[clang] CWG 2354: prohibit alignas for enums

Reviewed By: aaron.ballman, erichkeane

Differential Revision: https://reviews.llvm.org/D121723

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -3022,8 +3022,9 @@ def err_aligned_attribute_argument_not_int : Error<
 def err_align_value_attribute_argument_not_int : Error<
   "'align_value' attribute requires integer constant">;
 def err_alignas_attribute_wrong_decl_type : Error<
-  "%0 attribute cannot be applied to a %select{function parameter|"
-  "variable with 'register' storage class|'catch' variable|bit-field}1">;
+  "%0 attribute cannot be applied to %select{a function parameter|"
+  "a variable with 'register' storage class|a 'catch' variable|a bit-field|"
+  "an enumeration}1">;
 def err_alignas_missing_on_definition : Error<
   "%0 must be specified on definition if it is specified on any declaration">;
 def note_alignas_on_declaration : Note<"declared with %0 attribute here">;

clang/lib/Sema/SemaDeclAttr.cpp

@@ -4290,6 +4290,9 @@ void Sema::AddAlignedAttr(Decl *D, const AttributeCommonInfo &CI, Expr *E,
     //   declared with the register storage class specifier. An
     //   alignment-specifier may also be applied to the declaration of a class
     //   or enumeration type.
+    // CWG 2354:
+    //   CWG agreed to remove permission for alignas to be applied to
+    //   enumerations.
     // C11 6.7.5/2:
     //   An alignment attribute shall not be specified in a declaration of
     //   a typedef, or a bit-field, or a function, or a parameter, or an
@@ -4305,6 +4308,9 @@ void Sema::AddAlignedAttr(Decl *D, const AttributeCommonInfo &CI, Expr *E,
     } else if (const auto *FD = dyn_cast<FieldDecl>(D)) {
       if (FD->isBitField())
         DiagKind = 3;
+    } else if (const auto *ED = dyn_cast<EnumDecl>(D)) {
+      if (ED->getLangOpts().CPlusPlus)
+        DiagKind = 4;
     } else if (!isa<TagDecl>(D)) {
       Diag(AttrLoc, diag::err_attribute_wrong_decl_type) << &TmpAttr
         << (TmpAttr.isC11() ? ExpectedVariableOrField

clang/test/CXX/dcl.dcl/dcl.attr/dcl.align/p5.cpp

@@ -11,13 +11,6 @@ alignas(1) int n7 alignas(2), // expected-error {{less than minimum alignment}}
 alignas(8) int n9 alignas(2); // ok, overaligned
 alignas(1) extern int n10; // expected-error {{less than minimum alignment}}
 
-enum alignas(1) E1 {}; // expected-error {{requested alignment is less than minimum alignment of 4 for type 'E1'}}
-enum alignas(1) E2 : char {}; // ok
-enum alignas(4) E3 { e3 = 0 }; // ok
-enum alignas(4) E4 { e4 = 1ull << 33 }; // expected-error {{requested alignment is less than minimum alignment of 8 for type 'E4'}}
-enum alignas(8) E5 {};
-static_assert(alignof(E5) == 8, "");
-
 typedef __attribute__((aligned(16))) int IntAlign16;
 enum E6 : IntAlign16 {};
 static_assert(alignof(E6) == 4, "");
@@ -62,15 +55,6 @@ template struct X<2, 2, short>;
 template struct X<16, 8, S1>;
 template struct X<4, 4, S1>; // expected-note {{instantiation}}
 
-template<int N, typename T>
-struct Y {
-  enum alignas(N) E : T {}; // expected-error {{requested alignment is less than minimum}}
-};
-template struct Y<1, char>;
-template struct Y<2, char>;
-template struct Y<1, short>; // expected-note {{instantiation}}
-template struct Y<2, short>;
-
 template<int N, typename T>
 void f() {
   alignas(N) T v; // expected-error {{requested alignment is less than minimum}}

clang/test/CXX/dcl.dcl/dcl.attr/dcl.align/p6.cpp

@@ -27,21 +27,6 @@ int n8; // expected-error {{'alignas' must be specified on definition if it is s
 int n9; // expected-error {{'alignas' must be specified on definition if it is specified on any declaration}}
 alignas(4) extern int n9; // expected-note {{declared with 'alignas' attribute here}}
 
-
-enum alignas(2) E : char; // expected-note {{declared with 'alignas' attribute here}}
-enum E : char {}; // expected-error {{'alignas' must be specified on definition if it is specified on any declaration}}
-
-enum alignas(4) F : char; // expected-note {{previous declaration is here}}
-enum alignas(2) F : char; // expected-error {{redeclaration has different alignment requirement (2 vs 4)}}
-
-enum G : char;
-enum alignas(8) G : char {};
-enum G : char;
-
-enum H : char {}; // expected-error {{'alignas' must be specified on definition if it is specified on any declaration}}
-enum alignas(1) H : char; // expected-note {{declared with 'alignas' attribute here}}
-
-
 struct S;
 struct alignas(16) S; // expected-note {{declared with 'alignas' attribute here}}
 struct S;
@@ -73,14 +58,5 @@ alignas(O) alignas(P) char X<M, N, O, P>::Buffer[32]; // expected-error {{redecl
 char *x1848 = X<1,8,4,8>::Buffer; // ok
 char *x1248 = X<1,2,4,8>::Buffer; // expected-note {{in instantiation of}}
 
-template<int M, int N, int O, int P> struct Y {
-  enum alignas(M) alignas(N) E : char;
-};
-template<int M, int N, int O, int P>
-enum alignas(O) alignas(P) Y<M,N,O,P>::E : char { e };
-int y1848 = Y<1,8,4,8>::e;
-// FIXME: We should reject this.
-int y1248 = Y<1,2,4,8>::e;
-
 // Don't crash here.
 alignas(4) struct Incomplete incomplete; // expected-error {{incomplete type}} expected-note {{forward declaration}}

clang/test/CXX/drs/dr2354.cpp

@@ -0,0 +1,10 @@
+// RUN: %clang_cc1 -x c++ -verify %s 
+
+// dr2354: 15
+
+namespace DR2354 {
+
+enum alignas(64) A {};        // expected-error {{'alignas' attribute cannot be applied to an enumeration}}
+enum struct alignas(64) B {}; // expected-error {{'alignas' attribute cannot be applied to an enumeration}}
+
+} // namespace DR2354

clang/www/cxx_dr_status.html

@@ -13938,7 +13938,7 @@ and <I>POD class</I></td>
     <td><a href="https://wg21.link/cwg2354">2354</a></td>
     <td>CD5</td>
     <td>Extended alignment and object representation</td>
-    <td class="none" align="center">Unknown</td>
+    <td class="unreleased" align="center">Clang 15</td>
   </tr>
   <tr class="open" id="2355">
     <td><a href="https://wg21.link/cwg2355">2355</a></td>