[Clang][C++23] Implement P2448R2: Relaxing some constexpr restrictions (#77753)

Per
https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2022/p2448r2.html
function/constructor/destructor can be marked `constexpr` even though it
never produces a constant expression.
Non-literal types as return types and parameter types of functions
marked `constexpr` are also allowed.
Since this is not a DR, the diagnostic messages are still preserved for
C++ standards older than C++23.

Author: Fznamznon

clang/docs/ReleaseNotes.rst

@@ -97,6 +97,8 @@ C++23 Feature Support
 - Implemented `P2718R0: Lifetime extension in range-based for loops <https://wg21.link/P2718R0>`_. Also
   materialize temporary object which is a prvalue in discarded-value expression.
 
+- Implemented `P2448R2: Relaxing some constexpr restrictions <https://wg21.link/P2448R2>`_.
+
 C++2c Feature Support
 ^^^^^^^^^^^^^^^^^^^^^
 

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -9607,13 +9607,10 @@ def err_defaulted_copy_assign_not_ref : Error<
   "the parameter for an explicitly-defaulted copy assignment operator must be an "
   "lvalue reference type">;
 def err_incorrect_defaulted_constexpr : Error<
-  "defaulted definition of %sub{select_special_member_kind}0 "
-  "is not constexpr">;
+  "defaulted definition of %sub{select_special_member_kind}0 cannot be marked %select{constexpr|consteval}1 "
+  "before C++23">;
 def err_incorrect_defaulted_constexpr_with_vb: Error<
   "%sub{select_special_member_kind}0 cannot be 'constexpr' in a class with virtual base class">;
-def err_incorrect_defaulted_consteval : Error<
-  "defaulted declaration of %sub{select_special_member_kind}0 "
-  "cannot be consteval because implicit definition is not constexpr">;
 def warn_defaulted_method_deleted : Warning<
   "explicitly defaulted %sub{select_special_member_kind}0 is implicitly "
   "deleted">, InGroup<DefaultedFunctionDeleted>;
@@ -9724,21 +9721,12 @@ def note_defaulted_comparison_cannot_deduce_undeduced_auto : Note<
   "%select{|member|base class}0 %1 declared here">;
 def note_defaulted_comparison_cannot_deduce_callee : Note<
   "selected 'operator<=>' for %select{|member|base class}0 %1 declared here">;
-def ext_defaulted_comparison_constexpr_mismatch : Extension<
+def err_defaulted_comparison_constexpr_mismatch : Error<
   "defaulted definition of %select{%sub{select_defaulted_comparison_kind}1|"
-  "three-way comparison operator}0 that is "
-  "declared %select{constexpr|consteval}2 but"
-  "%select{|for which the corresponding implicit 'operator==' }0 "
-  "invokes a non-constexpr comparison function is a C++23 extension">,
-  InGroup<DiagGroup<"c++23-default-comp-relaxed-constexpr">>;
-def warn_cxx23_compat_defaulted_comparison_constexpr_mismatch : Warning<
-  "defaulted definition of %select{%sub{select_defaulted_comparison_kind}1|"
-  "three-way comparison operator}0 that is "
-  "declared %select{constexpr|consteval}2 but"
-  "%select{|for which the corresponding implicit 'operator==' }0 "
-  "invokes a non-constexpr comparison function is incompatible with C++ "
-  "standards before C++23">,
-  InGroup<CXXPre23Compat>, DefaultIgnore;
+  "three-way comparison operator}0 cannot be "
+  "declared %select{constexpr|consteval}2 because "
+  "%select{it|for which the corresponding implicit 'operator==' }0 "
+  "invokes a non-constexpr comparison function ">;
 def note_defaulted_comparison_not_constexpr : Note<
   "non-constexpr comparison function would be used to compare "
   "%select{|member %1|base class %1}0">;

clang/lib/AST/DeclCXX.cpp

@@ -400,10 +400,11 @@ CXXRecordDecl::setBases(CXXBaseSpecifier const * const *Bases,
 
