[GROOVY-3011] Compile error when using generic types with upper bounds

• Type: Bug
• Status: Closed
• Priority: Major
• Resolution: Fixed
• Affects Version/s: 1.5.6, 1.6-beta-1
• Fix Version/s: 1.5.7, 1.6-beta-2
• Environment: Groovy 1.5.6 (standalone or inside Grails 1.0.3), Groovy 1.6-beta-1, Sun Java SE 1.6.0_07 on 64bit Linux (OpenSUSE 11)
• Assignee: Unassigned
• Reporter: Martin Gerlach
• Created: 20/Aug/08 09:44
• Updated: 12/Oct/08 08:46
• Resolved: 21/Aug/08 04:15

Description

I am experiencing the following problem with Groovy 1.5.6 and 1.6-beta-1:

Groovy does not seem to accept subtypes of direct subtypes of SomeType in constructs like <T extends SomeType>.

The exact constellation for reproducing the error is as follows:

Directory structure

.
+- groovy
|  +- foo
|     +- bar
|        +- BarMaping.groovy
|        +- MyTypeMapping.groovy
|        +- MySubTypeMapping.groovy
+- java
   +- foo
      +- example
      |  +- MyType.java (interface extending Bar)
      |  +- MySubType.java (interface extending MyType)
      +- Bar.java
      +- Baz.java

Bar.java

package foo;

public interface Bar {

    Baz getId();

    void setId(Baz id);
}

Baz.java is just a POJO

MyType.java

package foo.example;

import foo.Bar;

public interface MyType extends Bar {

    String getTitle();

    void setTitle(String title);
}

MySubType.java

package foo.example;

public interface MySubType extends MyType {
}

BarMapping.groovy

package foo.bar

import foo.Bar
import foo.Baz

abstract class BarMapping<T extends Bar> {

    Baz id

    BarMapping(T bar) {
        mapFrom(bar)
    }

    def mapFrom(T bar) {
        id = bar?.id
    }

    def mapTo(T bar) {
        
    }
}

MyTypeMapping.groovy

package foo.bar

import foo.example.MyType

class MyTypeMapping extends BarMapping<MyType> {

    String title

    MyTypeMapping(MyType x) {
        super(x)
    }

    def mapFrom(MyType x) {
        super.mapFrom(x)
        title = x?.title
    }

    def mapTo(MyType x) {
        x.title = title
    }
}

MySubTypeMapping.groovy

package foo.bar

import foo.example.MySubType

class MySubTypeMapping extends BarMapping<MySubType> {

    String title

    MySubTypeMapping(MySubType x) {
        super(x)
    }

    def mapFrom(MySubType x) {
        super.mapFrom(x)
        title = x?.title
    }

    def mapTo(MySubType x) {
        x.title = title
    }
}

Now, the following lines will produce the compiler error:

> export CLASSPATH={absolute_path_to_this_dir}/java:{absolute_path_to_this_dir}/groovy
> cd java
> javac foo/*.java foo/example/*.java
> cd ../groovy
> groovyc foo/bar/*.groovy
org.codehaus.groovy.control.MultipleCompilationErrorsException: startup failed, foo/bar/MySubTypeMapping.groovy: 20: The type MySubType is not a valid substitute for the bounded parameter <T extends foo.Bar>
 @ line 20, column 32.
   class MySubTypeMapping extends BarMapping<MySubType> {
                                  ^

1 error

This means, MyTypeMapping.groovy compiles whereas MySubTypeMapping.groovy does not compile.

An equivalent Java hierarchy compiles without problems.

Inside a Grails application, the error message is:

> grails compile

Welcome to Grails 1.0.3 - http://grails.org/
Licensed under Apache Standard License 2.0
Grails home is set to: /martin/dev/grails-1.0.3

Base Directory: /martin/dev/projects/generics
Note: No plugin scripts found
Running script /martin/dev/grails-1.0.3/scripts/Compile.groovy
Environment set to development
    [mkdir] Created dir: /neofonie/home/gerlach/.grails/1.0.3/projects/generics/classes
  [groovyc] Compiling 13 source files to /neofonie/home/gerlach/.grails/1.0.3/projects/generics/classes
    [javac] Compiling 12 source files to /neofonie/home/gerlach/.grails/1.0.3/projects/generics/classes
Compilation error: org.codehaus.groovy.control.MultipleCompilationErrorsException: startup failed, /martin/dev/projects/generics/src/groovy/foo/bar/MySubTypeMapping.groovy: 20: The type MySubType is not a valid substitute for the bounded parameter <T extends foo.Bar>
 @ line 20, column 32.
   class MySubTypeMapping extends BarMapping<MySubType> {
                                  ^

1 error

This is the error message with groovy 1.6-beta-1

> groovyc foo/bar/*.groovy
java.lang.reflect.InvocationTargetException
        at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
        at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
        at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
        at java.lang.reflect.Method.invoke(Method.java:597)
        at org.codehaus.groovy.tools.GroovyStarter.rootLoader(GroovyStarter.java:108)
        at org.codehaus.groovy.tools.GroovyStarter.main(GroovyStarter.java:130)
Caused by: org.codehaus.groovy.control.MultipleCompilationErrorsException: startup failed, foo/bar/MySubTypeMapping.groovy: 20: The type MySubType is not a valid substitute for the bounded parameter <T extends foo.Bar>
 @ line 20, column 32.
   class MySubTypeMapping extends BarMapping<MySubType> {
                                  ^

1 error

        at org.codehaus.groovy.control.ErrorCollector.failIfErrors(ErrorCollector.java:296)
        at org.codehaus.groovy.control.CompilationUnit.applyToSourceUnits(CompilationUnit.java:785)
        at org.codehaus.groovy.control.CompilationUnit.compile(CompilationUnit.java:435)
        at org.codehaus.groovy.control.CompilationUnit.compile(CompilationUnit.java:414)
        at org.codehaus.groovy.tools.FileSystemCompiler.compile(FileSystemCompiler.java:63)
        at org.codehaus.groovy.tools.FileSystemCompiler.main(FileSystemCompiler.java:246)
        ... 6 more

Some more observations:

• If I use BarMapping<T> instead of BarMapping<T extends Bar>, the code compiles without any problem - we only loose the possibility to check the generic type at compile time.

• When using IntellijIDEA, everything seems fine even when using <T extends Bar>, I can even execute the unit tests included in the attached archive.