[Clang] Implement C++26 Attributes for Structured Bindings (P0609R3) (#…

…89906)

https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2024/p0609r3.pdf

We support this feature in all language mode.

maybe_unused applied to a binding makes the whole declaration unused.

clang/docs/LanguageExtensions.rst

@@ -1493,6 +1493,7 @@ Conditional ``explicit``                     __cpp_conditional_explicit       C+
 ``if consteval``                             __cpp_if_consteval               C++23         C++20
 ``static operator()``                        __cpp_static_call_operator       C++23         C++03
 Attributes on Lambda-Expressions                                              C++23         C++11
+Attributes on Structured Bindings            __cpp_structured_bindings        C++26         C++03
 ``= delete ("should have a reason");``       __cpp_deleted_function           C++26         C++03
 -------------------------------------------- -------------------------------- ------------- -------------
 Designated initializers (N494)                                                C99           C89

clang/docs/ReleaseNotes.rst

@@ -143,6 +143,8 @@ C++2c Feature Support
 
 - Implemented `P2573R2: = delete("should have a reason"); <https://wg21.link/P2573R2>`_
 
+- Implemented `P0609R3: Attributes for Structured Bindings <https://wg21.link/P0609R3>`_
+
 
 Resolutions to C++ Defect Reports
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

clang/include/clang/Basic/Attr.td

@@ -3211,7 +3211,7 @@ def ObjCRequiresPropertyDefs : InheritableAttr {
 def Unused : InheritableAttr {
   let Spellings = [CXX11<"", "maybe_unused", 201603>, GCC<"unused">,
                    C23<"", "maybe_unused", 202106>];
-  let Subjects = SubjectList<[Var, ObjCIvar, Type, Enum, EnumConstant, Label,
+  let Subjects = SubjectList<[Var, Binding, ObjCIvar, Type, Enum, EnumConstant, Label,
                               Field, ObjCMethod, FunctionLike]>;
   let Documentation = [WarnMaybeUnusedDocs];
 }

clang/include/clang/Basic/DiagnosticParseKinds.td

@@ -478,6 +478,15 @@ def ext_decomp_decl_empty : ExtWarn<
   "ISO C++17 does not allow a decomposition group to be empty">,
   InGroup<DiagGroup<"empty-decomposition">>;
 
+// C++26 structured bindings
+def ext_decl_attrs_on_binding : ExtWarn<
+  "an attribute specifier sequence attached to a structured binding declaration "
+  "is a C++2c extension">, InGroup<CXX26>;
+def warn_cxx23_compat_decl_attrs_on_binding : Warning<
+  "an attribute specifier sequence attached to a structured binding declaration "
+  "is incompatible with C++ standards before C++2c">,
+  InGroup<CXXPre26Compat>, DefaultIgnore;
+
 /// Objective-C parser diagnostics
 def err_expected_minus_or_plus : Error<
   "method type specifier must start with '-' or '+'">;

clang/include/clang/Sema/DeclSpec.h

@@ -36,6 +36,7 @@
 #include "llvm/ADT/SmallVector.h"
 #include "llvm/Support/Compiler.h"
 #include "llvm/Support/ErrorHandling.h"
+#include <optional>
 
 namespace clang {
   class ASTContext;
@@ -1790,6 +1791,7 @@ class DecompositionDeclarator {
   struct Binding {
     IdentifierInfo *Name;
     SourceLocation NameLoc;
+    std::optional<ParsedAttributes> Attrs;
   };
 
 private:
@@ -2339,10 +2341,10 @@ class Declarator {
   }
 
   /// Set the decomposition bindings for this declarator.
-  void
-  setDecompositionBindings(SourceLocation LSquareLoc,
-                           ArrayRef<DecompositionDeclarator::Binding> Bindings,
-                           SourceLocation RSquareLoc);
+  void setDecompositionBindings(
+      SourceLocation LSquareLoc,
+      MutableArrayRef<DecompositionDeclarator::Binding> Bindings,
+      SourceLocation RSquareLoc);
 
   /// AddTypeInfo - Add a chunk to this declarator. Also extend the range to
   /// EndLoc, which should be the last token of the chunk.

clang/include/clang/Sema/ParsedAttr.h

@@ -948,6 +948,7 @@ class ParsedAttributes : public ParsedAttributesView {
   ParsedAttributes(AttributeFactory &factory) : pool(factory) {}
   ParsedAttributes(const ParsedAttributes &) = delete;
   ParsedAttributes &operator=(const ParsedAttributes &) = delete;
+  ParsedAttributes(ParsedAttributes &&G) = default;
 
