[clang] Improve _Alignas on a struct declaration diagnostic (#65638)

Fixes https://github.com/llvm/llvm-project/issues/58637. 

Adds `isAlignas()` method on `AttributeCommonInfo` which accounts for
C++ `alignas` as well as C11 `_Alignas`.

The method is used to improve diagnostic in C when `_Alignas` is used in
C at the wrong location.  This corrects the previously suggested move
of `_Alignas` past the declaration specifier, now warns attribute
`_Alignas` is ignored.

Based on https://reviews.llvm.org/D141177.

clang/docs/ReleaseNotes.rst

@@ -292,6 +292,11 @@ Attribute Changes in Clang
   When viewing ``S::FruitKind`` in a debugger, it will behave as if the member
   was declared as type ``E`` rather than ``unsigned``.
 
+- Clang now warns you that the ``_Alignas`` attribute on declaration specifiers
+  is ignored, changed from the former incorrect suggestion to move it past
+  declaration specifiers. (`#58637 <https://github.com/llvm/llvm-project/issues/58637>`_)
+
+
 Improvements to Clang's diagnostics
 -----------------------------------
 - Clang constexpr evaluator now prints template arguments when displaying

clang/include/clang/Basic/AttributeCommonInfo.h

@@ -200,6 +200,15 @@ public:
 
   bool isC23Attribute() const { return SyntaxUsed == AS_C23; }
 
+  bool isAlignas() const {
+    // FIXME: In the current state, the IsAlignas member variable is only true
+    // with the C++  `alignas` keyword but not `_Alignas`. The following
+    // expression works around the otherwise lost information so it will return
+    // true for `alignas` or `_Alignas` while still returning false for things
+    // like  `__attribute__((aligned))`.
+    return (getParsedKind() == AT_Aligned && isKeywordAttribute());
+  }
+
   /// The attribute is spelled [[]] in either C or C++ mode, including standard
   /// attributes spelled with a keyword, like alignas.
   bool isStandardAttributeSyntax() const {

clang/lib/Sema/SemaDecl.cpp

@@ -5346,16 +5346,21 @@ Decl *Sema::ParsedFreeStandingDeclSpec(Scope *S, AccessSpecifier AS,
         TypeSpecType == DeclSpec::TST_interface ||
         TypeSpecType == DeclSpec::TST_union ||
         TypeSpecType == DeclSpec::TST_enum) {
-      for (const ParsedAttr &AL : DS.getAttributes())
-        Diag(AL.getLoc(), AL.isRegularKeywordAttribute()
-                              ? diag::err_declspec_keyword_has_no_effect
-                              : diag::warn_declspec_attribute_ignored)
-            << AL << GetDiagnosticTypeSpecifierID(DS);
-      for (const ParsedAttr &AL : DeclAttrs)
-        Diag(AL.getLoc(), AL.isRegularKeywordAttribute()
-                              ? diag::err_declspec_keyword_has_no_effect
-                              : diag::warn_declspec_attribute_ignored)
+
+      auto EmitAttributeDiagnostic = [this, &DS](const ParsedAttr &AL) {
+        unsigned DiagnosticId = diag::warn_declspec_attribute_ignored;
+        if (AL.isAlignas() && !getLangOpts().CPlusPlus)
+          DiagnosticId = diag::warn_attribute_ignored;
+        else if (AL.isRegularKeywordAttribute())
+          DiagnosticId = diag::err_declspec_keyword_has_no_effect;
+        else
+          DiagnosticId = diag::warn_declspec_attribute_ignored;
+        Diag(AL.getLoc(), DiagnosticId)
             << AL << GetDiagnosticTypeSpecifierID(DS);
+      };
+
+      llvm::for_each(DS.getAttributes(), EmitAttributeDiagnostic);
+      llvm::for_each(DeclAttrs, EmitAttributeDiagnostic);
     }
   }
 

clang/test/C/drs/dr4xx.c

@@ -164,11 +164,7 @@ void dr444(void) {
   /* FIXME: This should be accepted as per this DR. */
   int j = (_Alignas(int) int){12}; /* expected-error {{expected expression}} */
 
- /* FIXME: The diagnostic in this case is really bad; moving the specifier to
-  * where the diagnostic recommends causes a different, more inscrutable error
-  * about anonymous structures.
-  */
-  _Alignas(int) struct T { /* expected-warning {{attribute '_Alignas' is ignored, place it after "struct" to apply attribute to type declaration}} */
+  _Alignas(int) struct T { /* expected-warning {{'_Alignas' attribute ignored}} */
     int i;
   };
 

clang/test/Parser/c1x-alignas.c

@@ -9,5 +9,6 @@ char c4 _Alignas(32); // expected-error {{expected ';' after top level declarato
 
 char _Alignas(_Alignof(int)) c5;
 
+_Alignas(int) struct c6; // expected-warning {{'_Alignas' attribute ignored}}
 // CHECK-EXT: '_Alignas' is a C11 extension
 // CHECK-EXT: '_Alignof' is a C11 extension

clang/test/Parser/c2x-alignas.c

@@ -0,0 +1,11 @@
+// RUN: %clang_cc1 -std=c23 -fsyntax-only -verify %s
+
+_Alignas(int) struct c1; // expected-warning {{'_Alignas' attribute ignored}}
+
+// FIXME: `alignas` enters into C++ parsing code and never reaches the
+// declaration specifier attribute diagnostic infrastructure.
+// 
+// Fixing this will require the C23 notions of `alignas` being a keyword and
+// `_Alignas` being an alternate spelling integrated into the parsing
+// infrastructure.
+alignas(int) struct c1; // expected-error {{misplaced attributes; expected attributes here}}

clang/test/Parser/cxx0x-attributes.cpp

@@ -451,3 +451,5 @@ namespace P2361 {
                                  // expected-warning {{use of the 'deprecated' attribute is a C++14 extension}}
 [[nodiscard("\123")]] int b(); // expected-error{{invalid escape sequence '\123' in an unevaluated string literal}}
 }
+
+alignas(int) struct AlignAsAttribute {}; // expected-error {{misplaced attributes; expected attributes here}}