Add diagnostic for non-objc functions declared as open inside extension

Author: mininny

include/swift/AST/DiagnosticsSema.def

@@ -1504,6 +1504,9 @@ ERROR(access_control_extension_open,none,
 ERROR(access_control_open_bad_decl,none,
       "only classes and overridable class members can be declared 'open';"
       " use 'public'", ())
+ERROR(access_control_non_objc_open_func,none,
+      "non-@objc %select{properties|methods}0 in extensions cannot be overriden; use 'public' instead",
+      (bool))
 
 ERROR(invalid_decl_attribute,none,
       "'%0' attribute cannot be applied to this declaration", (DeclAttribute))

lib/Sema/TypeCheckAttr.cpp

@@ -795,6 +795,15 @@ void AttributeChecker::visitAccessControlAttr(AccessControlAttr *attr) {
           .fixItRemove(attr->getRange());
       }
     }
+
+    if (auto VD = dyn_cast<ValueDecl>(D)) {
+      if (!VD->isObjC() && attr->getAccess() == AccessLevel::Open) {
+        diagnose(attr->getLocation(), diag::access_control_non_objc_open_func,
+                 isa<FuncDecl>(VD))
+            .fixItReplace(attr->getRange(), "public");
+        attr->setInvalid();
+      }
+    }
   }
 
   if (attr->getAccess() == AccessLevel::Open) {

test/attr/open.swift

@@ -134,3 +134,21 @@ open class OpenSubClass : OpenSuperClass {
   open override subscript(index: MarkerForNonOpenSubscripts) -> Int { return 0 }
 
 }
+
+open class InvalidOpenExtension {
+  open func openMethod() { } // No error
+}
+extension InvalidOpenExtension {
+  open func nonObjcOpenMethod() { } // expected-error {{non-@objc methods in extensions cannot be overriden; use 'public' instead}} {{3-7=public}}
+  open var nonObjcOpenVar: Int { 3 }  // expected-error {{non-@objc properties in extensions cannot be overriden; use 'public' instead}} {{3-7=public}}
+  @objc open func objcOpenMethod() { } // No error
+  @objc open var objcOpenVar: Int { 3 } // No error
+}
+
+@objcMembers
+open class ValidOpenExtension { }
+extension ValidOpenExtension {
+    open func objcOpenMethod() { } // No error
+    open var objcOpenVar: Int { 3 } // No error
+}
+