Add a test for the rethrows behavior of async sequences and iterators

Author: DougGregor

test/Concurrency/async_sequence_rethrows.swift

@@ -0,0 +1,42 @@
+// RUN: %target-swift-frontend  -disable-availability-checking %s -emit-sil -o /dev/null -verify
+
+// REQUIRES: concurrency
+
+// Tests for the use of 'rethrows' on generic functions that have AsyncSequence
+// and AsyncIteratorProtocol requirements.
+
+func f1(_ seq: some AsyncSequence) async rethrows {
+  for try await _ in seq { }
+}
+
+func f2(_ seq: some AsyncSequence, body: () throws -> Void) async rethrows {
+  for try await _ in seq {
+    try body()
+  }
+}
+
+func f3(_ seq: some AsyncSequence, _ seq2: some AsyncSequence) async rethrows {
+  for try await _ in seq {
+  }
+
+  for try await _ in seq2 {
+  }
+}
+
+enum HomeworkError: Error {
+case dogAteIt
+}
+
+func testCalls(x: some AsyncSequence<Int, Never>, y: any AsyncSequence<Int, any Error>) async throws {
+  await f1(x)
+  try await f1(y)
+
+  await f2(x) { print("Hello") }
+  try await f2(y) { print("Hello") }
+
+  try await f2(x) { throw HomeworkError.dogAteIt }
+  try await f2(y) { throw HomeworkError.dogAteIt }
+
+  await f3(x, x)
+  try await f3(x, y)
+}