Problems constraining an element's type in Profile A to a profiled type (Profile B) with child elements that have slices when Profile A's parent has constrained child element's of the element type first

[cmoesel]

This is a tricky one that is best explained via an example.

If you have a profile that constrains a child element of one of its element's that is a complex type (e.g., a Patient profile that constrains meta.lastUpdated):

Profile: BaseParentPatientProfile
Parent: Patient
* meta.lastUpdated MS // force parent to expand all child elements of meta

And you define a profile on that same complex type (e.g., a profile on Meta) that introduces slicing on one of the complex type's children (e.g., slicing on Meta.security) and at least one slice (e.g., Meta.security[ABC]):

Profile: MetaWithABCSecuritySlice
Parent: Meta
* security ^slicing.discriminator.type = #value
* security ^slicing.discriminator.path = "system"
* security ^slicing.rules = #closed
* security ^slicing.description = "Slice on system value"
* security contains ABC 1..1
* security[ABC].system = "http://example.org/system/abc"

And then you define a child profile of the first parent profile that constrains the complex type element to the profile of the the complex element (e.g., a profile of BaseParentPatientProfile that constrains meta to MetaWithABCSecuritySlice) and then reference the slice defined in the complex type's profile (e.g., meta.security[ABC]):

Profile: BrokenPatientProfile
Parent: BaseParentPatientProfile
* meta only MetaWithABCSecuritySlice
* meta.security[ABC] MS

Then SUSHI will fail to find the element corresponding to the slice defined in the complex type's profile:

error No element found at path meta.security[ABC] for FlagRule in BrokenPatientProfile, skipping rule

I think this is because the parent profile (e.g., BaseParentPatientProfile) has to expand all the children of the complex type element (e.g., children of Patient.meta) to apply the constraint on a child element (e.g., Patient.meta.lastUpdated). When the type constraint (e.g., meta only MetaWithABCSecuritySlice) is applied to that element in a child profile, SUSHI does not merge the type's profile elements with the already expanded child elements from the parent profile -- so when it tries to reference a child element from the type's profile in the context of the profile that applied the type, it can't find it.

A potential workaround is to reintroduce the slicing and slices from the complex type profile into the profile that applies the complex type. E.g.:

Profile: WorkaroundPatientProfile
Parent: BaseParentPatientProfile
* meta only MetaWithABCSecuritySlice
* meta.security ^slicing.discriminator.type = #value
* meta.security ^slicing.discriminator.path = "system"
* meta.security ^slicing.rules = #closed
* meta.security ^slicing.description = "Slice on system value"
* meta.security contains ABC 1..1
* meta.security[ABC] MS

This causes some redundancy in the definitions, but I don't think it is harmful as long as the slicing definition and slices are a perfect match to what was in the complex type's profile.

Full Example on FSH Online

NOTE: I suspect the correct fix for this is that when a type constraint is applied to an element whose children have already been expanded, that type's elements should be merged with the existing expanded elements. I'm not sure how difficult such a merge would be.