The following test fails with "Should be null":

    value final class Test90Value {
        public final __NotFlattened MyValue1 valueField;

        public Test90Value() {
            valueField = MyValue1.createDefaultDontInline();
        }

        public Test90Value setValueField(MyValue1 valueField) {
            return __WithField(this.valueField, valueField);
        }
    }

    // Test scalarization of default value type with non-flattenable field
    @Test()
    public Test90Value test90(boolean b) {
        Test90Value vt1 = Test90Value.default;
        if ((Object)vt1.valueField != null) {
            throw new RuntimeException("Should be null");
        }
        Test90Value vt2 = vt1.setValueField(testValue1);
        return b ? vt1 : vt2;
    }

    @DontCompile
    public void test90_verifier(boolean warmup) {
        test90(true);
        test90(false);
    }

The problem is in valuetypenode.cpp:510 where we should check for
vt->field_is_flattenable(i).