Using shacl to validate a property that has at most one value in its properties

I'm trying to create a shacl based on the ontology that my organization is developing (in dutch): https://wegenenverkeer.data.vlaanderen.be/

The objects described have attributes (properties), that have a specified datatype. The datatype can a primitive (like string or decimal) or complex, which means the property will have properties itself (nested properties). For example: an asset object A will have an attribute assetId which is a complex datatype DtcIdentificator, which consists of two properties itself. I have succesfully created a shacl that validates objects by creating multiple shapes and nesting them.

I now run into the problem of what we call union datatypes. These are a special kind of complex datatypes. They are still nested datatypes: the attribute with the union datatypes will have multiple properties but only exactly zero or one of those properties may have a value. If the attribute has 2 properties with values, it is invalid. How can I create such a constraint in shacl?

Example (in dutch): https://wegenenverkeer.data.vlaanderen.be/doc/implementatiemodel/union-datatypes/#Afmeting%20verkeersbord
A traffic sign (Verkeersbord, see https://wegenenverkeer.data.vlaanderen.be/doc/implementatiemodel/signalisatie/#Verkeersbord) can have a property afmeting (size) of the datatype DtuAfmetingVerkeersbord.
If an asset A of this type would exist, I could define its size as (in dotnotation):

A.afmeting.rond.waarde = 700

-or-

A.afmeting.driehoekig.waarde = 400

Both are valid ways of using the afmeting property, however, if they are both used for the same object, this becomes invalid, as only one property of A.afmeting may have a value.

I have tried using the union constraint in shacl, but soon found out that that has nothing to do with what we call "union datatypes"

I think the reason you are struggling is because this kind of problem is usually modelled differently. Basically you have different types of Traffic signs and these signs can have measurements. With the model as you described, A.afmeting.rond.waarde captures 2 ideas using 1 property: (a) the type and (b) the size. From your question, this seems to be the intend. However, this is usually not how this kind of problem is addressed.

A more intuitive design is for Traffic sign to have 2 different properties: (a) type and (b) a measurement. The Traffic sign types are achthoekig, driehoekig, etc. Then you can use SHACL to check that a traffic sign has either both or no properties for a traffic sign.

I have implemented a solution for this, using a combination of 'or' and 'and' constraints:

imel:DtuTestUnionTypeUnionConstraint a sh:NodeShape ;
     rdfs:comment "union constraint of https://wegenenverkeer.data.vlaanderen.be/ns/implementatieelement#DtuTestUnionType" ;
     sh:or ( [ sh:and ( [ sh:maxCount 0 ;
          sh:path imel:DtuTestUnionType.unionKwantWrd ]
          [ sh:maxCount 0 ;
          sh:path imel:DtuTestUnionType.unionString ] ) ]
     [ sh:and ( [ sh:minCount 1 ;
          sh:path imel:DtuTestUnionType.unionKwantWrd ]
          [ sh:maxCount 0 ;
          sh:path imel:DtuTestUnionType.unionString ] ) ]
     [ sh:and ( [ sh:maxCount 0 ;
          sh:path imel:DtuTestUnionType.unionKwantWrd ]
          [ sh:minCount 1 ;
          sh:path imel:DtuTestUnionType.unionString ] ) ] ) ;
     sh:targetObjectsOf imel:DtuTestUnionType .