[#GROOVY-5418] Compilation error with covariant return types (incompatible types) - jira.codehaus.org

Details

Type: Bug
Status: Closed
Priority: Major
Resolution: Fixed
Affects Version/s: 1.8.6
Fix Version/s: 2.0.4, 1.8.9
Component/s: Compiler
Labels:
None
Environment:
Linux, jdk 1.6.0u31 (also reproducible with 1.7)

Number of attachments :
0

Description

Given the following files:

pkg/Base.java

package pkg;
public interface Base {
    Base doSomething();
}

pkg/Child.java

package pkg;
public interface Child extends Base {
    Child doSomething();
}

pkg/GChild.groovy

package pkg;
class GChild implements Child {
    Child doSomething() {
        return this
    }
}

When I try to compile this, groovyc complains about incompatible return types:

$ javac pkg/*.java
$ groovyc pkg/*.groovy
org.codehaus.groovy.control.MultipleCompilationErrorsException: startup failed:
pkg/GChild.groovy: 3: The return type of pkg.Child doSomething() in pkg.GChild is incompatible with pkg.Base doSomething() in pkg.Base
. At [3:2]  @ line 3, column 2.
        Child doSomething() {
    ^

1 error

$ javac -version
javac 1.6.0_31
$ groovyc -version
Groovy compiler version 1.8.6
Copyright 2003-2011 The Codehaus. http://groovy.codehaus.org/

$

A very similar (almost identical) issue has already been reported here: https://jira.codehaus.org/browse/GROOVY-2829 (marked as fixed in 1.5.7 and 1.6-beta2)

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Raffael Herzog added a comment - 25/Apr/12 8:24 AM

Playing around with the example, I found that really needs to be this situation to reproduce the problem:

Using classes instead of interfaces: Works
Using Number/Integer instead of Base/Child as return types: Works
And, interesting: Returning GChild instead of Child in GChild.groovy: Works

As a workaround, I can do the following if returning GChild isn't an option:

pkg/AbstractChild.java

package pkg;
public abstract class AbstractChild implements Child {
    protected abstract Object doIt();
    public Child doSomething() {
        return (Child)doIt();
    }
}

pkg/GChild.groovy

package pkg;
class GChild extends AbstractChild {
    // NOTE: including "implements Child" above will reproduce the problem!
    Object doIt() {
        return null
    }
}

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Raffael Herzog added a comment - 25/Apr/12 8:24 AM Playing around with the example, I found that really needs to be this situation to reproduce the problem: Using classes instead of interfaces: Works Using Number/Integer instead of Base/Child as return types: Works And, interesting: Returning GChild instead of Child in GChild.groovy: Works As a workaround, I can do the following if returning GChild isn't an option: pkg/AbstractChild.java package pkg; public abstract class AbstractChild implements Child { protected abstract Object doIt(); public Child doSomething() { return (Child)doIt(); } } pkg/GChild.groovy package pkg; class GChild extends AbstractChild { // NOTE: including " implements Child" above will reproduce the problem! Object doIt() { return null } }

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Knute Axelson added a comment - 13/Sep/12 5:19 PM - edited

I'm having the same problem. See sample code below. If you compile and run with Java it works and prints "this should work". If you compile and run with Groovy (tried with 1.8.6 and 2.0.2), you get

The return type of java.util.List doSomething() in BugExample$C is incompatible with java.lang.Iterable doSomething() in BugExample$A
. At [13:9] at line: 13, column: 9

BugExample.java/groovy

import java.util.List;

public class BugExample {
    static interface A {
        Iterable doSomething();
    }

    static interface B extends A {
        List doSomething();
    }

    static class C implements B {
        public List doSomething() {
            System.out.println("this should work");
            return null;
        }
    }

    public static void main(String[] args) {
        new C().doSomething();
    }
}

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Knute Axelson added a comment - 13/Sep/12 5:19 PM - edited I'm having the same problem. See sample code below. If you compile and run with Java it works and prints "this should work". If you compile and run with Groovy (tried with 1.8.6 and 2.0.2), you get The return type of java.util.List doSomething() in BugExample$C is incompatible with java.lang.Iterable doSomething() in BugExample$A . At [13:9] at line: 13, column: 9 BugExample.java/groovy import java.util.List; public class BugExample { static interface A { Iterable doSomething(); } static interface B extends A { List doSomething(); } static class C implements B { public List doSomething() { System .out.println( " this should work" ); return null ; } } public static void main( String [] args) { new C().doSomething(); } }

Compilation error with covariant return types (incompatible types)

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