       // C++11 [class.ctor]p6:
       //   If that user-written default constructor would satisfy the
-      //   requirements of a constexpr constructor, the implicitly-defined
-      //   default constructor is constexpr.
+      //   requirements of a constexpr constructor/function(C++23), the
+      //   implicitly-defined default constructor is constexpr.
       if (!BaseClassDecl->hasConstexprDefaultConstructor())
-        data().DefaultedDefaultConstructorIsConstexpr = false;
+        data().DefaultedDefaultConstructorIsConstexpr =
+            C.getLangOpts().CPlusPlus23;
 
       // C++1z [class.copy]p8:
       //   The implicitly-declared copy constructor for a class X will have
@@ -548,7 +549,8 @@ void CXXRecordDecl::addedClassSubobject(CXXRecordDecl *Subobj) {
   //   -- for every subobject of class type or (possibly multi-dimensional)
   //      array thereof, that class type shall have a constexpr destructor
   if (!Subobj->hasConstexprDestructor())
-    data().DefaultedDestructorIsConstexpr = false;
+    data().DefaultedDestructorIsConstexpr =
+        getASTContext().getLangOpts().CPlusPlus23;
 
   // C++20 [temp.param]p7:
   //   A structural type is [...] a literal class type [for which] the types
@@ -1297,7 +1299,8 @@ void CXXRecordDecl::addedMember(Decl *D) {
             !FieldRec->hasConstexprDefaultConstructor() && !isUnion())
           // The standard requires any in-class initializer to be a constant
           // expression. We consider this to be a defect.
-          data().DefaultedDefaultConstructorIsConstexpr = false;
+          data().DefaultedDefaultConstructorIsConstexpr =
+              Context.getLangOpts().CPlusPlus23;
 
         // C++11 [class.copy]p8:
         //   The implicitly-declared copy constructor for a class X will have

clang/lib/Sema/SemaDeclCXX.cpp

@@ -1715,6 +1715,8 @@ static bool CheckLiteralType(Sema &SemaRef, Sema::CheckConstexprKind Kind,
 static bool CheckConstexprDestructorSubobjects(Sema &SemaRef,
                                                const CXXDestructorDecl *DD,
                                                Sema::CheckConstexprKind Kind) {
+  assert(!SemaRef.getLangOpts().CPlusPlus23 &&
+         "this check is obsolete for C++23");
   auto Check = [&](SourceLocation Loc, QualType T, const FieldDecl *FD) {
     const CXXRecordDecl *RD =
         T->getBaseElementTypeUnsafe()->getAsCXXRecordDecl();
@@ -1746,6 +1748,8 @@ static bool CheckConstexprDestructorSubobjects(Sema &SemaRef,
 static bool CheckConstexprParameterTypes(Sema &SemaRef,
                                          const FunctionDecl *FD,
                                          Sema::CheckConstexprKind Kind) {
+  assert(!SemaRef.getLangOpts().CPlusPlus23 &&
+         "this check is obsolete for C++23");
   unsigned ArgIndex = 0;
   const auto *FT = FD->getType()->castAs<FunctionProtoType>();
   for (FunctionProtoType::param_type_iterator i = FT->param_type_begin(),
@@ -1767,6 +1771,8 @@ static bool CheckConstexprParameterTypes(Sema &SemaRef,
 /// true. If not, produce a suitable diagnostic and return false.
 static bool CheckConstexprReturnType(Sema &SemaRef, const FunctionDecl *FD,
                                      Sema::CheckConstexprKind Kind) {
+  assert(!SemaRef.getLangOpts().CPlusPlus23 &&
+         "this check is obsolete for C++23");
   if (CheckLiteralType(SemaRef, Kind, FD->getLocation(), FD->getReturnType(),
                        diag::err_constexpr_non_literal_return,
                        FD->isConsteval()))
@@ -1856,25 +1862,28 @@ bool Sema::CheckConstexprFunctionDefinition(const FunctionDecl *NewFD,
       }
     }
 
