[Clang] Implement CWG 2707 "Deduction guides cannot have a trailing requires-clause" (#110473)

Closes #98595

Author: zyn0217

clang/docs/ReleaseNotes.rst

@@ -205,6 +205,9 @@ Resolutions to C++ Defect Reports
 - Reject explicit object parameters with type ``void`` (``this void``).
   (`CWG2915: Explicit object parameters of type void <https://cplusplus.github.io/CWG/issues/2915.html>`_).
 
+- Clang now allows trailing requires clause on explicit deduction guides.
+  (`CWG2707: Deduction guides cannot have a trailing requires-clause <https://cplusplus.github.io/CWG/issues/2707.html>`_).
+
 C Language Changes
 ------------------
 

clang/include/clang/AST/DeclCXX.h

@@ -1965,9 +1965,11 @@ class CXXDeductionGuideDecl : public FunctionDecl {
                         ExplicitSpecifier ES,
                         const DeclarationNameInfo &NameInfo, QualType T,
                         TypeSourceInfo *TInfo, SourceLocation EndLocation,
-                        CXXConstructorDecl *Ctor, DeductionCandidate Kind)
+                        CXXConstructorDecl *Ctor, DeductionCandidate Kind,
+                        Expr *TrailingRequiresClause)
       : FunctionDecl(CXXDeductionGuide, C, DC, StartLoc, NameInfo, T, TInfo,
-                     SC_None, false, false, ConstexprSpecKind::Unspecified),
+                     SC_None, false, false, ConstexprSpecKind::Unspecified,
+                     TrailingRequiresClause),
         Ctor(Ctor), ExplicitSpec(ES) {
     if (EndLocation.isValid())
       setRangeEnd(EndLocation);
@@ -1987,7 +1989,8 @@ class CXXDeductionGuideDecl : public FunctionDecl {
          ExplicitSpecifier ES, const DeclarationNameInfo &NameInfo, QualType T,
          TypeSourceInfo *TInfo, SourceLocation EndLocation,
          CXXConstructorDecl *Ctor = nullptr,
-         DeductionCandidate Kind = DeductionCandidate::Normal);
+         DeductionCandidate Kind = DeductionCandidate::Normal,
+         Expr *TrailingRequiresClause = nullptr);
 
   static CXXDeductionGuideDecl *CreateDeserialized(ASTContext &C,
                                                    GlobalDeclID ID);

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -3050,8 +3050,6 @@ def note_is_deducible_constraint_evaluated_to_false : Note<
   "cannot deduce template arguments for %0 from %1">;
 def err_constrained_virtual_method : Error<
   "virtual function cannot have a requires clause">;
-def err_trailing_requires_clause_on_deduction_guide : Error<
-  "deduction guide cannot have a requires clause">;
 def err_constrained_non_templated_function
     : Error<"non-templated function cannot have a requires clause">;
 def err_non_temp_spec_requires_clause : Error<

clang/lib/AST/DeclCXX.cpp

@@ -2211,17 +2211,18 @@ CXXDeductionGuideDecl *CXXDeductionGuideDecl::Create(
     ASTContext &C, DeclContext *DC, SourceLocation StartLoc,
     ExplicitSpecifier ES, const DeclarationNameInfo &NameInfo, QualType T,
     TypeSourceInfo *TInfo, SourceLocation EndLocation, CXXConstructorDecl *Ctor,
-    DeductionCandidate Kind) {
-  return new (C, DC) CXXDeductionGuideDecl(C, DC, StartLoc, ES, NameInfo, T,
-                                           TInfo, EndLocation, Ctor, Kind);
+    DeductionCandidate Kind, Expr *TrailingRequiresClause) {
+  return new (C, DC)
+      CXXDeductionGuideDecl(C, DC, StartLoc, ES, NameInfo, T, TInfo,
+                            EndLocation, Ctor, Kind, TrailingRequiresClause);
 }
 
 CXXDeductionGuideDecl *
 CXXDeductionGuideDecl::CreateDeserialized(ASTContext &C, GlobalDeclID ID) {
   return new (C, ID) CXXDeductionGuideDecl(
       C, nullptr, SourceLocation(), ExplicitSpecifier(), DeclarationNameInfo(),
       QualType(), nullptr, SourceLocation(), nullptr,
-      DeductionCandidate::Normal);
+      DeductionCandidate::Normal, nullptr);
 }
 
 RequiresExprBodyDecl *RequiresExprBodyDecl::Create(

clang/lib/Sema/SemaDecl.cpp

@@ -9293,15 +9293,12 @@ static FunctionDecl *CreateNewFunctionDecl(Sema &SemaRef, Declarator &D,
         TrailingRequiresClause);
 
