[AST] Handle generic actor system when trying to produce a ref to decodeNextArgument

Author: xedin

lib/AST/DistributedDecl.cpp

@@ -256,7 +256,7 @@ swift::getAssociatedDistributedInvocationDecoderDecodeNextArgumentFunction(
     return nullptr;
 
   auto systemTy = getConcreteReplacementForProtocolActorSystemType(thunk);
-  if (!systemTy)
+  if (!systemTy || systemTy->is<GenericTypeParamType>())
     return nullptr;
 
   auto decoderTy =

lib/SIL/IR/SILFunctionBuilder.cpp

@@ -275,14 +275,15 @@ void SILFunctionBuilder::addFunctionAttributes(
       F->setDynamicallyReplacedFunction(replacedFunc);
     }
   } else if (constant.isDistributedThunk()) {
-    auto decodeFuncDecl =
+    // It's okay for `decodeFuncDecl` to be null because system could be
+    // generic.
+    if (auto decodeFuncDecl =
             getAssociatedDistributedInvocationDecoderDecodeNextArgumentFunction(
-                decl);
-    assert(decodeFuncDecl && "decodeNextArgument function not found!");
-
-    auto decodeRef = SILDeclRef(decodeFuncDecl);
-    auto *adHocFunc = getOrCreateDeclaration(decodeFuncDecl, decodeRef);
-    F->setReferencedAdHocRequirementWitnessFunction(adHocFunc);
+                decl)) {
+      auto decodeRef = SILDeclRef(decodeFuncDecl);
+      auto *adHocFunc = getOrCreateDeclaration(decodeFuncDecl, decodeRef);
+      F->setReferencedAdHocRequirementWitnessFunction(adHocFunc);
+    }
   }
 }