[Distributed] ban typed throws in distributed funcs

They don't yield a correct error type as we didn't implement it, so
rather allow it and risk crashes, ban it until we get the time to
implement it.

The real solution is to adjust typed throws error inference to do an
union of the thrown error of the func and the type thrown by the
distributed actor system remote call -- which today always would be (E |
Error) -> Error...

We could add a new associated type to DAS and then we could make it more
proper...

resolves rdar://136467528

Author: ktoso

include/swift/AST/DiagnosticsSema.def

@@ -5656,6 +5656,9 @@ ERROR(distributed_actor_func_unsupported_specifier, none,
 ERROR(distributed_actor_func_variadic, none,
       "cannot declare variadic argument %0 in %kind1",
       (DeclName, const ValueDecl *))
+ERROR(distributed_actor_func_typed_throws, none,
+      "cannot declare distributed function function with typed throws",
+      ())
 NOTE(actor_mutable_state,none,
      "mutation of this %0 is only permitted within the actor",
      (DescriptiveDeclKind))

lib/Sema/TypeCheckDistributed.cpp

@@ -618,6 +618,19 @@ bool CheckDistributedFunctionRequest::evaluate(
     return true;
   }
 
+  // TODO: rdar://136467591 Currently typed throws were not implemented for distributed methods
+  if (func->hasThrows()) {
+    if (auto thrownError = func->getEffectiveThrownErrorType()) {
+      // Basically we only support throwing `any Error` out of a distributed
+      // function because then the effective error thrown by thunk calls naturally
+      // is correct and the same `any Error`
+      if (thrownError.has_value() &&
+          !(*thrownError)->isEqual(C.getErrorExistentialType())) {
+        func->diagnose(diag::distributed_actor_func_typed_throws);
+      }
+    }
+  }
+
   return false;
 }
 

test/Distributed/distributed_actor_unsupported_typed_throws.swift

@@ -0,0 +1,30 @@
+// RUN: %target-swift-frontend -typecheck -verify -target %target-swift-5.7-abi-triple -I %t 2>&1 %s
+
+// UNSUPPORTED: back_deploy_concurrency
+// REQUIRES: concurrency
+// REQUIRES: distributed
+
+import Distributed
+
+typealias DefaultDistributedActorSystem = LocalTestingDistributedActorSystem
+
+distributed actor Foo {
+  distributed func alwaysThrows() throws(FooError) { // expected-error{{cannot declare distributed function function with typed throws}}
+    throw FooError()
+  }
+}
+
+struct FooError: Codable, Error { }
+struct RemoteInvocationError: Codable, Error { }
+
+func test(foo: Foo) async throws {
+  do {
+    try await foo.alwaysThrows() // actually, this is throws(Error) because network errors etc
+    fatalError("Should not reach here")
+    // FIXME: the following warning is showing why we had to ban typed throws;
+    //        the error type must instead be (FooError | Error (from the distributed thunk))
+    //        rdar://136467591
+  } catch let error as RemoteInvocationError { // expected-warning{{cast from 'FooError' to unrelated type 'RemoteInvocationError' always fails}}
+    print("error = \(error)")
+  }
+}