Improve overload resolution for KnowledgeSource protocols (#16)

# Improve overload resolution for KnowledgeSource protocols

## ♻️ Current situation & Problem
A `SharedRepository` typically has multiple overloads for a range of
different `KnowledgeSource` protocols. A knowledge source might provide
default values or might be computed. Typically, you only adopt a single
knowledge source protocol. However, you cannot prevent users from
adopting multiple. This PR improves the overload resolution in these
scenarios. All of the specialized KnowledgeSource protocols inherit the
base `KnowledgeSource` protocol. Therefore, Swift resolution will
automatically prefer the more specialized type. Problems arise if you
accidentally (or on purpose) conform to a computed KnowledgeSource
variant and the `DefaultProvidingKnowledgeSource`. In this case we
resolve the conflict and generally prefer the computed knowledge source
protocol. We consider a computed knowledge source more specialized than
only providing a default value.

## ⚙️ Release Notes 
* Improve overload resolution for knowledge source protocols in certain
situations


## 📚 Documentation
--


## ✅ Testing
Added some  unit testing to verify overload behavior.

## 📝 Code of Conduct & Contributing Guidelines 

By submitting creating this pull request, you agree to follow our [Code
of
Conduct](https://github.com/StanfordSpezi/.github/blob/main/CODE_OF_CONDUCT.md)
and [Contributing
Guidelines](https://github.com/StanfordSpezi/.github/blob/main/CONTRIBUTING.md):
- [x] I agree to follow the [Code of
Conduct](https://github.com/StanfordSpezi/.github/blob/main/CODE_OF_CONDUCT.md)
and [Contributing
Guidelines](https://github.com/StanfordSpezi/.github/blob/main/CONTRIBUTING.md).

Sources/SpeziFoundation/SharedRepository/SendableSharedRepository.swift

@@ -61,11 +61,13 @@ public protocol SendableSharedRepository<Anchor>: Sendable {
     /// A subscript to retrieve or set a `KnowledgeSource`.
     /// - Parameter source: The ``KnowledgeSource`` type.
     /// - Returns: The stored ``KnowledgeSource/Value`` or `nil` if not present.
+    @_disfavoredOverload
     subscript<Source: KnowledgeSource<Anchor>>(_ source: Source.Type) -> Source.Value? where Source.Value: Sendable { get set }
 
     /// A subscript to retrieve or set a `DefaultProvidingKnowledgeSource`.
     /// - Parameter source: The ``DefaultProvidingKnowledgeSource`` type.
     /// - Returns: The stored ``KnowledgeSource/Value`` or the ``DefaultProvidingKnowledgeSource/defaultValue``.
+    @_disfavoredOverload
     subscript<Source: DefaultProvidingKnowledgeSource<Anchor>>(_ source: Source.Type) -> Source.Value where Source.Value: Sendable { get }
 
     /// A subscript to retrieve a `ComputedKnowledgeSource`.
@@ -119,6 +121,7 @@ extension SendableSharedRepository {
 
 
     /// Default subscript implementation delegating to ``get(_:)`` or ``set(_:value:)``.
+    @_disfavoredOverload
     public subscript<Source: KnowledgeSource<Anchor>>(_ source: Source.Type) -> Source.Value? where Source.Value: Sendable {
         get {
             get(source)
@@ -142,6 +145,7 @@ extension SendableSharedRepository {
     }
 
     /// Default subscript implementation delegating to ``get(_:)`` or providing a ``DefaultProvidingKnowledgeSource/defaultValue``.
+    @_disfavoredOverload
     public subscript<Source: DefaultProvidingKnowledgeSource<Anchor>>(_ source: Source.Type) -> Source.Value where Source.Value: Sendable {
         self.get(source) ?? source.defaultValue
     }

Sources/SpeziFoundation/SharedRepository/SharedRepository.swift

@@ -62,11 +62,13 @@ public protocol SharedRepository<Anchor> {
     /// A subscript to retrieve or set a `KnowledgeSource`.
     /// - Parameter source: The ``KnowledgeSource`` type.
     /// - Returns: The stored ``KnowledgeSource/Value`` or `nil` if not present.
+    @_disfavoredOverload
     subscript<Source: KnowledgeSource<Anchor>>(_ source: Source.Type) -> Source.Value? { get set }
 
