assert false: "never called"
        }))

        expect:
        propertyWithBadValue.toString() == "property(java.lang.String, map(java.lang.String provider(?) check-type()))"
        providerWithNoValue().toString() == "property(java.lang.String, undefined)"
    }

    def "can set null value to discard value"() {
        given:
        def property = property()
        property.set(someValue())

    @Override
    AbstractProperty<T, ? extends ValueSupplier> providerWithValue(T value) {
        return propertyWithNoValue().value(value)
    }

    @Override
    AbstractProperty<T, ? extends ValueSupplier> providerWithNoValue() {
        return propertyWithNoValue()
    }

    @Override
    abstract AbstractProperty<T, ? extends ValueSupplier> propertyWithNoValue()

abstract class PropertySpec<T> extends ProviderSpec<T> {
    @Override
    PropertyInternal<T> providerWithValue(T value) {
        return propertyWithNoValue().value(value)
    }

    @Override
    PropertyInternal<T> providerWithNoValue() {
        return propertyWithNoValue()
    }

    /**
     * Returns a property with _no_ value.
     */
    abstract PropertyInternal<T> propertyWithNoValue()

    @Override