Fix some Swift 5.6 compatibility issues (apple#516)

* Use existential CodingKey parameters consistently

Swift 5.7 supports implicit opening for existentials, so these
conversions from `CodingKey` parameters to pass to methods that
are generic over `CodingKey` work fine. Prior to Swift 5.7, however,
these don't compile, with the message that `CodingKey` doesn't conform
to itself.

* Bump the required Swift version for the count-lines test

The overload resolution for the `static func main()` in an `@main`
type still had issues in Swift 5.6, such that a package with a min.
platform below that which works for concurrency backdeployment doesn't
properly resolve the AsyncParsableCommand `main()` function. In
Swift 5.7, this is properly resolved, so just the availability on
the main type is sufficient.

This change just skips the test of `count-lines` prior to Swift 5.7,
so that we can maintain the open platform minimum for the package
as a whole.

.gitignore

@@ -6,3 +6,4 @@
 .vscode
 .*.sw?
 *.docc-build
+*.vs

Package.swift

@@ -68,19 +68,3 @@ var package = Package(
             resources: [.copy("CountLinesTest.txt")]),
     ]
 )
-
-#if swift(>=5.6) && os(macOS)
-package.targets.append(contentsOf: [
-    // Examples
-    .executableTarget(
-        name: "count-lines",
-        dependencies: ["ArgumentParser"],
-        path: "Examples/count-lines"),
-
-    // Tools
-    .executableTarget(
-        name: "changelog-authors",
-        dependencies: ["ArgumentParser"],
-        path: "Tools/changelog-authors"),
-    ])
-#endif

Sources/ArgumentParser/Parsing/InputKey.swift

@@ -72,7 +72,7 @@ struct InputKey: Hashable {
   /// - Parameter codingKey: The base ``CodingKey``
   /// - Parameter path: The list of ``CodingKey`` values that lead to this one. May be empty.
   @inlinable
-  init<C: CodingKey>(codingKey: C, path: [CodingKey]) {
+  init(codingKey: CodingKey, path: [CodingKey]) {
     self.init(name: codingKey.stringValue, parent: Parent(InputKey(path: path)))
   }
 
@@ -113,7 +113,7 @@ extension InputKey {
   ///
   /// - Parameter codingKey: The key to clean.
   /// - Returns: The cleaned key.
-  static func clean<C: CodingKey>(codingKey: C) -> String {
+  static func clean(codingKey: CodingKey) -> String {
     clean(codingKey: codingKey.stringValue)
   }
 }

Tests/ArgumentParserExampleTests/CountLinesExampleTests.swift

@@ -9,7 +9,7 @@
 //
 //===----------------------------------------------------------------------===//
 
-#if os(macOS) && swift(>=5.6)
+#if os(macOS) && swift(>=5.7)
 
 import XCTest
 import ArgumentParserTestHelpers