[Diagnostics] Improve diagnostics on invalid open extensions

include/swift/AST/DiagnosticsSema.def

@@ -1499,7 +1499,7 @@ ERROR(access_control_extension_more,none,
       "be declared %select{%error|fileprivate|internal|public|%error}2",
       (AccessLevel, DescriptiveDeclKind, AccessLevel))
 ERROR(access_control_extension_open,none,
-      "extensions cannot use 'open' as their default access; use 'public'",
+      "extensions cannot be declared 'open'; declare individual members as 'open' instead",
       ())
 ERROR(access_control_open_bad_decl,none,
       "only classes and overridable class members can be declared 'open';"

lib/Sema/TypeCheckAttr.cpp

@@ -737,8 +737,25 @@ void AttributeChecker::visitAccessControlAttr(AccessControlAttr *attr) {
 
   if (auto extension = dyn_cast<ExtensionDecl>(D)) {
     if (attr->getAccess() == AccessLevel::Open) {
-      diagnose(attr->getLocation(), diag::access_control_extension_open)
-        .fixItReplace(attr->getRange(), "public");
+      auto diag =
+          diagnose(attr->getLocation(), diag::access_control_extension_open);
+      diag.fixItRemove(attr->getRange());
+      for (auto Member : extension->getMembers()) {
+        if (auto *VD = dyn_cast<ValueDecl>(Member)) {
+          if (VD->getAttrs().hasAttribute<AccessControlAttr>())
+            continue;
+
+          StringRef accessLevel = VD->isObjC() ? "open " : "public ";
+
+          if (auto *FD = dyn_cast<FuncDecl>(VD))
+            diag.fixItInsert(FD->getFuncLoc(), accessLevel);
+
+          if (auto *VAD = dyn_cast<VarDecl>(VD))
+            diag.fixItInsert(VAD->getParentPatternBinding()->getLoc(),
+                             accessLevel);
+        }
+      }
+
       attr->setInvalid();
       return;
     }

test/attr/open.swift

@@ -134,3 +134,12 @@ open class OpenSubClass : OpenSuperClass {
   open override subscript(index: MarkerForNonOpenSubscripts) -> Int { return 0 }
 
 }
+
+class InvalidOpenExtensionClass { }
+
+open extension InvalidOpenExtensionClass {  // expected-error {{extensions cannot be declared 'open'; declare individual members as 'open' instead}} {{1-6=}} {{3-3=public }} {{3-3=public }}
+  func C() { } // Insert public
+  private func A() { } // OK
+  var F: Int { 3 } // Insert public
+  private var G: Int { 3 } // Okay
+}

test/attr/open_objc.swift

@@ -0,0 +1,31 @@
+// RUN: %empty-directory(%t)
+// RUN: %target-typecheck-verify-swift -sdk %clang-importer-sdk -I %t
+// REQUIRES: objc_interop
+
+import Foundation
+
+class InvalidOpenExtensionClass { }
+
+open extension InvalidOpenExtensionClass {  // expected-error {{extensions cannot be declared 'open'; declare individual members as 'open' instead}} {{1-6=}} {{3-3=public }} {{9-9=open }} {{9-9=open }} {{3-3=public }}
+  private func A() { } // OK
+  @objc func B() { } // Insert open 
+  func C() { } // Insert public
+  @objc private var D: Int { 3 } // Okay
+  @objc var E: Int { 3 } // Insert open
+  var F: Int { 3 } // Insert public
+  private var G: Int { 3 } // Okay
+}
+
+@objc
+@objcMembers
+class InvalidOpenExtensionObjcClass: NSObject { }
+
+open extension InvalidOpenExtensionObjcClass {  // expected-error {{extensions cannot be declared 'open'; declare individual members as 'open' instead}} {{1-6=}} {{3-3=open }} {{9-9=open }} {{9-9=open }} {{3-3=open }}
+  private func A() { } // OK
+  @objc func B() { } // Insert open 
+  func C() { } // Insert open
+  @objc private var D: Int { 3 } // Okay
+  @objc var E: Int { 3 } // Insert open
+  var F: Int { 3 } // Insert open
+  private var G: Int { 3 } // Okay
+}