Summary

The javac compiler is not in sync with section §15.12.3 of the JSL 23. In particular the javac compiler is not correctly determining the compile-time result of some polymorphic methods.

Problem

The javac compiler is not in sync with section §15.12.3 of the JLS 23. The compile-time result of method invocations like:

import java.lang.invoke.VarHandle;
class PolymorphicMethodTest<V> {
    VarHandle vh;
    V method(Object obj) {
        return (V)vh.getAndSet(this, obj);
    }
}

are incorrectly determined by javac. Currently the compile-time result of this invocation is V but according to the spec it should be the type erasure of type variable V which is Object. As a result an unchecked cast warning should be issued in this case.

Solution

The proposed solution is to sync the javac compiler with section §15.12.3 of the JLS 23. In particular javac should be updated so that the compile-time result of all polymorphic methods invocations is determined correctly.

The new behavior will be present for --source 24 and later, as this change can generate new warnings in production code and tools maintainers could need some time to adapt to the new behavior.

Specification

Of note for this CSR is section §15.12.3 Compile-Time Step 3: Is the Chosen Method Appropriate? of the JLS 23. We are in particular interested in this text:

• If the compile-time declaration for the method invocation is a signature 
  polymorphic method, then:
    ...
    › Otherwise, if the method invocation expression is the operand of a cast
      expression (§15.16), the compile-time result is the erasure of the type of
      the cast expression (§4.6).

The mentioned type erasure is not currently being done by the javac compiler. After the proposed change, the erasure will be done.