Avoid runtime casts to parameterized protocol types, which require newer OS versions

Sources/Testing/Test+Macro.swift

@@ -113,7 +113,7 @@ extension Test {
     sourceLocation: SourceLocation
   ) -> Self {
     let typeName = _typeName(containingType, qualified: false)
-    return Self(name: typeName, displayName: displayName, traits: traits, sourceLocation: sourceLocation, containingType: containingType, testCases: nil)
+    return Self(name: typeName, displayName: displayName, traits: traits, sourceLocation: sourceLocation, containingType: containingType)
   }
 }
 

Sources/Testing/Test.Case.Generator.swift

@@ -265,6 +265,53 @@ extension Test.Case.Generator: Sequence {
   }
 }
 
-// MARK: - TestCases
+// MARK: - Type-erasing to Sequence<Test.Case>
 
-extension Test.Case.Generator: TestCases {}
+/// A type-erased protocol describing a sequence of ``Test/Case`` instances.
+///
+/// This protocol is necessary because it is not currently possible to express
+/// `Sequence<Test.Case> & Sendable` as an existential (`any`)
+/// ([96960993](rdar://96960993)). It is also not possible to have a value of
+/// an underlying generic sequence type without specifying its generic
+/// parameters.
+private protocol _TestCases: Sequence<Test.Case> & Sendable {}
+
+extension Test.Case.Generator: _TestCases {}
+
+extension Test {
+  /// A type-erasing wrapper for a `_TestCases`-conforming type.
+  ///
+  /// See the documentation for the `_TestCases` protocol explaining why this
+  /// type erasure is necessary.
+  struct Cases: Sequence, Sendable {
+    /// The type-erased sequence of test cases this instance wraps.
+    private var _sequence: any _TestCases
+
+    init<S>(_ testCases: Test.Case.Generator<S>) {
+      _sequence = testCases
+    }
+
+    /// A type-erasing wrapper for an iterator of a `_TestCases`-conforming
+    /// type.
+    struct Iterator: IteratorProtocol {
+      /// The type-erased test case iterator this instance wraps.
+      private var _iterator: any IteratorProtocol<Test.Case>
+
+      fileprivate init(iterator: any IteratorProtocol<Test.Case>) {
+        _iterator = iterator
+      }
+
+      mutating func next() -> Test.Case? {
+        _iterator.next()
+      }
+    }
+
+    func makeIterator() -> Iterator {
+      Iterator(iterator: _sequence.makeIterator())
+    }
+
+    var underestimatedCount: Int {
+      _sequence.underestimatedCount
+    }
+  }
+}

Sources/Testing/Test.Case.swift

@@ -96,12 +96,3 @@ extension Test {
     public var secondName: String?
   }
 }
-
-/// A type-erased protocol describing a sequence of ``Test/Case`` instances.
-///
-/// This protocol is necessary because it is not currently possible to express
-/// `Sequence<Test.Case> & Sendable` as an existential (`any`)
-/// ([96960993](rdar://96960993)). It is also not possible to have a value of
-/// an underlying generic sequence type without specifying its generic
-/// parameters.
-protocol TestCases: Sequence<Test.Case> & Sendable {}

Sources/Testing/Test.swift

@@ -95,19 +95,17 @@ public struct Test: Sendable {
   public var xcTestCompatibleSelector: __XCTestCompatibleSelector?
 
   /// Storage for the ``testCases`` property.
-  private var _testCases: (any TestCases)?
+  private var _testCases: Test.Cases?
 
   /// The set of test cases associated with this test, if any.
   ///
   /// For parameterized tests, each test case is associated with a single
   /// combination of parameterized inputs. For non-parameterized tests, a single
   /// test case is synthesized. For test suite types (as opposed to test
   /// functions), the value of this property is `nil`.
-  ///
-  /// The value of this property is guaranteed to be `Sendable`.
   @_spi(ExperimentalParameterizedTesting)
-  public var testCases: (any Sequence<Test.Case>)? {
-    _testCases as? any Sequence<Test.Case>
+  public var testCases: (some Sequence<Test.Case> & Sendable)? {
+    _testCases
   }
 
   /// Whether or not this test is parameterized.
@@ -139,23 +137,39 @@ public struct Test: Sendable {
     containingType != nil && testCases == nil
   }
 
+  /// Initialize an instance of this type representing a test suite type.
   init(
+    name: String,
+    displayName: String? = nil,
+    traits: [any Trait],
+    sourceLocation: SourceLocation,
+    containingType: Any.Type
+  ) {
+    self.name = name
+    self.displayName = displayName
+    self.traits = traits
+    self.sourceLocation = sourceLocation
+    self.containingType = containingType
+  }
+
+  /// Initialize an instance of this type representing a test function.
+  init<S>(
     name: String,
     displayName: String? = nil,
     traits: [any Trait],
     sourceLocation: SourceLocation,
     containingType: Any.Type? = nil,
     xcTestCompatibleSelector: __XCTestCompatibleSelector? = nil,
-    testCases: (any TestCases)? = nil,
-    parameters: [ParameterInfo]? = nil
+    testCases: Test.Case.Generator<S>,
+    parameters: [ParameterInfo]
   ) {
     self.name = name
     self.displayName = displayName
     self.traits = traits
     self.sourceLocation = sourceLocation
     self.containingType = containingType
     self.xcTestCompatibleSelector = xcTestCompatibleSelector
-    self._testCases = testCases
+    self._testCases = Test.Cases(testCases)
     self.parameters = parameters
   }
 }