Support typed throws (#52)

* Support typed throws

* Update Sources/ConcurrencyExtras/Result.swift

Co-authored-by: Stephen Celis <[email protected]>

* Constrain change to compiler(>=6)

* Also gate the test to Swift 6

* Include func g in ifdef

* wip

---------

Co-authored-by: Stephen Celis <[email protected]>
Co-authored-by: Stephen Celis <[email protected]>

Sources/ConcurrencyExtras/Result.swift

@@ -1,14 +1,27 @@
-extension Result where Failure == Swift.Error {
-  /// Creates a new result by evaluating an async throwing closure, capturing the returned value as
-  /// a success, or any thrown error as a failure.
-  ///
-  /// - Parameter body: A throwing closure to evaluate.
-  @_transparent
-  public init(catching body: () async throws -> Success) async {
-    do {
-      self = .success(try await body())
-    } catch {
-      self = .failure(error)
+#if compiler(>=6)
+  extension Result {
+    /// Creates a new result by evaluating an async throwing closure, capturing the returned value as
+    /// a success, or any thrown error as a failure.
+    ///
+    /// - Parameter body: A throwing closure to evaluate.
+    @_transparent
+    public init(catching body: () async throws(Failure) -> Success) async {
+      do {
+        self = .success(try await body())
+      } catch {
+        self = .failure(error)
+      }
     }
   }
-}
+#else
+  extension Result where Failure == Swift.Error {
+    @_transparent
+    public init(catching body: () async throws -> Success) async {
+      do {
+        self = .success(try await body())
+      } catch {
+        self = .failure(error)
+      }
+    }
+  }
+#endif

Tests/ConcurrencyExtrasTests/ResultTests.swift

@@ -0,0 +1,62 @@
+import ConcurrencyExtras
+import XCTest
+
+final class ResultTests: XCTestCase {
+  struct SomeError: Error, Equatable { }
+  func f(_ value: Int) async throws -> Int {
+    if value == 42 {
+      return 42
+    } else {
+      throw SomeError()
+    }
+  }
+
+  func testBasics() async {
+    do {
+      let res = await Result { try await f(42) }
+      XCTAssertEqual(try res.get(), 42)
+    }
+    do {
+      let res = await Result { try await f(0) }
+      do {
+        _ = try res.get()
+      } catch let error as SomeError {
+        XCTAssertEqual(error, SomeError())
+      } catch {
+        XCTFail("unexpected error: \(error)")
+      }
+    }
+  }
+
+  #if compiler(>=6)
+    func g(_ value: Int) async throws(SomeError) -> Int {
+      if value == 42 {
+        return 42
+      } else {
+        throw SomeError()
+      }
+    }
+
+    func testTypedThrows() async {
+      let res = await Result { () async throws(SomeError) -> Int in try await g(0) }
+      do {
+        _ = try res.get()
+      } catch {
+        XCTAssertEqual(error, SomeError())
+      }
+    }
+
+    func h() async throws(SomeError) -> Int {
+      throw SomeError()
+    }
+
+    func testTypedThrowsInference() async {
+      let res = await Result(catching: h)
+      do {
+        _ = try res.get()
+      } catch {
+        XCTAssertEqual(error, SomeError())
+      }
+    }
+  #endif
+}