[clang] Implement CWG2428 "Deprecating a concept" (#92295)

This patch allows attributes to be attached to C++20 concepts,
implementing
[CWG2428](https://cplusplus.github.io/CWG/issues/2428.html).

clang/docs/ReleaseNotes.rst

@@ -226,6 +226,9 @@ Resolutions to C++ Defect Reports
 - Clang now diagnoses declarative nested-name-specifiers with pack-index-specifiers.
   (`CWG2858: Declarative nested-name-specifiers and pack-index-specifiers <https://cplusplus.github.io/CWG/issues/2858.html>`_).
 
+- Clang now allows attributes on concepts.
+  (`CWG2428: Deprecating a concept <https://cplusplus.github.io/CWG/issues/2428.html>`_).
+
 - P0522 implementation is enabled by default in all language versions, and
   provisional wording for CWG2398 is implemented.
 

clang/include/clang/Sema/Sema.h

@@ -2022,6 +2022,8 @@ public:
   void CheckTCBEnforcement(const SourceLocation CallExprLoc,
                            const NamedDecl *Callee);
 
+  void CheckConstrainedAuto(const AutoType *AutoT, SourceLocation Loc);
+
 private:
   void CheckArrayAccess(const Expr *BaseExpr, const Expr *IndexExpr,
                         const ArraySubscriptExpr *ASE = nullptr,
@@ -9356,7 +9358,8 @@ public:
   Decl *ActOnConceptDefinition(Scope *S,
                                MultiTemplateParamsArg TemplateParameterLists,
                                const IdentifierInfo *Name,
-                               SourceLocation NameLoc, Expr *ConstraintExpr);
+                               SourceLocation NameLoc, Expr *ConstraintExpr,
+                               const ParsedAttributesView &Attrs);
 
   void CheckConceptRedefinition(ConceptDecl *NewDecl, LookupResult &Previous,
                                 bool &AddToScope);

clang/lib/Parse/ParseTemplate.cpp

@@ -167,9 +167,13 @@ Parser::DeclGroupPtrTy Parser::ParseTemplateDeclarationOrSpecialization(
                                   LastParamListWasEmpty);
 
   // Parse the actual template declaration.
-  if (Tok.is(tok::kw_concept))
-    return Actions.ConvertDeclToDeclGroup(
-        ParseConceptDefinition(TemplateInfo, DeclEnd));
+  if (Tok.is(tok::kw_concept)) {
+    Decl *ConceptDecl = ParseConceptDefinition(TemplateInfo, DeclEnd);
+    // We need to explicitly pass ConceptDecl to ParsingDeclRAIIObject, so that
+    // delayed diagnostics (e.g. warn_deprecated) have a Decl to work with.
+    ParsingTemplateParams.complete(ConceptDecl);
+    return Actions.ConvertDeclToDeclGroup(ConceptDecl);
+  }
 
   return ParseDeclarationAfterTemplate(
       Context, TemplateInfo, ParsingTemplateParams, DeclEnd, AccessAttrs, AS);
@@ -316,7 +320,8 @@ Parser::ParseConceptDefinition(const ParsedTemplateInfo &TemplateInfo,
   const IdentifierInfo *Id = Result.Identifier;
   SourceLocation IdLoc = Result.getBeginLoc();
 
-  DiagnoseAndSkipCXX11Attributes();
+  ParsedAttributes Attrs(AttrFactory);
+  MaybeParseAttributes(PAKM_GNU | PAKM_CXX11, Attrs);
 
   if (!TryConsumeToken(tok::equal)) {
     Diag(Tok.getLocation(), diag::err_expected) << tok::equal;
@@ -335,8 +340,8 @@ Parser::ParseConceptDefinition(const ParsedTemplateInfo &TemplateInfo,
   ExpectAndConsumeSemi(diag::err_expected_semi_declaration);
   Expr *ConstraintExpr = ConstraintExprResult.get();
   return Actions.ActOnConceptDefinition(getCurScope(),
-                                        *TemplateInfo.TemplateParams,
-                                        Id, IdLoc, ConstraintExpr);
+                                        *TemplateInfo.TemplateParams, Id, IdLoc,
+                                        ConstraintExpr, Attrs);
 }
 
 /// ParseTemplateParameters - Parses a template-parameter-list enclosed in

clang/lib/Sema/SemaChecking.cpp

@@ -8092,6 +8092,12 @@ void Sema::checkCall(NamedDecl *FDecl, const FunctionProtoType *Proto,
     diagnoseArgDependentDiagnoseIfAttrs(FD, ThisArg, Args, Loc);
 }
 
+void Sema::CheckConstrainedAuto(const AutoType *AutoT, SourceLocation Loc) {
+  if (ConceptDecl *Decl = AutoT->getTypeConstraintConcept()) {
+    DiagnoseUseOfDecl(Decl, Loc);
+  }
+}
+
 /// CheckConstructorCall - Check a constructor call for correctness and safety
 /// properties not enforced by the C type system.
 void Sema::CheckConstructorCall(FunctionDecl *FDecl, QualType ThisType,

clang/lib/Sema/SemaDecl.cpp

@@ -7573,6 +7573,11 @@ NamedDecl *Sema::ActOnVariableDeclarator(
     tryToFixVariablyModifiedVarType(TInfo, R, D.getIdentifierLoc(),
                                     /*DiagID=*/0);
 
