See bottom most para in http://mail.openjdk.java.net/pipermail/valhalla-dev/2018-February/003803.html

ATM, javac allows only a static method flagged as a value factory to update instance fields of a value type - which operations don't really update an existing instances fields but implement copy on write semantics by virtue of code generation using withfield opcode.

This ticket is to eliminate the static value factory mechanism altogether and allow any method in the same nest to perform the withfield operation.