[C23] Fix handling of alignas (#81637)

In C++, alignas is an attribute specifier, while in C23, it's an alias
of _Alignas, which is a type specifier/qualifier. This means that they
parse differently in some circumstances.

Fixes https://github.com/llvm/llvm-project/issues/81472

1 files changed, 22 insertions, 8 deletions

diff --git a/clang/lib/Parse/ParseDecl.cpp b/clang/lib/Parse/ParseDecl.cpp
index 94696d8..9640d7e 100644
--- a/clang/lib/Parse/ParseDecl.cpp
+++ b/clang/lib/Parse/ParseDecl.cpp
@@ -3518,8 +3518,24 @@ void Parser::ParseDeclarationSpecifiers(
       DS.Finish(Actions, Policy);
       return;
 
-    case tok::l_square:
+    // alignment-specifier
+    case tok::kw__Alignas:
+      if (!getLangOpts().C11)
+        Diag(Tok, diag::ext_c11_feature) << Tok.getName();
+      [[fallthrough]];
     case tok::kw_alignas:
+      // _Alignas and alignas (C23, not C++) should parse the same way. The C++
+      // parsing for alignas happens through the usual attribute parsing. This
+      // ensures that an alignas specifier can appear in a type position in C
+      // despite that not being valid in C++.
+      if (getLangOpts().C23 || Tok.getKind() == tok::kw__Alignas) {
+        if (Tok.getKind() == tok::kw_alignas)
+          Diag(Tok, diag::warn_c23_compat_keyword) << Tok.getName();
+        ParseAlignmentSpecifier(DS.getAttributes());
+        continue;
+      }
+      [[fallthrough]];
+    case tok::l_square:
       if (!isAllowedCXX11AttributeSpecifier())
         goto DoneWithDeclSpec;
 
@@ -4234,13 +4250,6 @@ void Parser::ParseDeclarationSpecifiers(
       isInvalid = DS.setFunctionSpecNoreturn(Loc, PrevSpec, DiagID);
       break;
 
-    // alignment-specifier
-    case tok::kw__Alignas:
-      if (!getLangOpts().C11)
-        Diag(Tok, diag::ext_c11_feature) << Tok.getName();
-      ParseAlignmentSpecifier(DS.getAttributes());
-      continue;
-
     // friend
     case tok::kw_friend:
       if (DSContext == DeclSpecContext::DSC_class)
@@ -5857,6 +5866,11 @@ bool Parser::isDeclarationSpecifier(
   case tok::kw__Atomic:
     return true;
 
+  case tok::kw_alignas:
+    // alignas is a type-specifier-qualifier in C23, which is a kind of
+    // declaration-specifier. Outside of C23 mode (including in C++), it is not.
+    return getLangOpts().C23;
+
     // GNU ObjC bizarre protocol extension: <proto1,proto2> with implicit 'id'.
   case tok::less:
     return getLangOpts().ObjC;