   AttributePool &getPool() const { return pool; }
 

clang/lib/AST/DeclBase.cpp

@@ -1115,7 +1115,9 @@ int64_t Decl::getID() const {
 
 const FunctionType *Decl::getFunctionType(bool BlocksToo) const {
   QualType Ty;
-  if (const auto *D = dyn_cast<ValueDecl>(this))
+  if (const auto *D = dyn_cast<BindingDecl>(this))
+    return nullptr;
+  else if (const auto *D = dyn_cast<ValueDecl>(this))
     Ty = D->getType();
   else if (const auto *D = dyn_cast<TypedefNameDecl>(this))
     Ty = D->getUnderlyingType();

clang/lib/Frontend/InitPreprocessor.cpp

@@ -703,7 +703,7 @@ static void InitializeCPlusPlusFeatureTestMacros(const LangOptions &LangOpts,
     Builder.defineMacro("__cpp_nested_namespace_definitions", "201411L");
     Builder.defineMacro("__cpp_variadic_using", "201611L");
     Builder.defineMacro("__cpp_aggregate_bases", "201603L");
-    Builder.defineMacro("__cpp_structured_bindings", "201606L");
+    Builder.defineMacro("__cpp_structured_bindings", "202403L");
     Builder.defineMacro("__cpp_nontype_template_args",
                         "201411L"); // (not latest)
     Builder.defineMacro("__cpp_fold_expressions", "201603L");

clang/lib/Parse/ParseDecl.cpp

@@ -7038,18 +7038,23 @@ void Parser::ParseDirectDeclarator(Declarator &D) {
 void Parser::ParseDecompositionDeclarator(Declarator &D) {
   assert(Tok.is(tok::l_square));
 
+  TentativeParsingAction PA(*this);
+  BalancedDelimiterTracker T(*this, tok::l_square);
+  T.consumeOpen();
+
+  if (isCXX11AttributeSpecifier())
+    DiagnoseAndSkipCXX11Attributes();
+
   // If this doesn't look like a structured binding, maybe it's a misplaced
   // array declarator.
-  // FIXME: Consume the l_square first so we don't need extra lookahead for
-  // this.
-  if (!(NextToken().is(tok::identifier) &&
-        GetLookAheadToken(2).isOneOf(tok::comma, tok::r_square)) &&
-      !(NextToken().is(tok::r_square) &&
-        GetLookAheadToken(2).isOneOf(tok::equal, tok::l_brace)))
+  if (!(Tok.is(tok::identifier) &&
+        NextToken().isOneOf(tok::comma, tok::r_square, tok::kw_alignas,
+                            tok::l_square)) &&
+      !(Tok.is(tok::r_square) &&
+        NextToken().isOneOf(tok::equal, tok::l_brace))) {
+    PA.Revert();
     return ParseMisplacedBracketDeclarator(D);
-
-  BalancedDelimiterTracker T(*this, tok::l_square);
-  T.consumeOpen();
+  }
 
   SmallVector<DecompositionDeclarator::Binding, 32> Bindings;
   while (Tok.isNot(tok::r_square)) {
@@ -7074,13 +7079,27 @@ void Parser::ParseDecompositionDeclarator(Declarator &D) {
       }
     }
 
+    if (isCXX11AttributeSpecifier())
+      DiagnoseAndSkipCXX11Attributes();
+
     if (Tok.isNot(tok::identifier)) {
       Diag(Tok, diag::err_expected) << tok::identifier;
       break;
     }
 
-    Bindings.push_back({Tok.getIdentifierInfo(), Tok.getLocation()});
+    IdentifierInfo *II = Tok.getIdentifierInfo();
+    SourceLocation Loc = Tok.getLocation();
     ConsumeToken();
+
+    ParsedAttributes Attrs(AttrFactory);
+    if (isCXX11AttributeSpecifier()) {
+      Diag(Tok, getLangOpts().CPlusPlus26
+                    ? diag::warn_cxx23_compat_decl_attrs_on_binding
+                    : diag::ext_decl_attrs_on_binding);
+      MaybeParseCXX11Attributes(Attrs);
+    }
+
+    Bindings.push_back({II, Loc, std::move(Attrs)});
   }
 
   if (Tok.isNot(tok::r_square))
@@ -7095,6 +7114,8 @@ void Parser::ParseDecompositionDeclarator(Declarator &D) {
     T.consumeClose();
   }
 
+  PA.Commit();
+
   return D.setDecompositionBindings(T.getOpenLocation(), Bindings,
                                     T.getCloseLocation());
 }

clang/lib/Sema/DeclSpec.cpp

@@ -293,7 +293,7 @@ DeclaratorChunk DeclaratorChunk::getFunction(bool hasProto,
 
 void Declarator::setDecompositionBindings(
     SourceLocation LSquareLoc,
-    ArrayRef<DecompositionDeclarator::Binding> Bindings,
+    MutableArrayRef<DecompositionDeclarator::Binding> Bindings,
     SourceLocation RSquareLoc) {
   assert(!hasName() && "declarator given multiple names!");
 
