Index: clang/lib/Lex/PPDirectives.cpp
===================================================================
--- clang/lib/Lex/PPDirectives.cpp
+++ clang/lib/Lex/PPDirectives.cpp
@@ -150,6 +150,30 @@
                             MacroName);
 }
 
+static bool isLanguageDefinedBuiltin(const SourceManager &SourceMgr,
+                                     const MacroInfo *MI,
+                                     const StringRef MacroName) {
+  // If this is a macro with special handling (like __LINE__) then it's language
+  // defined.
+  if (MI->isBuiltinMacro())
+    return true;
+  // Builtin macros are defined in the builtin file
+  if (!SourceMgr.isWrittenInBuiltinFile(MI->getDefinitionLoc()))
+    return false;
+  // C defines macros starting with __STDC, and C++ defines macros starting with
+  // __STDCPP
+  if (MacroName.startswith("__STDC"))
+    return true;
+  // C++ defines the __cplusplus macro
+  if (MacroName == "__cplusplus")
+    return true;
+  // C++ defines various feature-test macros starting with __cpp
+  if (MacroName.startswith("__cpp"))
+    return true;
+  // Anything else isn't language-defined
+  return false;
+}
+
 static MacroDiag shouldWarnOnMacroDef(Preprocessor &PP, IdentifierInfo *II) {
   const LangOptions &Lang = PP.getLangOpts();
   StringRef Text = II->getName();
@@ -3106,9 +3130,7 @@
 
       // 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() ||
-          (SourceMgr.isWrittenInBuiltinFile(OtherMI->getDefinitionLoc()) &&
-           !isFeatureTestMacro(MacroNameTok.getIdentifierInfo()->getName())))
+      if (isLanguageDefinedBuiltin(SourceMgr, OtherMI, II->getName()))
         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.
@@ -3189,11 +3211,8 @@
       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)))
+    // C++ [cpp.predefined]p4, but allow it as an extension.
+    if (isLanguageDefinedBuiltin(SourceMgr, MI, II->getName()))
       Diag(MacroNameTok, diag::ext_pp_undef_builtin_macro);
 
     if (MI->isWarnIfUnused())
Index: clang/test/Preprocessor/macro-reserved.c
===================================================================
--- clang/test/Preprocessor/macro-reserved.c
+++ clang/test/Preprocessor/macro-reserved.c
@@ -7,6 +7,7 @@
 #define _HAVE_X 0
 #define X__Y
 #define __STDC__ 1 // expected-warning {{redefining builtin macro}}
+#define __clang__ 1
 
 #undef for
 #undef final
@@ -15,6 +16,7 @@
 #undef _HAVE_X
 #undef X__Y
 #undef __STDC_HOSTED__ // expected-warning {{undefining builtin macro}}
+#undef __INT32_TYPE__
 
 // allowlisted definitions
 #define while while