Diagnose root classes with @objcImpl

@objcImpl, like @objc, cannot be used to implement root classes. Diagnose an attempt to do so.

Fixes rdar://109130979.

Author: beccadax

include/swift/AST/DiagnosticsSema.def

@@ -1718,6 +1718,10 @@ ERROR(attr_objc_implementation_must_be_imported,none,
       "defined by a Swift 'class' declaration, not an imported Objective-C "
       "'@interface' declaration",
       (ValueDecl *))
+ERROR(attr_objc_implementation_must_have_super,none,
+      "'@_objcImplementation' cannot be used to implement root %kind0; declare "
+      "its superclass in the header",
+      (ValueDecl *))
 ERROR(attr_objc_implementation_category_not_found,none,
       "could not find category %0 on Objective-C class %1; make sure your "
       "umbrella or bridging header imports the header that declares it",

lib/AST/Decl.cpp

@@ -1825,7 +1825,8 @@ llvm::Optional<Identifier>
 ExtensionDecl::getCategoryNameForObjCImplementation() const {
   assert(isObjCImplementation());
 
-  auto attr = getAttrs().getAttribute<ObjCImplementationAttr>();
+  auto attr = getAttrs()
+                  .getAttribute<ObjCImplementationAttr>(/*AllowInvalid=*/true);
   if (attr->isCategoryNameInvalid())
     return llvm::None;
 

lib/Sema/TypeCheckAttr.cpp

@@ -1503,6 +1503,13 @@ visitObjCImplementationAttr(ObjCImplementationAttr *attr) {
     return;
   }
 
+  if (!CD->hasSuperclass()) {
+    diagnoseAndRemoveAttr(attr, diag::attr_objc_implementation_must_have_super,
+                          CD);
+    CD->diagnose(diag::decl_declared_here, CD);
+    return;
+  }
+
   if (!attr->isCategoryNameInvalid() && !ED->getImplementedObjCDecl()) {
     diagnose(attr->getLocation(),
              diag::attr_objc_implementation_category_not_found,

test/decl/ext/Inputs/objc_implementation.h

@@ -168,6 +168,10 @@
 
 @end
 
+@interface ObjCImplRootClass
+
+@end
+
 
 
 struct ObjCStruct {

test/decl/ext/objc_implementation.swift

@@ -456,6 +456,10 @@ protocol EmptySwiftProto {}
 @_objcImplementation(WTF) extension SwiftClass {} // expected
 // expected-error@-1 {{'@_objcImplementation' cannot be used to extend class 'SwiftClass' because it was defined by a Swift 'class' declaration, not an imported Objective-C '@interface' declaration}} {{1-27=}}
 
+@_objcImplementation extension ObjCImplRootClass {
+  // expected-error@-1 {{'@_objcImplementation' cannot be used to implement root class 'ObjCImplRootClass'; declare its superclass in the header}}
+}
+
 func usesAreNotAmbiguous(obj: ObjCClass) {
   obj.method(fromHeader1: 1)
   obj.method(fromHeader2: 2)