To facilitate prototyping in the 'valhalla/type-restrictions' branch:

Declare a type annotation ('@Target(TYPE_USE)') named 'java.lang.invoke.RestrictedType' whose 'value' element has type String.

When javac generates code for a field whose type is annotated by @RestrictedType("QFoo;"), it generates a RestrictedField attribute pointing to a Utf8 constant representing the given String.

There is no need to validate the string—just generate the attribute, and the JVM will interpret it.

This capability is orthogonal to '-XXflattenWithTypeRestrictions'. That flag supports experiments with different compilation strategies for primitive value types. The @RestrictedType annotation supports ad hoc attribute generation, for more fine-grained control.

There is no need to support interop between the two approaches—so, for example, when *reading* a class file, if '-XXflattenWithTypeRestrictions' is set, it's okay to assume that the RestrictedField attribute stores that actual Java-language type of the field.