t900.scala:4: error: type mismatch;
 found   : Foo.this.x.type (with underlying type Foo.this.bar)
 required: AnyRef
Note that bar is unbounded, which means AnyRef is not a known parent.
Such types can participate in value classes, but instances
cannot appear in singleton types or in reference comparisons.
        def break(): x.type
                     ^
one error found