JEP 401 states:

It is an error if the operand type of a synchronized statement is any subtype of PrimitiveObject, including any primitive reference type.

// --
This program compiles fine, but fails at runtime:

public primitive class X {
    public static void main(String [] args) {
        X.ref x = X.default;
        synchronized (x) {}
    }
}

// when run:
Exception in thread "main" java.lang.IllegalMonitorStateException
at X.main(X.java:4)