[clang] CWG 2354: prohibit alignas for enums

[erichkeane]

Please just note in the comment for the C++11 bit that it is modified by CWG2354, otherwise I think this is good.

clang/lib/Sema/SemaDeclAttr.cpp

4291

Can you add a comment up near here updating this?

[Endill]

Fix warning about clang-format being missing

[aaron.ballman]

LGTM aside from a tiny testing nit. It's interesting that WG14 doesn't seem to have a similar DR reported them that I can find (nothing for you to do about it).

clang/test/CXX/drs/dr2354.cpp

2

[Endill]

Add a quote from CWG 2354 after a quote from C++11

[Endill]

clang/lib/Sema/SemaDeclAttr.cpp

4291

Like this? I can also quote the same paragraph from C++20, where this DR is already applied.

[Endill]

Can somebody commit this? I don't have commit access.

[ChuanqiXu]

In D121723#3384962, @Endill wrote:

Can somebody commit this? I don't have commit access.

Hi, what's your prefered name and email addressed? I would love to commit it.

[Endill]

In D121723#3384982, @ChuanqiXu wrote:

In D121723#3384962, @Endill wrote:

Can somebody commit this? I don't have commit access.

Hi, what's your prefered name and email addressed? I would love to commit it.

Vlad Serebrennikov
[e-mail]

clang/include/clang/Basic/DiagnosticSemaKinds.td

: Error<"%0 attribute is not supported on targets missing %1;"
             " specify an appropriate -march= or -mcpu=">;
 // The err_*_attribute_argument_not_int are separate because they're used by
 // VerifyIntegerConstantExpression.
 def err_aligned_attribute_argument_not_int : Error<
   "'aligned' attribute requires integer constant">;
 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">;
 def err_alignas_mismatch : Error<
   "redeclaration has different alignment requirement (%1 vs %0)">;
 def err_alignas_underaligned : Error<
   "requested alignment is less than minimum alignment of %1 for type %0">;
 def warn_aligned_attr_underaligned : Warning<err_alignas_underaligned.Text>,

clang/lib/Sema/SemaDeclAttr.cpp

void Sema::AddAlignedAttr(Decl *D, const AttributeCommonInfo &CI, Expr *E,
 
   // C++11 alignas(...) and C11 _Alignas(...) have additional requirements.
   if (TmpAttr.isAlignas()) {
     // C++11 [dcl.align]p1:
     //   An alignment-specifier may be applied to a variable or to a class
     //   data member, but it shall not be applied to a bit-field, a function
     //   parameter, the formal parameter of a catch clause, or a variable
     //   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
     //   object declared with the register storage-class specifier.
     int DiagKind = -1;
     if (isa<ParmVarDecl>(D)) {
       DiagKind = 0;
     } else if (const auto *VD = dyn_cast<VarDecl>(D)) {
       if (VD->getStorageClass() == SC_Register)
         DiagKind = 1;
       if (VD->isExceptionVariable())
         DiagKind = 2;
     } 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
                             : ExpectedVariableFieldOrTag);
       return;
     }
     if (DiagKind != -1) {
       Diag(AttrLoc, diag::err_alignas_attribute_wrong_decl_type)

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

 // RUN: %clang_cc1 -std=c++11 -triple x86_64-linux-gnu -verify %s
 
 alignas(1) int n1; // expected-error {{requested alignment is less than minimum alignment of 4 for type 'int'}}
 alignas(1) alignas(2) int n2; // expected-error {{less than minimum alignment}}
 alignas(1) alignas(2) alignas(4) int n3; // ok
 alignas(1) alignas(2) alignas(0) int n4; // expected-error {{less than minimum alignment}}
 alignas(1) alignas(2) int n5 alignas(4); // ok
 alignas(1) alignas(4) int n6 alignas(2); // ok
 alignas(1) int n7 alignas(2), // expected-error {{less than minimum alignment}}
                n8 alignas(4); // ok
 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, "");
 
 struct S1 {
   alignas(8) int n;
 };
 struct alignas(2) S2 { // expected-error {{requested alignment is less than minimum alignment of 4 for type 'S2'}}

 template struct X<1, 2, char>; // expected-note {{instantiation}}
 template struct X<1, 1, short>; // expected-note {{instantiation}}
 template struct X<2, 1, short>; // expected-note {{instantiation}}
 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}}
 };
 template void f<1, char>();
 template void f<2, char>();
 template void f<1, short>(); // expected-note {{instantiation}}
 template void f<2, short>();

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

 alignas(8) int n7;
 
 alignas(8) extern int n8; // expected-note {{declared with 'alignas' attribute here}}
 int n8; // expected-error {{'alignas' must be specified on definition if it is specified on any declaration}}
 
 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;
 struct S { int n; }; // expected-error {{'alignas' must be specified on definition if it is specified on any declaration}}
 
 struct alignas(2) T;
 struct alignas(2) T { char c; }; // expected-note {{previous declaration is here}}
 struct T;

 template<int M, int N, int O, int P> struct X {
   alignas(M) alignas(N) static char Buffer[32]; // expected-note {{previous declaration is here}}
 };
 template<int M, int N, int O, int P>
 alignas(O) alignas(P) char X<M, N, O, P>::Buffer[32]; // expected-error {{redeclaration has different alignment requirement (8 vs 2)}}
 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

+// 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

<tr id="2353">
     <td>CD5</td>
     <td>Potential results of a member access expression for a static data member</td>
     <td class="full" align="center">Clang 9</td>
   </tr>
   <tr id="2354">
     <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>
     <td>extension</td>
     <td>Deducing <I>noexcept-specifier</I>s</td>
     <td align="center">Extension</td>
   </tr>
   <tr id="2356">