[Distributed] Report error rather than crash no ad-hoc requirements are implemented

lib/Sema/DerivedConformanceDistributedActor.cpp

@@ -51,6 +51,15 @@ bool DerivedConformance::canDeriveDistributedActor(
 bool DerivedConformance::canDeriveDistributedActorSystem(
     NominalTypeDecl *nominal, DeclContext *dc) {
   auto &C = nominal->getASTContext();
+
+  // Make sure ad-hoc requirements that we'll use in synthesis are present, before we try to use them.
+  // This leads to better error reporting because we already have errors happening (missing witnesses).
+  if (auto handlerType = getDistributedActorSystemResultHandlerType(nominal)) {
+    if (!C.getOnReturnOnDistributedTargetInvocationResultHandler(
+        handlerType->getAnyNominal()))
+      return false;
+  }
+
   return C.getLoadedModule(C.Id_Distributed);
 }
 

test/Distributed/distributed_imcomplete_system_dont_crash.swift

@@ -0,0 +1,39 @@
+// RUN: %empty-directory(%t)
+// RUN: %target-swift-frontend -typecheck -verify -disable-availability-checking 2>&1 %s
+// REQUIRES: concurrency
+// REQUIRES: distributed
+
+import Distributed
+
+public final class CompletelyHollowActorSystem: DistributedActorSystem {
+  // expected-error@-1{{type 'CompletelyHollowActorSystem' does not conform to protocol 'DistributedActorSystem'}}
+  // expected-note@-2{{protocol 'DistributedActorSystem' requires function 'remoteCallVoid' with signature:}}
+  // expected-error@-3{{class 'CompletelyHollowActorSystem' is missing witness for protocol requirement 'remoteCall'}}
+  // expected-note@-4{{protocol 'DistributedActorSystem' requires function 'remoteCall' with signature:}}
+  // expected-error@-5{{class 'CompletelyHollowActorSystem' is missing witness for protocol requirement 'remoteCallVoid'}}
+
+  public typealias ActorID = String
+  public typealias InvocationEncoder = Encoder
+  // expected-note@-1{{possibly intended match 'CompletelyHollowActorSystem.InvocationEncoder' (aka 'CompletelyHollowActorSystem.Encoder') does not conform to 'DistributedTargetInvocationEncoder'}}
+  public typealias InvocationDecoder = Decoder
+  // expected-note@-1{{possibly intended match 'CompletelyHollowActorSystem.InvocationDecoder' (aka 'CompletelyHollowActorSystem.Decoder') does not conform to 'DistributedTargetInvocationDecoder'}}
+
+  public typealias SerializationRequirement = Codable
+
+  public func actorReady<Act>(_ actor: Act) where Act : DistributedActor, ActorID == Act.ID {
+
+  }
+
+  public struct Encoder: InvocationEncoder {
+
+  }
+
+  public struct Decoder: InvocationDecoder {
+
+  }
+
+  public struct ResultHandler: DistributedTargetInvocationResultHandler {
+    // expected-error@-1{{type 'CompletelyHollowActorSystem.ResultHandler' does not conform to protocol 'DistributedTargetInvocationResultHandler'}}
+  }
+
+}