com.sun.tools.javac.comp.Check#checkRefType(com.sun.tools.javac.util.JCDiagnostic.DiagnosticPosition, com.sun.tools.javac.code.Type)

as it stands on lworld branch tip reads:

if (t.isReference() && !types.isValue(t))
    return t;
else
    return typeTagError(pos,
                                diags.fragment(Fragments.TypeReqRef),
                                t);

This code was carried over from the original valhalla prototype, where values were rooted at a point other than j.l.O and so such forbiddance made sense. (values cannot be type arguments unless a class was anyfied)

As it stands it forbids various operations on value types that are legitimate in the lworld such as:

    - Value types can be type arguments in generic type parameterizations.
    - Value types can be explicit type witnesses in generic method invocations
    - Value types can be wildcard bounds (even if only with limited utility on account of being final classes - a la String)
    - Value types can be intersection (cast) type components
    - Value instances can be enclosing instances.

(Whether values should be restricted to top level classes, or can also be
inner local nested is an open design question - but javac already supports
them as non-top level classes also, but that support is partial and broken in ways that will be addressed by a fix to this ticket)