     /// A subscript to retrieve or set a `DefaultProvidingKnowledgeSource`.
     /// - Parameter source: The ``DefaultProvidingKnowledgeSource`` type.
     /// - Returns: The stored ``KnowledgeSource/Value`` or the ``DefaultProvidingKnowledgeSource/defaultValue``.
+    @_disfavoredOverload
     subscript<Source: DefaultProvidingKnowledgeSource<Anchor>>(_ source: Source.Type) -> Source.Value { get }
 
     /// A subscript to retrieve a `ComputedKnowledgeSource`.
@@ -120,6 +122,7 @@ extension SharedRepository {
 
 
     /// Default subscript implementation delegating to ``get(_:)`` or ``set(_:value:)``.
+    @_disfavoredOverload
     public subscript<Source: KnowledgeSource<Anchor>>(_ source: Source.Type) -> Source.Value? {
         get {
             get(source)
@@ -140,6 +143,7 @@ extension SharedRepository {
     }
 
     /// Default subscript implementation delegating to ``get(_:)`` or providing a ``DefaultProvidingKnowledgeSource/defaultValue``.
+    @_disfavoredOverload
     public subscript<Source: DefaultProvidingKnowledgeSource<Anchor>>(_ source: Source.Type) -> Source.Value {
         self.get(source) ?? source.defaultValue
     }

Tests/SpeziFoundationTests/SendableSharedRepositoryTests.swift

@@ -188,4 +188,10 @@ final class SendableSharedRepositoryTests: XCTestCase {
         optionalComputedValue = nil
         XCTAssertEqual(repository[OptionalComputedTestStruct<_StoreComputePolicy, Repository>.self], 4)
     }
+
+    @MainActor
+    func testComputedKnowledgeSourcePreferred() {
+        let value = repository[ComputedDefaultTestStruct<_StoreComputePolicy, Repository>.self]
+        XCTAssertEqual(value, computedValue)
+    }
 }

Tests/SpeziFoundationTests/SharedRepositoryTests.swift

@@ -53,7 +53,7 @@ struct DefaultedTestStruct: DefaultProvidingKnowledgeSource, TestTypes {
 }
 
 
-struct ComputedTestStruct<Policy: ComputedKnowledgeSourceStoragePolicy, Repository>: ComputedKnowledgeSource {
+struct ComputedTestStruct<Policy: ComputedKnowledgeSourceStoragePolicy, Repository>: KnowledgeSource, ComputedKnowledgeSource {
     typealias Anchor = TestAnchor
     typealias Value = Int
     typealias StoragePolicy = Policy
@@ -65,6 +65,23 @@ struct ComputedTestStruct<Policy: ComputedKnowledgeSourceStoragePolicy, Reposito
     }
 }
 
+struct ComputedDefaultTestStruct<Policy: ComputedKnowledgeSourceStoragePolicy, Repository>: ComputedKnowledgeSource, DefaultProvidingKnowledgeSource {
+    typealias Anchor = TestAnchor
+    typealias Value = Int
+    typealias StoragePolicy = Policy
+
+    static var defaultValue: Int {
+        XCTFail("\(Self.self) access result in default value execution!")
+        return -1
+    }
+
+    static func compute(from repository: Repository) -> Int {
+        MainActor.assumeIsolated {
+            computedValue
+        }
+    }
+}
+
 
 struct OptionalComputedTestStruct<Policy: ComputedKnowledgeSourceStoragePolicy, Repository>: OptionalComputedKnowledgeSource {
     typealias Anchor = TestAnchor
@@ -261,4 +278,10 @@ final class SharedRepositoryTests: XCTestCase {
         optionalComputedValue = nil
         XCTAssertEqual(repository[OptionalComputedTestStruct<_StoreComputePolicy, Repository>.self], 4)
     }
+
+    @MainActor
+    func testComputedKnowledgeSourcePreferred() {
+        let value = repository[ComputedDefaultTestStruct<_StoreComputePolicy, Repository>.self]
+        XCTAssertEqual(value, computedValue)
+    }
 }