[Concurrency] Implicitly strip optionals for return type of translated "async throws".

Review over a large number of APIs has found that most of the time, the
result type of an Objective-C completion handler method that becomes
"async throws" should be optional. Change the default behavior to
match this, and replace _Nullable_on_error with _Nullable_result to
capture the case where the result should be optional.

lib/ClangImporter/ImportType.cpp

@@ -2065,7 +2065,13 @@ static Type decomposeCompletionHandlerType(
         paramIdx == *info.completionHandlerErrorParamIndex())
       continue;
 
-    resultTypeElts.push_back(param.getPlainType());
+    // If there is an error parameter, remove nullability.
+    Type paramType = param.getPlainType();
+    // TODO: Clang should gain a nullability form that overrides this.
+    if (info.completionHandlerErrorParamIndex())
+      paramType = paramType->lookThroughAllOptionalTypes();
+
+    resultTypeElts.push_back(paramType);
   }
 
   switch (resultTypeElts.size()) {

lib/Sema/TypeCheckDeclObjC.cpp

@@ -1946,8 +1946,10 @@ bool swift::fixDeclarationName(InFlightDiagnostic &diag, const ValueDecl *decl,
   }
 
   // Fix the argument names that need fixing.
-  assert(name.getArgumentNames().size()
+  if (name.getArgumentNames().size()
-          == targetName.getArgumentNames().size());
+          != targetName.getArgumentNames().size())
+    return false;
+  
   auto params = func->getParameters();
   for (unsigned i = 0, n = name.getArgumentNames().size(); i != n; ++i) {
     auto origArg = name.getArgumentNames()[i];

test/ClangImporter/objc_async.swift

@@ -8,8 +8,8 @@ import ObjCConcurrency
 func testSlowServer(slowServer: SlowServer) async throws {
   let _: Int = await slowServer.doSomethingSlow("mail")
   let _: Bool = await slowServer.checkAvailability()
-  let _: String = try await slowServer.findAnswer() ?? "nope"
+  let _: String = try await slowServer.findAnswer()
-  let _: String = await try slowServer.findAnswerFailingly() ?? "nope"
+  let _: String = await try slowServer.findAnswerFailingly()
   let _: Void = await slowServer.doSomethingFun("jump")
   let _: (Int) -> Void = slowServer.completionHandler
 

test/IDE/print_clang_objc_async.swift

@@ -10,13 +10,13 @@
 // CHECK-DAG:     func doSomethingSlow(_ operation: String, completionHandler handler: @escaping (Int) -> Void)
 // CHECK-DAG:     func doSomethingSlow(_ operation: String) async -> Int
 // CHECK-DAG:     func doSomethingDangerous(_ operation: String, completionHandler handler: ((String?, Error?) -> Void)? = nil)
-// CHECK-DAG:     func doSomethingDangerous(_ operation: String) async throws -> String?
+// CHECK-DAG:     func doSomethingDangerous(_ operation: String) async throws -> String
 // CHECK-DAG:     func checkAvailability(completionHandler: @escaping (Bool) -> Void)
 // CHECK-DAG:     func checkAvailability() async -> Bool
 // CHECK-DAG:     func findAnswer(completionHandler handler: @escaping (String?, Error?) -> Void)
-// CHECK-DAG:     func findAnswer() async throws -> String?
+// CHECK-DAG:     func findAnswer() async throws -> String
 // CHECK-DAG:     func findAnswerFailingly(completionHandler handler: @escaping (String?, Error?) -> Void) throws
-// CHECK-DAG:     func findAnswerFailingly() async throws -> String?
+// CHECK-DAG:     func findAnswerFailingly() async throws -> String
 // CHECK-DAG:     func doSomethingFun(_ operation: String) async
 // CHECK: {{^[}]$}}
 

test/decl/protocol/conforms/objc_async.swift

@@ -7,7 +7,7 @@ import ObjCConcurrency
 
 // Conform via async method
 class C1: ConcurrentProtocol {
-  func askUser(toSolvePuzzle puzzle: String) async throws -> String? { nil }
+  func askUser(toSolvePuzzle puzzle: String) async throws -> String { "" }
 
   func askUser(toJumpThroughHoop hoop: String) async -> String { "hello" }
 }
@@ -26,7 +26,7 @@ class C2: ConcurrentProtocol {
 // Conform via both; this is an error
 class C3: ConcurrentProtocol {
   // expected-note@+1{{method 'askUser(toSolvePuzzle:)' declared here}}
-  func askUser(toSolvePuzzle puzzle: String) async throws -> String? { nil }
+  func askUser(toSolvePuzzle puzzle: String) async throws -> String { "" }
 
   // expected-error@+1{{'askUser(toSolvePuzzle:completionHandler:)' with Objective-C selector 'askUserToSolvePuzzle:completionHandler:' conflicts with method 'askUser(toSolvePuzzle:)' with the same Objective-C selector}}
   func askUser(toSolvePuzzle puzzle: String, completionHandler: ((String?, Error?) -> Void)?) {