Section 9.4 says "Comparison operators are defined only when the two operands have the same type, except for years and months duration
and days and time duration, which can be compared for equality. Notice, however, that “with the exception of the zerolength
duration, no instance of xs:dayTimeDuration can ever be equal to an instance of xs:yearMonthDuration.”
Thus stating that `P0D == P0Y` despite the fact they are different types and do not comply with the type-lattice equivalency. This is as per the XPATH operation `op:duration-equal` that the spec says equality here conforms to.
It is not crystal clear if this applies to FEEL as well as SFEEL though the same sections does say:
> The semantics of S-FEEL expressions are defined in this section, in terms of the semantics of the XML Schema datatypes
and the XQuery 1.0 and XPath 2.0 Data Model datatypes, and in terms of the corresponding functions and operators
defined by XQuery 1.0 and XPath 2.0 Functions and Operators (prefixed by “op:” below). A complete stand-alone
specification of the semantics is to be found in clause 10.3.2, as part of the definition of FEEL. Within the scope of SFEEL,
the two definitions are equivalent and equally normative.
So, "Within the scope of SFEEL, the two definitions are equivalent and equally normative." seems to indicate that it does hold true for FEEL.
My recommendation is that `P0D == P0Y` does apply to FEEL because it would make sense to a business person. `1 Apple != 1 Orange`, but zero apples is really the same as zero oranges, or zero anything for that matter. And, in practice, zero days really is the same as zero years.