[clang] [Clang] disallow constexpr with auto and explicit type in C23 (PR #163469)

[Oleksandr T. via cfe-commits]

https://github.com/a-tarasyuk created https://github.com/llvm/llvm-project/pull/163469

Fixes #163090

--- 

This PR addresses the issue of Clang not diagnosing the invalid combination of `constexpr`,
`auto`, and an explicit type

```c
constexpr auto int x = 0
```

>From 3856379ddbb59c6dcd813765571a67f635034a3c Mon Sep 17 00:00:00 2001
From: Oleksandr Tarasiuk <oleksandr.tarasiuk at outlook.com>
Date: Wed, 15 Oct 2025 01:42:15 +0300
Subject: [PATCH] [Clang] disallow constexpr with auto and explicit type in C23

---
 clang/docs/ReleaseNotes.rst  | 2 ++
 clang/lib/Sema/DeclSpec.cpp  | 2 +-
 clang/test/Parser/c2x-auto.c | 2 ++
 3 files changed, 5 insertions(+), 1 deletion(-)

diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index edb872c1f388d..de745e54a0cbd 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -269,6 +269,8 @@ 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``).
diff --git a/clang/lib/Sema/DeclSpec.cpp b/clang/lib/Sema/DeclSpec.cpp
index 184d31ecd1e40..482cb1fe703fe 100644
--- a/clang/lib/Sema/DeclSpec.cpp
+++ b/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());
diff --git a/clang/test/Parser/c2x-auto.c b/clang/test/Parser/c2x-auto.c
index b878a5b7c42d4..b33b267841132 100644
--- a/clang/test/Parser/c2x-auto.c
+++ b/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;
 }