@@ -317,7 +317,7 @@ void Declarator::setDecompositionBindings(
           new DecompositionDeclarator::Binding[Bindings.size()];
       BindingGroup.DeleteBindings = true;
     }
-    std::uninitialized_copy(Bindings.begin(), Bindings.end(),
+    std::uninitialized_move(Bindings.begin(), Bindings.end(),
                             BindingGroup.Bindings);
   }
 }

clang/lib/Sema/SemaDecl.cpp

@@ -1974,7 +1974,7 @@ static bool ShouldDiagnoseUnusedDecl(const LangOptions &LangOpts,
     // it is, by the bindings' expressions).
     bool IsAllPlaceholders = true;
     for (const auto *BD : DD->bindings()) {
-      if (BD->isReferenced())
+      if (BD->isReferenced() || BD->hasAttr<UnusedAttr>())
         return false;
       IsAllPlaceholders = IsAllPlaceholders && BD->isPlaceholderVar(LangOpts);
     }

clang/lib/Sema/SemaDeclCXX.cpp

@@ -910,6 +910,8 @@ Sema::ActOnDecompositionDeclarator(Scope *S, Declarator &D,
 
     auto *BD = BindingDecl::Create(Context, DC, B.NameLoc, VarName);
 
+    ProcessDeclAttributeList(S, BD, *B.Attrs);
+
     // Find the shadowed declaration before filtering for scope.
     NamedDecl *ShadowedDecl = D.getCXXScopeSpec().isEmpty()
                                   ? getShadowedDeclaration(BD, Previous)

clang/test/Lexer/cxx-features.cpp

@@ -222,7 +222,7 @@
 #error "wrong value for __cpp_aggregate_bases"
 #endif
 
-#if check(structured_bindings, 0, 0, 0, 201606, 201606, 201606, 201606)
+#if check(structured_bindings, 0, 0, 0, 202403L, 202403L, 202403L, 202403L)
 #error "wrong value for __cpp_structured_bindings"
 #endif
 

clang/test/Parser/cxx1z-decomposition.cpp

@@ -1,6 +1,7 @@
-// RUN: %clang_cc1 -std=c++17 %s -verify=expected,cxx17 -fcxx-exceptions
-// RUN: %clang_cc1 -std=c++2b %s -verify=expected,cxx2b -fcxx-exceptions
-// RUN: not %clang_cc1 -std=c++17 %s -emit-llvm-only -fcxx-exceptions
+// RUN: %clang_cc1 -std=c++17 %s -triple x86_64-unknown-linux-gnu -verify=expected,cxx17,pre2c -fcxx-exceptions
+// RUN: %clang_cc1 -std=c++2b %s -triple x86_64-unknown-linux-gnu -verify=expected,cxx2b,pre2c,post2b -fcxx-exceptions
+// RUN: %clang_cc1 -std=c++2c %s -triple x86_64-unknown-linux-gnu -verify=expected,cxx2c,post2b -fcxx-exceptions
+// RUN: not %clang_cc1 -std=c++17 %s -triple x86_64-unknown-linux-gnu -emit-llvm-only -fcxx-exceptions
 
 struct S { int a, b, c; };
 
@@ -58,7 +59,7 @@ namespace OtherDecl {
 namespace GoodSpecifiers {
   void f() {
     int n[1];
-    const volatile auto &[a] = n; // cxx2b-warning {{volatile qualifier in structured binding declaration is deprecated}}
+    const volatile auto &[a] = n; // post2b-warning {{volatile qualifier in structured binding declaration is deprecated}}
   }
 }
 
@@ -97,8 +98,8 @@ namespace BadSpecifiers {
     S [a] = s; // expected-error {{cannot be declared with type 'S'}}
     decltype(auto) [b] = s; // expected-error {{cannot be declared with type 'decltype(auto)'}}
     auto ([c2]) = s; // cxx17-error {{decomposition declaration cannot be declared with parenthese}} \
-                     // cxx2b-error {{use of undeclared identifier 'c2'}} \
-                     // cxx2b-error {{expected body of lambda expression}} \
+                     // post2b-error {{use of undeclared identifier 'c2'}} \
+                     // post2b-error {{expected body of lambda expression}} \
 
     // FIXME: This error is not very good.
     auto [d]() = s; // expected-error {{expected ';'}} expected-error {{expected expression}}
@@ -119,9 +120,6 @@ namespace BadSpecifiers {
     [[]] auto [ok_3] = s;
     alignas(S) auto [ok_4] = s;
 
