Remove support for ObjC, update release note

Author: AaronBallman

clang/docs/ReleaseNotes.rst

@@ -155,7 +155,7 @@ C Language Changes
   diagnoses implicit conversion from ``void *`` to another pointer type as
   being incompatible with C++. (#GH17792)
 - Added ``-Wc++-keyword``, grouped under ``-Wc++-compat``, which diagnoses when
-  a C++ keyword is used as an identifier in C. Partially addresses #GH21898.
+  a C++ keyword is used as an identifier in C. (#GH21898)
 - Added ``-Wc++-hidden-decl``, grouped under ``-Wc++-compat``, which diagnoses
   use of tag types which are visible in C but not visible in C++ due to scoping
   rules. e.g.,

clang/lib/Basic/IdentifierTable.cpp

@@ -287,7 +287,12 @@ static void AddKeyword(StringRef Keyword,
   // Don't add this keyword if disabled in this language and isn't otherwise
   // special.
   if (AddResult == KS_Disabled) {
-    if (IsKeywordInCpp(Flags))
+    // We do not consider any identifiers to be C++ keywords when in
+    // Objective-C because @ effectively introduces a custom grammar where C++
+    // keywords can be used (and similar for selectors). We could enable this
+    // for Objective-C, but it would require more logic to ensure we do not
+    // issue compatibility diagnostics in these cases.
+    if (!LangOpts.ObjC && IsKeywordInCpp(Flags))
       MarkIdentifierAsKeywordInCpp(Table, Keyword);
     return;
   }

clang/lib/Lex/Preprocessor.cpp

@@ -836,23 +836,8 @@ bool Preprocessor::HandleIdentifier(Token &Identifier) {
 
   // If this identifier would be a keyword in C++, diagnose as a compatibility
   // issue.
-  if (II.IsKeywordInCPlusPlus() && !DisableMacroExpansion) {
-    // Objective-C is special in that something like @class is two tokens,
-    // where 'class' is an identifier. We don't want to diagnose @class as
-    // using an identifier reserved in C++, but we do still want to diagnose
-    // something like 'int class;'. In Objective-C, @ basically introduces an
-    // entirely new grammar, so no identier should be flagged as reserved.
-    bool Diagnose = true;
-    if (getLangOpts().ObjC) {
-      std::optional<Token> PrevTok =
-          Lexer::findPreviousToken(Identifier.getLocation(), getSourceManager(),
-                                   getLangOpts(), /*IncludeComments=*/false);
-      Diagnose = !PrevTok || !PrevTok->is(tok::at);
-    }
-
-    if (Diagnose)
-      Diag(Identifier, diag::warn_pp_identifier_is_cpp_keyword) << &II;
-  }
+  if (II.IsKeywordInCPlusPlus() && !DisableMacroExpansion)
+    Diag(Identifier, diag::warn_pp_identifier_is_cpp_keyword) << &II;
 
   // If this is an extension token, diagnose its use.
   // We avoid diagnosing tokens that originate from macro definitions.

clang/test/Sema/c++-keyword-in-objc.m

@@ -1,9 +1,12 @@
 // RUN: %clang_cc1 -fsyntax-only -verify -Wc++-keyword %s
+// expected-no-diagnostics
 
-// Show that @class is not diagnosed as use of a C++ keyword in Objective-C,
-// but that other uses of class are diagnosed.
+// Show that we do not diagnose any C++ keywords in Objective-C.
 
 @class Foo; // Okay, Objective-C @ keyword, not a regular identifier
 
-int class = 12; // expected-warning {{identifier 'class' conflicts with a C++ keyword}}
+// FIXME: it would be nice to diagnose this, but it is intentionally allowed
+// due to @ and selectors allowing C++ keywords in ways that are supposed to be
+// contextually compatible with C++.
+int class = 12;