[java] Fix #6146: ClassCastException in TypeTestUtil

[oowekyala]

#6146 (comment)

public class Foo {

  @Override
  public void bar() {
      Iterable<Bar> local = filter(unknown(), input -> input.debug());
  }

  public static <T> Iterable<T> filter(final Iterable<T> unfiltered,
    final java.util.function.Predicate<? super T> retainIfTrue) {
    return null;
  }
}

(thanks for the test case @zbynek)

In this example, the problem was that

1. We find the method filter is applicable
2. When inferring the invocation type, we first replace T with an inference variable 'a. We set the type of the lambda to be Predicate<? super 'a>, which means the type of its parameter input is 'a.
3. We encounter both the constraints T = (*unknown*) and T = Bar (from the argument unknown() and the context type Iterable<Bar> respectively). Those are considered incompatible so we fail.
4. After failing invocation, we don't clean up the type of the lambda or of input so the inference variable stay in the tree
5. Later on during rule execution, TypeTestUtil finds the inference variable and tries to cast its symbol it to JClassSymbol and fails

This fix has three parts

1. We now properly clean up the tree when invocation fails, that is here we will type the lambda as Predicate<(*error*)> and type input as (*error*), removing the 'a from AST nodes. This was the root problem really.
2. We also now consider constraints like T = (* unknown *) and T = Bar to be compatible. This improves type inference when types are missing from the classpath, and will allow the invocation in this example not to fail. To be complete, {T = (*unknown*), T = Bar} will solve to Bar, and {T = (*error*), T = Bar} will solve to (*error*), that is, the error type is contagious.
3. Lastly I made TypeTestUtil more defensive so that it cannot throw a ClassCastException anymore. This will hide future issues... 🤷

Related issues

• Fix [java] ClassCastException: class InferenceVarSym cannot be cast to class JClassSymbol #6146

Ready?

• Added unit tests for fixed bug/feature
• Passing all unit tests
• Complete build ./mvnw clean verify passes (checked automatically by github actions)
• Added (in-code) documentation (if needed)

[pmd-actions-helper]

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No regression tested rules have been changed.

(comment created at 2025-10-28 08:02:48+00:00 for 76126d5)

[adangel]

Thanks!