Fix build warning when @CasePathable is applied to enums with no ca…

…ses (#164)

* Fix build warning when `@CasePathable` is applied to enums with no cases

Fixes warnings due to unreachable code:

For example:

```swift
@CasePathable enum Foo {}
```

Produces this expansion (compiler warnings added as code comments by me)

```swift
enum Foo: CasePathable {
  public struct AllCasePaths: Sequence {
    public subscript(root: Foo) -> PartialCaseKeyPath<Foo> {
      return \.never // ⚠️ warning: will never be executed
      // 'root' is of type 'Foo' which cannot be constructed because it
      // is an enum with no cases
    }

    public func makeIterator() -> IndexingIterator<[PartialCaseKeyPath<Foo>]> {
      let allCasePaths: [PartialCaseKeyPath<Foo>] = []
      return allCasePaths.makeIterator()
    }
  }
  public static var allCasePaths: AllCasePaths { AllCasePaths() }
}
```

We can fix this warning by simply not producing the subscript definition
when the enum has no cases.

* Omit return statement for enums without cases

Sources/CasePathsMacros/CasePathableMacro.swift

@@ -91,11 +91,13 @@ extension CasePathableMacro: MemberMacro {
  "allCasePaths.append(\\.\(raw: $0.name.text))"
  }
 
+ let subscriptReturn = allCases.isEmpty ? #"\.never"# : #"return \.never"#
+
  return [
  """
  public struct AllCasePaths: Sequence {
  public subscript(root: \(enumName)) -> PartialCaseKeyPath<\(enumName)> {
- \(raw: rootSubscriptCases.map { "\($0.description)\n" }.joined())return \\.never
+ \(raw: rootSubscriptCases.map { "\($0.description)\n" }.joined())\(raw: subscriptReturn)
  }
  \(raw: casePaths.map(\.description).joined(separator: "\n"))
  public func makeIterator() -> IndexingIterator<[PartialCaseKeyPath<\(enumName)>]> {

Tests/CasePathsMacrosTests/CasePathableMacroTests.swift

@@ -111,6 +111,33 @@ final class CasePathableMacroTests: XCTestCase {
  }
  }
 
+ func testCasePathable_NoCases() {
+ assertMacro {
+ """
+ @CasePathable enum EnumWithNoCases {}
+ """
+ } expansion: {
+ #"""
+ enum EnumWithNoCases {
+
+ public struct AllCasePaths: Sequence {
+ public subscript(root: EnumWithNoCases) -> PartialCaseKeyPath<EnumWithNoCases> {
+ \.never
+ }
+
+ public func makeIterator() -> IndexingIterator<[PartialCaseKeyPath<EnumWithNoCases>]> {
+ let allCasePaths: [PartialCaseKeyPath<EnumWithNoCases>] = []
+ return allCasePaths.makeIterator()
+ }
+ }
+ public static var allCasePaths: AllCasePaths { AllCasePaths() }}
+
+ extension EnumWithNoCases: CasePaths.CasePathable {
+ }
+ """#
+ }
+ }
+
  func testCasePathable_ElementList() {
  assertMacro {
  """
@@ -348,7 +375,7 @@ final class CasePathableMacroTests: XCTestCase {
 
  public struct AllCasePaths: Sequence {
  public subscript(root: Foo) -> PartialCaseKeyPath<Foo> {
- return \.never
+ \.never
  }
 
  public func makeIterator() -> IndexingIterator<[PartialCaseKeyPath<Foo>]> {
@@ -371,7 +398,7 @@ final class CasePathableMacroTests: XCTestCase {
 
  public struct AllCasePaths: Sequence {
  public subscript(root: Foo) -> PartialCaseKeyPath<Foo> {
- return \.never
+ \.never
  }
 
  public func makeIterator() -> IndexingIterator<[PartialCaseKeyPath<Foo>]> {