break request-evaluator cycle when synthesizing actor init

When synthesizing the default initializer for an actor, we'd sometimes hit
a cycle when that initializer needs to chain to NSObject.init. The cycle
only happens because we ask if the initializer we're trying to synthesize
is a convenience init in a scenario which only applies to non-final classes.

Since all actors are effectively "final" classes, it's valid to workaround the
cycle by only asking that initializer question for non-final classes, thus
breaking the cycle.

Author: kavon

lib/AST/ASTContext.cpp

@@ -3072,10 +3072,14 @@ AnyFunctionType::Param swift::computeSelfParam(AbstractFunctionDecl *AFD,
     }
 
     // Convenience initializers have a dynamic 'self' in '-swift-version 5'.
+    //
+    // NOTE: it's important that we check if it's a convenience init only after
+    // confirming it's not semantically final, or else there can be a request
+    // evaluator cycle to determine the init kind for actors, which are final.
     if (Ctx.isSwiftVersionAtLeast(5)) {
-      if (wantDynamicSelf && CD->isConvenienceInit())
+      if (wantDynamicSelf)
         if (auto *classDecl = selfTy->getClassOrBoundGenericClass())
-          if (!classDecl->isSemanticallyFinal())
+          if (!classDecl->isSemanticallyFinal() && CD->isConvenienceInit())
             isDynamicSelf = true;
     }
   } else if (isa<DestructorDecl>(AFD)) {

test/Concurrency/actor_isolation_objc.swift

@@ -55,3 +55,7 @@ actor Dril: NSObject {
     return true
   }
 }
+
+
+// makes sure the synthesized init's delegation kind is determined correctly.
+actor Pumpkin: NSObject {}