   } else if (Name.getNameKind() == DeclarationName::CXXDeductionGuideName) {
-    if (TrailingRequiresClause)
-      SemaRef.Diag(TrailingRequiresClause->getBeginLoc(),
-                   diag::err_trailing_requires_clause_on_deduction_guide)
-          << TrailingRequiresClause->getSourceRange();
     if (SemaRef.CheckDeductionGuideDeclarator(D, R, SC))
       return nullptr;
-    return CXXDeductionGuideDecl::Create(SemaRef.Context, DC, D.getBeginLoc(),
-                                         ExplicitSpecifier, NameInfo, R, TInfo,
-                                         D.getEndLoc());
+    return CXXDeductionGuideDecl::Create(
+        SemaRef.Context, DC, D.getBeginLoc(), ExplicitSpecifier, NameInfo, R,
+        TInfo, D.getEndLoc(), /*Ctor=*/nullptr,
+        /*Kind=*/DeductionCandidate::Normal, TrailingRequiresClause);
   } else if (DC->isRecord()) {
     // If the name of the function is the same as the name of the record,
     // then this must be an invalid constructor that has a return type.

clang/lib/Sema/SemaTemplateInstantiateDecl.cpp

@@ -2233,7 +2233,7 @@ Decl *TemplateDeclInstantiator::VisitFunctionDecl(
         SemaRef.Context, DC, D->getInnerLocStart(),
         InstantiatedExplicitSpecifier, NameInfo, T, TInfo,
         D->getSourceRange().getEnd(), DGuide->getCorrespondingConstructor(),
-        DGuide->getDeductionCandidateKind());
+        DGuide->getDeductionCandidateKind(), TrailingRequiresClause);
     Function->setAccess(D->getAccess());
   } else {
     Function = FunctionDecl::Create(

clang/test/CXX/dcl/dcl.decl/p3.cpp

@@ -65,4 +65,4 @@ struct R {
 };
 
 template<typename T>
-R(T) -> R<T> requires true; // expected-error{{deduction guide cannot have a requires clause}}
+R(T) -> R<T> requires true;

clang/test/CXX/drs/cwg27xx.cpp

@@ -201,3 +201,31 @@ static_assert(false, f().s);
 #endif
 } // namespace cwg2798
 
+namespace cwg2707 { // cwg2707: 20
+
+#if __cplusplus >= 202002L
+
+template <class T, unsigned N> struct A {
+  T value[N];
+};
+
+template <typename... T>
+A(T...) -> A<int, sizeof...(T)> requires (sizeof...(T) == 2);
+
+// Brace elision is not allowed for synthesized CTAD guides if the array size
+// is value-dependent.
+// So this should pick up our explicit deduction guide.
+A a = {1, 2};
+
+A b = {3, 4, 5};
+// expected-error@-1 {{no viable constructor or deduction guide}} \
+// expected-note@-13 {{candidate function template not viable}} \
+// expected-note@-13 {{implicit deduction guide}} \
+// expected-note@-8 {{constraints not satisfied}} \
+// expected-note@-8 {{because 'sizeof...(T) == 2' (3 == 2) evaluated to false}} \
+// expected-note@-13 {{candidate function template not viable}} \
+// expected-note@-13 {{implicit deduction guide}}
+
+#endif
+
+} // namespace cwg2707

clang/www/cxx_dr_status.html

@@ -16089,7 +16089,7 @@ <h2 id="cxxdr">C++ defect report implementation status</h2>
     <td><a href="https://cplusplus.github.io/CWG/issues/2707.html">2707</a></td>
     <td>DRWP</td>
     <td>Deduction guides cannot have a trailing <I>requires-clause</I></td>
-    <td class="unknown" align="center">Unknown</td>
+    <td class="unreleased" align="center">Clang 20</td>
   </tr>
   <tr id="2708">
     <td><a href="https://cplusplus.github.io/CWG/issues/2708.html">2708</a></td>
@@ -17334,7 +17334,7 @@ <h2 id="cxxdr">C++ defect report implementation status</h2>
     <td><a href="https://cplusplus.github.io/CWG/issues/2913.html">2913</a></td>
     <td>tentatively ready</td>
     <td>Grammar for <I>deduction-guide</I> has <I>requires-clause</I> in the wrong position</td>
-    <td align="center">Not resolved</td>
+    <td class="unreleased" align="center">Clang 20</td>
   </tr>
   <tr class="open" id="2914">
     <td><a href="https://cplusplus.github.io/CWG/issues/2914.html">2914</a></td>