xsd:g{Day,Month,Year,..} should not be promotable to xsd:date (but xsd:date should stay demotable to xsd:g{..}) #393
Description
That the xsd g-types are promotable to xsd:date means that they can be compared with xsd:date.
The comparisons don't really make sense because to be promoted to xsd:date the missing components are filled in from the unix-epoch. The only direction we need is demotion from xsd:date to xsd g types (according to spec).
That the comparision works currently means for example that "1960-02-01"^^xsd:date < "--01"^^xsd:gMonth is true, which may be unexpected.
To facilitate this change we need to change the interfaces for promotion to allow for failure on promotion as well.