}
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 }
     ^
{code}

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

{code:title=Fixed Groovy}
class SecondImpl extends FirstImpl implements Second {
  DerivedItem getItem() { super.item }
}
{code}

The equivalent code compiles fine in Java:

{code:title=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 {}
}
{code}



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