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[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).
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‎clang/docs/ReleaseNotes.rst‎

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

Lines changed: 4 additions & 1 deletion
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@@ -2022,6 +2022,8 @@ class Sema final : public SemaBase {
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void CheckTCBEnforcement(const SourceLocation CallExprLoc,
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const NamedDecl *Callee);
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void CheckConstrainedAuto(const AutoType *AutoT, SourceLocation Loc);
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private:
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void CheckArrayAccess(const Expr *BaseExpr, const Expr *IndexExpr,
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const ArraySubscriptExpr *ASE = nullptr,
@@ -9356,7 +9358,8 @@ class Sema final : public SemaBase {
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Decl *ActOnConceptDefinition(Scope *S,
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MultiTemplateParamsArg TemplateParameterLists,
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const IdentifierInfo *Name,
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SourceLocation NameLoc, Expr *ConstraintExpr);
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SourceLocation NameLoc, Expr *ConstraintExpr,
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const ParsedAttributesView &Attrs);
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void CheckConceptRedefinition(ConceptDecl *NewDecl, LookupResult &Previous,
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bool &AddToScope);

‎clang/lib/Parse/ParseTemplate.cpp‎

Lines changed: 11 additions & 6 deletions
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@@ -167,9 +167,13 @@ Parser::DeclGroupPtrTy Parser::ParseTemplateDeclarationOrSpecialization(
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LastParamListWasEmpty);
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// Parse the actual template declaration.
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if (Tok.is(tok::kw_concept))
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return Actions.ConvertDeclToDeclGroup(
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ParseConceptDefinition(TemplateInfo, DeclEnd));
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if (Tok.is(tok::kw_concept)) {
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Decl *ConceptDecl = ParseConceptDefinition(TemplateInfo, DeclEnd);
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// We need to explicitly pass ConceptDecl to ParsingDeclRAIIObject, so that
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// delayed diagnostics (e.g. warn_deprecated) have a Decl to work with.
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ParsingTemplateParams.complete(ConceptDecl);
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return Actions.ConvertDeclToDeclGroup(ConceptDecl);
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}
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return ParseDeclarationAfterTemplate(
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Context, TemplateInfo, ParsingTemplateParams, DeclEnd, AccessAttrs, AS);
@@ -316,7 +320,8 @@ Parser::ParseConceptDefinition(const ParsedTemplateInfo &TemplateInfo,
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const IdentifierInfo *Id = Result.Identifier;
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SourceLocation IdLoc = Result.getBeginLoc();
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DiagnoseAndSkipCXX11Attributes();
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ParsedAttributes Attrs(AttrFactory);
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MaybeParseAttributes(PAKM_GNU | PAKM_CXX11, Attrs);
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321326
if (!TryConsumeToken(tok::equal)) {
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Diag(Tok.getLocation(), diag::err_expected) << tok::equal;
@@ -335,8 +340,8 @@ Parser::ParseConceptDefinition(const ParsedTemplateInfo &TemplateInfo,
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ExpectAndConsumeSemi(diag::err_expected_semi_declaration);
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Expr *ConstraintExpr = ConstraintExprResult.get();
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return Actions.ActOnConceptDefinition(getCurScope(),
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*TemplateInfo.TemplateParams,
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Id, IdLoc, ConstraintExpr);
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*TemplateInfo.TemplateParams, Id, IdLoc,
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ConstraintExpr, Attrs);
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}
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/// ParseTemplateParameters - Parses a template-parameter-list enclosed in

‎clang/lib/Sema/SemaChecking.cpp‎

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

‎clang/lib/Sema/SemaDecl.cpp‎

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

‎clang/lib/Sema/SemaTemplate.cpp‎

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@@ -5653,6 +5653,8 @@ Sema::CheckConceptTemplateId(const CXXScopeSpec &SS,
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/*UpdateArgsWithConversions=*/false))
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return ExprError();
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DiagnoseUseOfDecl(NamedConcept, ConceptNameInfo.getLoc());
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auto *CSD = ImplicitConceptSpecializationDecl::Create(
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Context, NamedConcept->getDeclContext(), NamedConcept->getLocation(),
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CanonicalConverted);
@@ -9859,7 +9861,8 @@ Decl *Sema::ActOnTemplateDeclarator(Scope *S,
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Decl *Sema::ActOnConceptDefinition(
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Scope *S, MultiTemplateParamsArg TemplateParameterLists,
9862-
const IdentifierInfo *Name, SourceLocation NameLoc, Expr *ConstraintExpr) {
9864+
const IdentifierInfo *Name, SourceLocation NameLoc, Expr *ConstraintExpr,
9865+
const ParsedAttributesView &Attrs) {
98639866
DeclContext *DC = CurContext;
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98659868
if (!DC->getRedeclContext()->isFileContext()) {
@@ -9921,6 +9924,9 @@ Decl *Sema::ActOnConceptDefinition(
99219924
ActOnDocumentableDecl(NewDecl);
99229925
if (AddToScope)
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PushOnScopeChains(NewDecl, S);
9927+
9928+
ProcessDeclAttributeList(S, NewDecl, Attrs);
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return NewDecl;
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}
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‎clang/lib/Sema/SemaType.cpp‎

