[TypeChecker] Reject @objc attribute on marker protocols

(cherry picked from commit 9dcb921)

Author: xedin

include/swift/AST/DiagnosticsSema.def

@@ -5906,8 +5906,9 @@ ERROR(invalid_objc_decl_context,none,
       "@objc can only be used with members of classes, @objc protocols, and "
       "concrete extensions of classes", ())
 ERROR(invalid_objc_decl,none,
-      "only classes (and their extensions), protocols, methods, initializers, "
-      "properties, and subscript declarations can be declared @objc", ())
+      "only classes (and their extensions), non-marker protocols, methods, "
+      "initializers, properties, and subscript declarations can be declared"
+      " @objc", ())
 ERROR(invalid_objc_swift_rooted_class,none,
       "only classes that inherit from NSObject can be declared @objc", ())
 NOTE(invalid_objc_swift_root_class_insert_nsobject,none,

lib/Sema/TypeCheckAttr.cpp

@@ -1303,9 +1303,12 @@ void AttributeChecker::visitObjCAttr(ObjCAttr *attr) {
 
   // Only certain decls can be ObjC.
   llvm::Optional<Diag<>> error;
-  if (isa<ClassDecl>(D) ||
-      isa<ProtocolDecl>(D)) {
+  if (isa<ClassDecl>(D)) {
     /* ok */
+  } else if (auto *P = dyn_cast<ProtocolDecl>(D)) {
+    if (P->isMarkerProtocol())
+      error = diag::invalid_objc_decl;
+    /* ok on non-marker protocols */
   } else if (auto Ext = dyn_cast<ExtensionDecl>(D)) {
     if (!Ext->getSelfClassDecl())
       error = diag::objc_extension_not_class;

test/attr/attr_marker_protocol.swift

@@ -67,3 +67,6 @@ protocol P10 { }
 
 extension Array: P10 where Element: P10, Element: P8 { }
 // expected-error@-1{{conditional conformance to non-marker protocol 'P10' cannot depend on conformance of 'Element' to marker protocol 'P8'}}
+
+@objc @_marker protocol P11 {}
+// expected-error@-1 {{only classes (and their extensions), non-marker protocols, methods, initializers, properties, and subscript declarations can be declared @objc}}