Summary

Javac is not marking method parameters as synthetic or mandated by default, when applicable, as mandated by the JLS in section 13.1. The compiler should be synchronized with the spec.

Problem

Javac is marking method parameters as synthetic or mandated only if the -parameters option is passed. The JLS states that a Java compiler should mark all method parameters as synthetic or mandated when applicable.

Solution

The proposed fix is to mark all method parameters as synthetic or mandated when applicable by emitting the MethodParameters attribute in the class file. This applies to all release / target versions since 8, when the MethodParameters attribute was added.

Specification

The related specification section is JLS 18 § 13.1, in particular:

A binary representation for a class or interface must also contain all of the following: 

[...]

11. A construct emitted by a Java compiler must be marked as synthetic if it does not
    correspond to a construct declared explicitly or implicitly in source code, unless
    the emitted construct is a class initialization method (JVMS §2.9).
12. A construct emitted by a Java compiler must be marked as mandated if it corresponds
    to a formal parameter declared implicitly in source code (§8.8.1, §8.8.9, §8.9.3,
    §15.9.5.1).