[Distributed] Don't crash when 'open' distributed actor is declared (#85170)

Author: ktoso

lib/AST/Decl.cpp

@@ -5565,10 +5565,18 @@ void ValueDecl::copyFormalAccessFrom(const ValueDecl *source,
     access = AccessLevel::FilePrivate;
 
   // Only certain declarations can be 'open'.
-  if (access == AccessLevel::Open && !isSyntacticallyOverridable()) {
-    assert(!isa<ClassDecl>(this) &&
-           "copying 'open' onto a class has complications");
-    access = AccessLevel::Public;
+  if (access == AccessLevel::Open) {
+    auto classDecl = dyn_cast<ClassDecl>(source);
+    if (classDecl && classDecl->isDistributedActor()) {
+      // don't copy 'public'; an 'open distributed actor' is illegal to begin with
+      return;
+    }
+
+    if (!isSyntacticallyOverridable()) {
+      assert(!isa<ClassDecl>(this) &&
+             "copying 'open' onto a class has complications");
+      access = AccessLevel::Public;
+    }
   }
 
   setAccess(access);

lib/Sema/TypeCheckDistributed.cpp

@@ -709,6 +709,13 @@ void TypeChecker::checkDistributedActor(SourceFile *SF, NominalTypeDecl *nominal
   if (!swift::ensureDistributedModuleLoaded(nominal))
     return;
 
+  if (nominal->getFormalAccess() == AccessLevel::Open) {
+    // we should have outright banned 'open' for all actors, but seems we didn't.
+    // distributed actor synthesis always previously crashed if someone were to
+    // declare one as open, so we're banning it now, rather than leave it crashing.
+    (void) nominal->diagnose(diag::access_control_open_bad_decl);
+  }
+
   auto &C = nominal->getASTContext();
   auto loc = nominal->getLoc();
   recordRequiredImportAccessLevelForDecl(C.getDistributedActorDecl(), nominal,

validation-test/compiler_crashers/ValueDecl-copyFormalAccessFrom-d1e149.swift

@@ -0,0 +1,7 @@
+// RUN: not %target-swift-frontend -typecheck %s
+// REQUIRES: OS=macosx
+import Distributed
+distributed open actor a {
+}
+
+open distributed actor DA: Comparable {}