UseVarForObject creates broken code when replacing type variable

[uhafner]

What version of OpenRewrite are you using?

I am using

• <rewrite-maven-plugin.version>5.39.2</rewrite-maven-plugin.version>
• <rewrite-testing-frameworks.version>2.17.1</rewrite-testing-frameworks.version>
• <rewrite-static-analysis.version>1.15.0</rewrite-static-analysis.version>
• <rewrite-migrate-java.version>2.23.0</rewrite-migrate-java.version>
• <rewrite-recommendations.version>1.8.4</rewrite-recommendations.version>

How are you running OpenRewrite?

I am using the Maven plugin, and my project is a single module project. I did run the command mvn -X clean rewrite:run.

See https://github.com/uhafner/codingstyle for details. The OpenRewrite configuration is visible in the pom.xml.

What is the smallest, simplest way to reproduce the problem?

package edu.hm.hafner.util;

import java.io.Serializable;

import org.junit.jupiter.api.DisplayName;
import org.junit.jupiter.api.Test;

public abstract class SerializableTest<T extends Serializable> extends ResourceTest {
    protected abstract T createSerializable();

    @Test
    @DisplayName("should be serializable: instance -> byte array -> instance")
    void shouldBeSerializable() {
        T serializableInstance = createSerializable();
    }
}

What did you expect to see?

Type variable should not be replaced by var.

package edu.hm.hafner.util;

import java.io.Serializable;

import org.junit.jupiter.api.DisplayName;
import org.junit.jupiter.api.Test;

public abstract class SerializableTest<T extends Serializable> extends ResourceTest {
    protected abstract T createSerializable();

    @Test
    @DisplayName("should be serializable: instance -> byte array -> instance")
    void shouldBeSerializable() {
        T serializableInstance = createSerializable();
    }
}

What did you see instead?

package edu.hm.hafner.util;

import java.io.Serializable;

import org.junit.jupiter.api.DisplayName;
import org.junit.jupiter.api.Test;

public abstract class SerializableTest<T extends Serializable> extends ResourceTest {
    protected abstract T createSerializable();

    @Test
    @DisplayName("should be serializable: instance -> byte array -> instance")
    void shouldBeSerializable() {
        var serializableInstance = __P__.<error>;
    }
}

What is the full stack trace of any errors you encountered?

No stack trace produced.

Are you interested in [contributing a fix to OpenRewrite]

Not right now.

[MBoegers]

In this case the derivation of the type seems broken. Such change is indeed not correct. The expectation seems right, I'll check what's missing. Thanks for reporting 🙏

[MBoegers]

We can reproduce this with the test below. The issue is caused be the type boundary (extends Serializable) at the generic parameter. Further exploration is needed here.

@Test
    @Issue("https://github.com/openrewrite/rewrite-migrate-java/issues/550")
    void genericType() {
        rewriteRun(
          java(
            """
              import java.io.Serializable;
              
              abstract class Outter<T extends Serializable> {
                 abstract T doIt();
                 void trigger() {
                    T x = doIt();
                 }
              }
              """, """
              import java.io.Serializable;
              
              abstract class Outter<T extends Serializable> {
                 abstract T doIt();
                 void trigger() {
                    var x = doIt();
                 }
              }
              """
          )
        );
    }

[MBoegers]

One additional question @uhafner, why are you expecting T instead of var? var is perfectly fine here, from the JLS view.

[uhafner]

One additional question @uhafner, why are you expecting T instead of var? var is perfectly fine here, from the JLS view.

You are right, it works with var as well (I never used var for type variables before).