Add forward-compatible flag to validator

[lmckenzi]

(from https://jira.hl7.org/browse/FHIR-13414)

The commitment of "normative" in common use is "no breaking changes". Acknowledging that the precise definition is TBD, one aspect I've heard frequently is the ability to add optional, non-modifier elements to existing resources. The intention as I understand it is for systems with normative definitions to ignore these new elements.

In practice today however, all validators reject resources with elements not included in the release of FHIR validated against. This means that a post-normative addition would be rejected by a validator using an earlier normative release.

Make the change and also provide documentation for other validators to follow (and/or test cases)

[grahamegrieve]

this seems like a very unlikely change to make for the validator. There'd really be no way to decide if the content was valid or not

[lmckenzi]

If a validator is being used in a production system, and the production system wants to be 'forward compatible', this is needed. It's fine to spit out warnings saying "hey, this thing isn't valid in the version of FHIR you're using, but might be in a future one". But they wouldn't want it to raise an error. That's definitely a validation thing.

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This issue has been automatically marked as stale because it has not had recent activity. It will be closed in 5 days if no further activity occurs. Thank you for your contributions.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed in 5 days if no further activity occurs. Thank you for your contributions.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed in 5 days if no further activity occurs. Thank you for your contributions.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed in 5 days if no further activity occurs. Thank you for your contributions.