DocumentModel contains tags relationship to TagModel. A View receives selectedTag. Predicate (as part of @Query) should filter documents that have given tag.

let selectedTag = TagModel()

#Predicate<DocumentModel> { document in
   document.tags.contains {
      $0.tagID == selectedTag.tagID
   }
}

Predicate doesn’t build because: Cannot convert to closure result type ‘any StandardPredicateExpression’.

Predicate macro expands to:

Foundation.Predicate<DocumentModel>({ document in
   PredicateExpressions.build_contains(
       PredicateExpressions.build_KeyPath(
           root: PredicateExpressions.build_Arg(document),
           keyPath: .tags
       )
   ) {
       PredicateExpressions.build_Equal(
           lhs: PredicateExpressions.build_KeyPath(
               root: PredicateExpressions.build_Arg($0),
               keyPath: .tagID
           ),
           rhs: PredicateExpressions.build_KeyPath(
               root: PredicateExpressions.build_Arg(selectedTag),
               keyPath: .tagID
           )
       )
   }
})

Full error message:
Cannot convert value of type 'PredicateExpressions.SequenceContainsWhere<PredicateExpressions.KeyPath<PredicateExpressions.Variable<DocumentModel>, [TagModel]>, PredicateExpressions.Equal<PredicateExpressions.KeyPath<PredicateExpressions.Variable<TagModel>, String>, PredicateExpressions.KeyPath<PredicateExpressions.Value<TagModel>, String>>>' to closure result type 'any StandardPredicateExpression<Bool>'

Apple Developer Documentation has example code using contains() and comparing strings:

let messagePredicate = #Predicate<Message> { message in
   message.recipients.contains {
       $0.firstName == message.sender.firstName
   }
}

How to filter documents that contain selected tag?