1 files changed, 24 insertions, 0 deletions
diff --git a/clang/lib/Lex/PPDirectives.cpp b/clang/lib/Lex/PPDirectives.cpp
index fd16168203a..298543e2e95 100644
--- a/clang/lib/Lex/PPDirectives.cpp
+++ b/clang/lib/Lex/PPDirectives.cpp
@@ -2266,6 +2266,30 @@ void Preprocessor::HandleDefineDirective(Token &DefineTok,
   // Finally, if this identifier already had a macro defined for it, verify that
   // the macro bodies are identical, and issue diagnostics if they are not.
   if (const MacroInfo *OtherMI=getMacroInfo(MacroNameTok.getIdentifierInfo())) {
+    // In Objective-C, ignore attempts to directly redefine the builtin
+    // definitions of the ownership qualifiers.  It's still possible to
+    // #undef them.
+    auto isObjCProtectedMacro = [](const IdentifierInfo *II) -> bool {
+      return II->isStr("__strong") ||
+             II->isStr("__weak") ||
+             II->isStr("__unsafe_unretained") ||
+             II->isStr("__autoreleasing");
+    };
+   if (getLangOpts().ObjC1 &&
+        SourceMgr.getFileID(OtherMI->getDefinitionLoc())
+          == getPredefinesFileID() &&
+        isObjCProtectedMacro(MacroNameTok.getIdentifierInfo())) {
+      // Warn if it changes the tokens.
+      if ((!getDiagnostics().getSuppressSystemWarnings() ||
+           !SourceMgr.isInSystemHeader(DefineTok.getLocation())) &&
+          !MI->isIdenticalTo(*OtherMI, *this,
+                             /*Syntactic=*/LangOpts.MicrosoftExt)) {
+        Diag(MI->getDefinitionLoc(), diag::warn_pp_objc_macro_redef_ignored);
+      }
+      assert(!OtherMI->isWarnIfUnused());
+      return;
+    }
+
     // It is very common for system headers to have tons of macro redefinitions
     // and for warnings to be disabled in system headers.  If this is the case,
     // then don't bother calling MacroInfo::isIdenticalTo.