discriminator [url] does not have fixed value, binding or existence assertions - hl7-fhir

Question

Given a profile that references an extension via URL as usual (IOW, 'sliced' by extension URL), if the definition of the extension does not include a fixedUri constraint then the HAPI validator (5.6.26) emits the following error:
Slicing cannot be evaluated: Could not match discriminator ([url]) for slice [...]
in profile [...] - the discriminator [url] does not have fixed value, binding or
existence assertions.
This seems exceedingly odd, since Extension.url is a mandatory field and as such has an implicit 'existence assertion'. Moreover, one would think that - by definition - it would have the fixed value specified in StructureDefinition.url for a top-level extension.
Note: the problem can easily be prevented by adding the aforementioned fixedUri definition to the extension profile, if there is the remotest chance that the extension might ever be involved in slicing by URL (repeating the URL specified in StructureDefinition.url).
{
"id": "Extension.url",
"path": "Extension.url",
"fixedUri": "https://foo/StructureDefinition/blah"
}
However, I would like to understand what is going on here. Can some shed light on this?

Answer 1

There is no "by definition". It's actually possible to have an extension whose canonical URL differs from the Extension.url - if you're creating a profile of an existing extension. The Extension data type does, however explicitly pre-define the slicing which is what requires url to be explicitly declared as a fixed value. In short, everything is functioning as intended. If you're defining a profile with a base of 'Extension', you'll have to define a fixedUrl.