Add TaggedStructuredQueries trait

Author: stephencelis

Sources/StructuredQueriesCore/Documentation.docc/Articles/DefiningYourSchema.md

@@ -331,6 +331,51 @@ With that you can insert reminders with notes like so:
   }
 }
 
+#### Tagged identifiers
+
+The [Tagged](https://github.com/pointfreeco/swift-tagged) library provides lightweight syntax for
+introducing type-safe identifiers (and more) to your models. StructuredQueries ships support for
+Tagged with a `TaggedStructuredQueries` package trait, which is available starting from Swift 6.1.
+
+To enable the trait, specify it in the Package.swift file that depends on StructuredQueries:
+
+```diff
+ .package(
+   url: "https://github.com/pointfreeco/swift-structured-queries",
+   from: "0.2.0",
++  traits: ["TaggedStructuredQueries"]
+ ),
+```
+
+This will allow you to introduce distinct `Tagged` identifiers throughout your schema:
+
+```diff
+ @Table
+ struct RemindersList: Identifiable {
+-  let id: Int
++  typealias ID = Tagged<Self, Int>
++  let id: ID
+   // ...
+ }
+ @Table
+ struct Reminder: Identifiable {
+-  let id: Int
++  typealias ID = Tagged<Self, Int>
++  let id: ID
+   // ...
+   var remindersList: Reminder.ID
+ }
+```
+
+This adds a new layer of type-safety when constructing queries. Previously comparing a
+`RemindersList.ID` to a `Reminder.ID` would compile just fine, even though it is a nonsensical thing
+to do. But now, such a comparison is a compile time error:
+
+```
+RemindersList.leftJoin(Reminder.all) {
+  $0.id == $1.id  // 🛑 Requires the types 'Reminder.ID' and 'RemindersList.ID' be equivalent
+}
+
 ### Primary keyed tables
 
 It is possible to tell let the `@Table` macro know which property of your data type is the primary

Sources/StructuredQueriesCore/Traits/Tagged.swift

@@ -14,12 +14,14 @@
   }
 
   extension Tagged: QueryRepresentable where RawValue: QueryRepresentable {
-    public init(queryOutput: RawValue.QueryOutput) {
-      self.init(RawValue(queryOutput: queryOutput))
+    public typealias QueryOutput = Tagged<Tag, RawValue.QueryOutput>
+
+    public var queryOutput: QueryOutput {
+      QueryOutput(rawValue: self.rawValue.queryOutput)
     }
 
-    public var queryOutput: RawValue.QueryOutput {
-      rawValue.queryOutput
+    public init(queryOutput: QueryOutput) {
+      self.init(rawValue: RawValue(queryOutput: queryOutput.rawValue))
     }
   }
 

Tests/StructuredQueriesTests/UpdateTests.swift

@@ -365,7 +365,7 @@ extension SnapshotTests {
         WHERE ("reminders"."id" = 1)
         RETURNING "dueDate"
         """
-      }results: {
+      } results: {
         """
         ┌─────┐
         │ nil │
@@ -383,7 +383,7 @@ extension SnapshotTests {
         WHERE ("reminders"."id" = 1)
         RETURNING "dueDate"
         """
-      }results: {
+      } results: {
         """
         ┌────────────────────────────────┐
         │ Date(2018-01-29T00:08:00.000Z) │