+  if (const AutoType *AutoT = R->getAs<AutoType>())
+    CheckConstrainedAuto(
+        AutoT,
+        TInfo->getTypeLoc().getContainedAutoTypeLoc().getConceptNameLoc());
+
   bool IsMemberSpecialization = false;
   bool IsVariableTemplateSpecialization = false;
   bool IsPartialSpecialization = false;

clang/lib/Sema/SemaTemplate.cpp

@@ -5653,6 +5653,8 @@ Sema::CheckConceptTemplateId(const CXXScopeSpec &SS,
           /*UpdateArgsWithConversions=*/false))
     return ExprError();
 
+  DiagnoseUseOfDecl(NamedConcept, ConceptNameInfo.getLoc());
+
   auto *CSD = ImplicitConceptSpecializationDecl::Create(
       Context, NamedConcept->getDeclContext(), NamedConcept->getLocation(),
       CanonicalConverted);
@@ -9859,7 +9861,8 @@ Decl *Sema::ActOnTemplateDeclarator(Scope *S,
 
 Decl *Sema::ActOnConceptDefinition(
     Scope *S, MultiTemplateParamsArg TemplateParameterLists,
-    const IdentifierInfo *Name, SourceLocation NameLoc, Expr *ConstraintExpr) {
+    const IdentifierInfo *Name, SourceLocation NameLoc, Expr *ConstraintExpr,
+    const ParsedAttributesView &Attrs) {
   DeclContext *DC = CurContext;
 
   if (!DC->getRedeclContext()->isFileContext()) {
@@ -9921,6 +9924,9 @@ Decl *Sema::ActOnConceptDefinition(
   ActOnDocumentableDecl(NewDecl);
   if (AddToScope)
     PushOnScopeChains(NewDecl, S);
+
+  ProcessDeclAttributeList(S, NewDecl, Attrs);
+
   return NewDecl;
 }
 

clang/lib/Sema/SemaType.cpp

@@ -6392,6 +6392,11 @@ TypeResult Sema::ActOnTypeName(Declarator &D) {
     CheckExtraCXXDefaultArguments(D);
   }
 
+  if (const AutoType *AutoT = T->getAs<AutoType>())
+    CheckConstrainedAuto(
+        AutoT,
+        TInfo->getTypeLoc().getContainedAutoTypeLoc().getConceptNameLoc());
+
   return CreateParsedType(T, TInfo);
 }
 

clang/test/CXX/drs/cwg24xx.cpp

@@ -45,6 +45,48 @@ void fallthrough(int n) {
 #endif
 }
 
+namespace cwg2428 { // cwg2428: 19
+#if __cplusplus >= 202002L
+template <typename>
+concept C [[deprecated]] = true; // #cwg2428-C
+
+template <typename>
+[[deprecated]] concept C2 = true;
+// expected-error@-1 {{expected unqualified-id}}
+
+template <typename T>
+concept C3 = C<T>;
+// expected-warning@-1 {{'C' is deprecated}}
+//   expected-note@#cwg2428-C {{'C' has been explicitly marked deprecated here}}
+
+template <typename T, C U>
+// expected-warning@-1 {{'C' is deprecated}}
+//   expected-note@#cwg2428-C {{'C' has been explicitly marked deprecated here}}
+requires C<T>
+// expected-warning@-1 {{'C' is deprecated}}
+//   expected-note@#cwg2428-C {{'C' has been explicitly marked deprecated here}}
+void f() {
+  bool b = C<int>;
+  // expected-warning@-1 {{'C' is deprecated}}
+  //   expected-note@#cwg2428-C {{'C' has been explicitly marked deprecated here}}
+};
+
+void g(C auto a) {};
+// expected-warning@-1 {{'C' is deprecated}}
+//   expected-note@#cwg2428-C {{'C' has been explicitly marked deprecated here}}
+
+template <typename T>
+auto h() -> C auto {
+// expected-warning@-1 {{'C' is deprecated}}
+//   expected-note@#cwg2428-C {{'C' has been explicitly marked deprecated here}}
+  C auto foo = T();
+  // expected-warning@-1 {{'C' is deprecated}}
+  //   expected-note@#cwg2428-C {{'C' has been explicitly marked deprecated here}}
+  return foo;
+}
+#endif
+} // namespace cwg2428
+
 namespace cwg2430 { // cwg2430: 2.7
 struct S {
   S f(S s) { return s; }

clang/test/SemaCXX/cxx-deprecated.cpp

@@ -1,4 +1,4 @@
-// RUN: %clang_cc1 -fsyntax-only -verify -std=c++1z %s
+// RUN: %clang_cc1 -fsyntax-only -verify -std=c++20 %s
 
 namespace [[deprecated]] {}  // expected-warning {{'deprecated' attribute on anonymous namespace ignored}}
 
@@ -27,3 +27,13 @@ namespace M = N; // expected-warning {{'N' is deprecated}}
 
 // Shouldn't diag:
 [[nodiscard, deprecated("")]] int PR37935();
+
+namespace cxx20_concept {
+template <typename>
+concept C __attribute__((deprecated)) = true; // #C
+
+template <C T>
+// expected-warning@-1 {{'C' is deprecated}}
+//   expected-note@#C {{'C' has been explicitly marked deprecated here}}
+void f();
+} // namespace cxx20_concept

clang/www/cxx_dr_status.html

@@ -14376,7 +14376,7 @@ and <I>POD class</I></td>
     <td><a href="https://cplusplus.github.io/CWG/issues/2428.html">2428</a></td>
     <td>C++23</td>
     <td>Deprecating a concept</td>
-    <td class="unknown" align="center">Unknown</td>
+    <td class="unreleased" align="center">Clang 19</td>
   </tr>
   <tr id="2429">
     <td><a href="https://cplusplus.github.io/CWG/issues/2429.html">2429</a></td>