Index: clang/lib/Lex/PPDirectives.cpp
===================================================================
--- clang/lib/Lex/PPDirectives.cpp
+++ clang/lib/Lex/PPDirectives.cpp
@@ -319,15 +319,6 @@
     return Diag(MacroNameTok, diag::err_defined_macro_name);
   }
 
-  if (isDefineUndef == MU_Undef) {
-    auto *MI = getMacroInfo(II);
-    if (MI && MI->isBuiltinMacro()) {
-      // Warn if undefining "__LINE__" and other builtins, per C99 6.10.8/4
-      // and C++ [cpp.predefined]p4], but allow it as an extension.
-      Diag(MacroNameTok, diag::ext_pp_undef_builtin_macro);
-    }
-  }
-
   // If defining/undefining reserved identifier or a keyword, we need to issue
   // a warning.
   SourceLocation MacroNameLoc = MacroNameTok.getLocation();
@@ -3004,6 +2995,14 @@
   MI->setTokens(Tokens, BP);
   return MI;
 }
+
+static bool isObjCProtectedMacro(const IdentifierInfo *II) {
+  return II->isStr("__strong") ||
+         II->isStr("__weak") ||
+         II->isStr("__unsafe_unretained") ||
+         II->isStr("__autoreleasing");
+}
+
 /// HandleDefineDirective - Implements \#define.  This consumes the entire macro
 /// line then lets the caller lex the next real token.
 void Preprocessor::HandleDefineDirective(
@@ -3075,13 +3074,7 @@
     // 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().ObjC &&
+    if (getLangOpts().ObjC &&
         SourceMgr.getFileID(OtherMI->getDefinitionLoc())
           == getPredefinesFileID() &&
         isObjCProtectedMacro(MacroNameTok.getIdentifierInfo())) {
@@ -3107,7 +3100,8 @@
 
       // Warn if defining "__LINE__" and other builtins, per C99 6.10.8/4 and
       // C++ [cpp.predefined]p4, but allow it as an extension.
-      if (OtherMI->isBuiltinMacro())
+      if (OtherMI->isBuiltinMacro() ||
+          SourceMgr.isWrittenInBuiltinFile(OtherMI->getDefinitionLoc()))
         Diag(MacroNameTok, diag::ext_pp_redef_builtin_macro);
       // Macros must be identical.  This means all tokens and whitespace
       // separation must be the same.  C99 6.10.3p2.
@@ -3187,6 +3181,14 @@
     if (!MI->isUsed() && MI->isWarnIfUnused())
       Diag(MI->getDefinitionLoc(), diag::pp_macro_not_used);
 
+    // Warn if undefining "__LINE__" and other builtins, per C99 6.10.8/4 and
+    // C++ [cpp.predefined]p4, but allow it as an extension. Don't warn if this
+    // is an Objective-C builtin macro though.
+    if ((MI->isBuiltinMacro() ||
+         SourceMgr.isWrittenInBuiltinFile(MI->getDefinitionLoc())) &&
+        !(getLangOpts().ObjC && isObjCProtectedMacro(II)))
+      Diag(MacroNameTok, diag::ext_pp_undef_builtin_macro);
+
     if (MI->isWarnIfUnused())
       WarnUnusedMacroLocs.erase(MI->getDefinitionLoc());
 
Index: clang/test/Preprocessor/macro-reserved.c
===================================================================
--- clang/test/Preprocessor/macro-reserved.c
+++ clang/test/Preprocessor/macro-reserved.c
@@ -6,6 +6,7 @@
 #define __cplusplus
 #define _HAVE_X 0
 #define X__Y
+#define __STDC__ 1 // expected-warning {{redefining builtin macro}}
 
 #undef for
 #undef final
@@ -13,6 +14,7 @@
 #undef __cplusplus
 #undef _HAVE_X
 #undef X__Y
+#undef __STDC_HOSTED__ // expected-warning {{undefining builtin macro}}
 
 // allowlisted definitions
 #define while while
Index: clang/test/Preprocessor/macro-reserved.cpp
===================================================================
--- clang/test/Preprocessor/macro-reserved.cpp
+++ clang/test/Preprocessor/macro-reserved.cpp
@@ -12,7 +12,7 @@
 #undef _HAVE_X
 #undef X__Y
 
-#undef __cplusplus
+#undef __cplusplus // expected-warning {{undefining builtin macro}}
 #define __cplusplus
 
 // allowlisted definitions