[GROOVY-9956] Cannot infer correct type argument when passing a subtype of the declared type argument

• Type: Bug
• Status: Closed
• Priority: Major
• Resolution: Fixed
• Affects Version/s: 3.0.7, 4.0.0-alpha-2
• Fix Version/s: 4.0.0-alpha-3, 3.0.11, 2.5.18
• Component/s: Static compilation, Static Type Checker
• Assignee: Eric Milles
• Reporter: Stefanos Chaliasos
• Created: 26/Feb/21 17:05
• Updated: 18/Jul/22 16:49
• Resolved: 03/Mar/21 00:24

Description

I have the following Groovy program.

class Foo<T> {
    T f;
    Foo (T f) {
      this.f = f;
    }

}

interface Bar{}
class Baz<T> implements Bar {}

@groovy.transform.TypeChecked
class Main  {
  public static void main(String[] args) {
    Foo<Bar> x1 = new Foo<Bar>(new Baz<Integer>()) // Compiles
    Foo<Bar> x2 = new Foo<>(new Baz<Integer>()) // Does not compile
  }
}

Actual Behavior

The program does not compile, and I get the following error.

org.codehaus.groovy.control.MultipleCompilationErrorsException: startup failed:
Main.groovy: 16: [Static type checking] - Incompatible generic argument types. Cannot assign Foo <Baz> to: Foo <Bar>
 @ line 16, column 19.
       Foo<Bar> x2 = new Foo<>(new Baz<Integer>())
                     ^

1 error

Expected Behavior

Compile successfully.

Affected Version

I have also tested it with the compiler from Master (commit: 666627b3be7718e3265fc42b473060bc73f42e2f).

Comment

If I remove the type parameter from Baz, then it only fails when compiled with the compiler from the Master.
Specifically, the following test case compiles with 3.0.7 and 4.0.0-alpha-2 compilers but fails with the same message as the previous test case when using the compiler from the Master.

class Foo<T> {
    T f;
    Foo (T f) {
      this.f = f;
    }

}

interface Bar{}
class Baz implements Bar {}

@groovy.transform.TypeChecked
class Main  {
  public static void main(String[] args) {
    Foo<Bar> x1 = new Foo<Bar>(new Baz()) // Compiles
    Foo<Bar> x2 = new Foo<>(new Baz()) // Does not compile
  }
}