[Clang] disallow constexpr with auto and explicit type in C23

clang/docs/ReleaseNotes.rst

@@ -271,6 +271,8 @@ Non-comprehensive list of changes in this release
   allocation functions with a token ID can be enabled via the
   ``-fsanitize=alloc-token`` flag.
 
+- Clang now rejects the invalid use of ``constexpr`` with ``auto`` and an explicit type. (#GH163090)
+
 New Compiler Flags
 ------------------
 - New option ``-fno-sanitize-debug-trap-reasons`` added to disable emitting trap reasons into the debug info when compiling with trapping UBSan (e.g. ``-fsanitize-trap=undefined``).

clang/lib/Sema/DeclSpec.cpp

@@ -1369,7 +1369,7 @@ void DeclSpec::Finish(Sema &S, const PrintingPolicy &Policy) {
 
   if (S.getLangOpts().C23 &&
       getConstexprSpecifier() == ConstexprSpecKind::Constexpr &&
-      StorageClassSpec == SCS_extern) {
+      (StorageClassSpec == SCS_extern || StorageClassSpec == SCS_auto)) {
     S.Diag(ConstexprLoc, diag::err_invalid_decl_spec_combination)
         << DeclSpec::getSpecifierName(getStorageClassSpec())
         << SourceRange(getStorageClassSpecLoc());

clang/test/Parser/c2x-auto.c

@@ -62,6 +62,8 @@ auto basic_usage(auto auto) {   // c23-error {{'auto' not allowed in function pr
 
   int auto_cxx_decl = auto(0);  // expected-error {{expected expression}}
 
+  constexpr auto int x = 0; // c23-error {{cannot combine with previous 'auto' declaration specifier}} \
+                               c17-error {{use of undeclared identifier 'constexpr'}}
   return c;
 }