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@@ -6392,6 +6392,11 @@ TypeResult Sema::ActOnTypeName(Declarator &D) {
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CheckExtraCXXDefaultArguments(D);
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}
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if (const AutoType *AutoT = T->getAs<AutoType>())
6396+
CheckConstrainedAuto(
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AutoT,
6398+
TInfo->getTypeLoc().getContainedAutoTypeLoc().getConceptNameLoc());
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63956400
return CreateParsedType(T, TInfo);
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}
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‎clang/test/CXX/drs/cwg24xx.cpp‎

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@@ -45,6 +45,48 @@ void fallthrough(int n) {
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#endif
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}
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namespace cwg2428 { // cwg2428: 19
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#if __cplusplus >= 202002L
50+
template <typename>
51+
concept C [[deprecated]] = true; // #cwg2428-C
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53+
template <typename>
54+
[[deprecated]] concept C2 = true;
55+
// expected-error@-1 {{expected unqualified-id}}
56+
57+
template <typename T>
58+
concept C3 = C<T>;
59+
// expected-warning@-1 {{'C' is deprecated}}
60+
// expected-note@#cwg2428-C {{'C' has been explicitly marked deprecated here}}
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62+
template <typename T, C U>
63+
// expected-warning@-1 {{'C' is deprecated}}
64+
// expected-note@#cwg2428-C {{'C' has been explicitly marked deprecated here}}
65+
requires C<T>
66+
// expected-warning@-1 {{'C' is deprecated}}
67+
// expected-note@#cwg2428-C {{'C' has been explicitly marked deprecated here}}
68+
void f() {
69+
bool b = C<int>;
70+
// expected-warning@-1 {{'C' is deprecated}}
71+
// expected-note@#cwg2428-C {{'C' has been explicitly marked deprecated here}}
72+
};
73+
74+
void g(C auto a) {};
75+
// expected-warning@-1 {{'C' is deprecated}}
76+
// expected-note@#cwg2428-C {{'C' has been explicitly marked deprecated here}}
77+
78+
template <typename T>
79+
auto h() -> C auto {
80+
// expected-warning@-1 {{'C' is deprecated}}
81+
// expected-note@#cwg2428-C {{'C' has been explicitly marked deprecated here}}
82+
C auto foo = T();
83+
// expected-warning@-1 {{'C' is deprecated}}
84+
// expected-note@#cwg2428-C {{'C' has been explicitly marked deprecated here}}
85+
return foo;
86+
}
87+
#endif
88+
} // namespace cwg2428
89+
4890
namespace cwg2430 { // cwg2430: 2.7
4991
struct S {
5092
S f(S s) { return s; }

‎clang/test/SemaCXX/cxx-deprecated.cpp‎

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@@ -1,4 +1,4 @@
1-
// RUN: %clang_cc1 -fsyntax-only -verify -std=c++1z %s
1+
// RUN: %clang_cc1 -fsyntax-only -verify -std=c++20 %s
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33
namespace [[deprecated]] {} // expected-warning {{'deprecated' attribute on anonymous namespace ignored}}
44

@@ -27,3 +27,13 @@ namespace M = N; // expected-warning {{'N' is deprecated}}
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2828
// Shouldn't diag:
2929
[[nodiscard, deprecated("")]] int PR37935();
30+
31+
namespace cxx20_concept {
32+
template <typename>
33+
concept C __attribute__((deprecated)) = true; // #C
34+
35+
template <C T>
36+
// expected-warning@-1 {{'C' is deprecated}}
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// expected-note@#C {{'C' has been explicitly marked deprecated here}}
38+
void f();
39+
} // namespace cxx20_concept

‎clang/www/cxx_dr_status.html‎

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@@ -14376,7 +14376,7 @@ <h2 id="cxxdr">C++ defect report implementation status</h2>
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<td><a href="https://cplusplus.github.io/CWG/issues/2428.html">2428</a></td>
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<td>C++23</td>
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<td>Deprecating a concept</td>
14379-
<td class="unknown" align="center">Unknown</td>
14379+
<td class="unreleased" align="center">Clang 19</td>
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</tr>
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<tr id="2429">
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<td><a href="https://cplusplus.github.io/CWG/issues/2429.html">2429</a></td>

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