[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 in C. (#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 +1369,8 @@ void DeclSpec::Finish(Sema &S, const PrintingPolicy &Policy) {
 
   if (S.getLangOpts().C23 &&
       getConstexprSpecifier() == ConstexprSpecKind::Constexpr &&
-      StorageClassSpec == SCS_extern) {
+      getTypeSpecType() != TST_unspecified &&
+      (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

@@ -130,3 +130,30 @@ void atomic(void) {
 void attributes(void) {
   auto ident [[clang::annotate("this works")]] = 12;  // c17-error {{type specifier missing, defaults to 'int'; ISO C99 and later do not support implicit int}}
 }
+
+/** GH163090 */
+constexpr auto int a1 = 0; // c23-error {{illegal storage class on file-scoped variable}} \
+                              c23-error {{cannot combine with previous 'auto' declaration specifier}} \
+                              c17-error {{illegal storage class on file-scoped variable}} \
+                              c17-error {{unknown type name 'constexpr'}}
+
+constexpr int auto a2 = 0; // c23-error {{cannot combine with previous 'int' declaration specifier}} \
+                              c17-error {{illegal storage class on file-scoped variable}} \
+                              c17-error {{unknown type name 'constexpr'}}
+
+auto int b1 = 0; // c23-error {{illegal storage class on file-scoped variable}} \
+                    c17-error {{illegal storage class on file-scoped variable}}
+
+int auto b2 = 0; // c23-error {{cannot combine with previous 'int' declaration specifier}} \
+                    c17-error {{illegal storage class on file-scoped variable}}
+
+void f() {
+  constexpr auto int c1 = 0; // c23-error {{cannot combine with previous 'auto' declaration specifier}} \
+                                c17-error {{use of undeclared identifier 'constexpr'}}
+
+  constexpr int auto c2 = 0; // c23-error {{cannot combine with previous 'int' declaration specifier}} \
+                                c17-error {{use of undeclared identifier 'constexpr'}}
+
+  auto int d1 = 0;
+  int auto d2 = 0; // c23-error {{cannot combine with previous 'int' declaration specifier}}
+}