Make a 'count the elements of a parameter pack' helper function

Author: grynspan

Diff for: Sources/Testing/ExitTests/ExitTest.CapturedValue.swift

@@ -144,13 +144,7 @@ extension Collection where Element == ExitTest.CapturedValue {
       // exit test handler or entry point is likely broken.
       guard let wrappedValue = capturedValues.first?.wrappedValue else {
         let actualCount = self.count
-        let expectedCount: Int = {
-          var result = 0
-          for _ in repeat (each T).self {
-            result += 1
-          }
-          return result
-        }()
+        let expectedCount = parameterPackCount(repeat (each T).self)
         fatalError("Found fewer captured values (\(actualCount)) than expected (\(expectedCount)) when passing them to the current exit test.")
       }
 

Diff for: Sources/Testing/Support/Additions/ArrayAdditions.swift

@@ -21,3 +21,21 @@ extension Array {
     self = optionalValue.map { [$0] } ?? []
   }
 }
+
+/// Get the number of elements in a parameter pack.
+///
+/// - Parameters:
+///   - pack: The parameter pack.
+///
+/// - Returns: The number of elements in `pack`.
+///
+/// - Complexity: O(_n_) where _n_ is the number of elements in `pack`. The
+///   compiler may be able to optimize this operation when the types of `pack`
+///   are statically known.
+func parameterPackCount<each T>(_ pack: repeat each T) -> Int {
+  var result = 0
+  for _ in repeat each pack {
+    result += 1
+  }
+  return result
+}