-    // ... but not after the identifier or declarator.
-    // FIXME: These errors are not very good.
-    auto [bad_attr_1 [[]]] = s; // expected-error {{attribute list cannot appear here}} expected-error 2{{}}
     auto [bad_attr_2] [[]] = s; // expected-error {{expected ';'}} expected-error {{}}
   }
 }
@@ -156,3 +154,50 @@ namespace Init {
     S [goodish4] { 4 }; // expected-error {{cannot be declared with type 'S'}}
   }
 }
+
+
+namespace attributes {
+
+struct S{
+    int a;
+    int b = 0;
+};
+
+void err() {
+    auto [[]] = S{0}; // expected-error {{expected unqualified-id}}
+    auto [ alignas(42) a, foo ] = S{0}; // expected-error {{an attribute list cannot appear here}}
+    auto [ c, [[]] d ] = S{0}; // expected-error {{an attribute list cannot appear here}}
+    auto [ e, alignas(42) f ] = S{0}; // expected-error {{an attribute list cannot appear here}}
+}
+
+void ok() {
+    auto [ a alignas(42) [[]], b alignas(42) [[]]] = S{0}; // expected-error 2{{'alignas' attribute only applies to variables, data members and tag types}} \
+                                                           // pre2c-warning  2{{an attribute specifier sequence attached to a structured binding declaration is a C++2c extension}}
+    auto [ c [[]] alignas(42), d [[]] alignas(42) [[]]] = S{0}; // expected-error 2{{'alignas' attribute only applies to variables, data members and tag types}} \
+                                                                // pre2c-warning  2{{an attribute specifier sequence attached to a structured binding declaration is a C++2c extension}}
+}
+
+
+auto [G1 [[deprecated]], G2 [[deprecated]]] = S{42}; // #deprecated-here
+// pre2c-warning@-1 2{{an attribute specifier sequence attached to a structured binding declaration is a C++2c extension}}
+
+int test() {
+  return G1 + G2; // expected-warning {{'G1' is deprecated}} expected-note@#deprecated-here {{here}} \
+                  // expected-warning {{'G2' is deprecated}} expected-note@#deprecated-here {{here}}
+}
+
+void invalid_attributes() {
+  // pre2c-warning@+1 {{an attribute specifier sequence attached to a structured binding declaration is a C++2c extension}}
+  auto [a alignas(42) // expected-error {{'alignas' attribute only applies to variables, data members and tag types}}
+      [[assume(true), // expected-error {{'assume' attribute cannot be applied to a declaration}}
+        carries_dependency, // expected-error {{'carries_dependency' attribute only applies to parameters, Objective-C methods, and functions}}
+        fallthrough,  // expected-error {{'fallthrough' attribute cannot be applied to a declaration}}
+        likely, // expected-error {{'likely' attribute cannot be applied to a declaration}}
+        unlikely, // expected-error {{'unlikely' attribute cannot be applied to a declaration}}
+        nodiscard,  // expected-warning {{'nodiscard' attribute only applies to Objective-C methods, enums, structs, unions, classes, functions, function pointers, and typedefs}}
+        noreturn,  // expected-error {{'noreturn' attribute only applies to functions}}
+        no_unique_address]], // expected-error {{'no_unique_address' attribute only applies to non-bit-field non-static data members}}
+    b] = S{0};
+}
+
+}

clang/test/SemaCXX/unused.cpp

@@ -102,11 +102,21 @@ namespace PR33839 {
     for (auto [x] : a) { // expected-warning {{unused variable '[x]'}}
     }
   }
-  void use() { 
+  void use() {
     f<int>(); // expected-note {{instantiation of}}
     g<true>();
     g<false>();
     h<int>(); // expected-note {{instantiation of}}
   }
 }
+
+namespace maybe_unused_binding {
+
+void test() {
+  struct X { int a, b; } x;
+  auto [a [[maybe_unused]], b] = x; // expected-warning {{an attribute specifier sequence attached to a structured binding declaration is a C++2c extension}}
+}
+
+}
+
 #endif

clang/www/cxx_status.html

@@ -187,7 +187,7 @@ <h2 id="cxx26">C++2c implementation status</h2>
  <tr>
   <td>Trivial infinite loops are not Undefined Behavior</td>
   <td><a href="https://wg21.link/P2809R3">P2809R3</a> (<a href="#dr">DR</a>)</td>
-  <td class="none" align="center">No</td>
+  <td class="unreleased" align="center">Clang 19</td>
  </tr>
  <tr>
   <td>Erroneous behaviour for uninitialized reads</td>