-    // - its return type shall be a literal type;
-    if (!CheckConstexprReturnType(*this, NewFD, Kind))
+    // - its return type shall be a literal type; (removed in C++23)
+    if (!getLangOpts().CPlusPlus23 &&
+        !CheckConstexprReturnType(*this, NewFD, Kind))
       return false;
   }
 
   if (auto *Dtor = dyn_cast<CXXDestructorDecl>(NewFD)) {
     // A destructor can be constexpr only if the defaulted destructor could be;
     // we don't need to check the members and bases if we already know they all
-    // have constexpr destructors.
-    if (!Dtor->getParent()->defaultedDestructorIsConstexpr()) {
+    // have constexpr destructors. (removed in C++23)
+    if (!getLangOpts().CPlusPlus23 &&
+        !Dtor->getParent()->defaultedDestructorIsConstexpr()) {
       if (Kind == CheckConstexprKind::CheckValid)
         return false;
       if (!CheckConstexprDestructorSubobjects(*this, Dtor, Kind))
         return false;
     }
   }
 
-  // - each of its parameter types shall be a literal type;
-  if (!CheckConstexprParameterTypes(*this, NewFD, Kind))
+  // - each of its parameter types shall be a literal type; (removed in C++23)
+  if (!getLangOpts().CPlusPlus23 &&
+      !CheckConstexprParameterTypes(*this, NewFD, Kind))
     return false;
 
   Stmt *Body = NewFD->getBody();
@@ -2457,7 +2466,8 @@ static bool CheckConstexprFunctionBody(Sema &SemaRef, const FunctionDecl *Dcl,
   // function", so is not checked in CheckValid mode.
   SmallVector<PartialDiagnosticAt, 8> Diags;
   if (Kind == Sema::CheckConstexprKind::Diagnose &&
-      !Expr::isPotentialConstantExpr(Dcl, Diags)) {
+      !Expr::isPotentialConstantExpr(Dcl, Diags) &&
+      !SemaRef.getLangOpts().CPlusPlus23) {
     SemaRef.Diag(Dcl->getLocation(),
                  diag::ext_constexpr_function_never_constant_expr)
         << isa<CXXConstructorDecl>(Dcl) << Dcl->isConsteval()
@@ -7535,21 +7545,23 @@ static bool defaultedSpecialMemberIsConstexpr(
 
   // C++1y [class.copy]p26:
   //   -- [the class] is a literal type, and
-  if (!Ctor && !ClassDecl->isLiteral())
+  if (!Ctor && !ClassDecl->isLiteral() && !S.getLangOpts().CPlusPlus23)
     return false;
 
   //   -- every constructor involved in initializing [...] base class
   //      sub-objects shall be a constexpr constructor;
   //   -- the assignment operator selected to copy/move each direct base
   //      class is a constexpr function, and
-  for (const auto &B : ClassDecl->bases()) {
-    const RecordType *BaseType = B.getType()->getAs<RecordType>();
-    if (!BaseType)
-      continue;
-    CXXRecordDecl *BaseClassDecl = cast<CXXRecordDecl>(BaseType->getDecl());
-    if (!specialMemberIsConstexpr(S, BaseClassDecl, CSM, 0, ConstArg,
-                                  InheritedCtor, Inherited))
-      return false;
+  if (!S.getLangOpts().CPlusPlus23) {
+    for (const auto &B : ClassDecl->bases()) {
+      const RecordType *BaseType = B.getType()->getAs<RecordType>();
+      if (!BaseType)
+        continue;
+      CXXRecordDecl *BaseClassDecl = cast<CXXRecordDecl>(BaseType->getDecl());
+      if (!specialMemberIsConstexpr(S, BaseClassDecl, CSM, 0, ConstArg,
+                                    InheritedCtor, Inherited))
+        return false;
+    }
   }
 
   //   -- every constructor involved in initializing non-static data members
@@ -7559,20 +7571,22 @@ static bool defaultedSpecialMemberIsConstexpr(
   //   -- for each non-static data member of X that is of class type (or array
   //      thereof), the assignment operator selected to copy/move that member is
   //      a constexpr function
-  for (const auto *F : ClassDecl->fields()) {
-    if (F->isInvalidDecl())
-      continue;
-    if (CSM == Sema::CXXDefaultConstructor && F->hasInClassInitializer())
-      continue;
-    QualType BaseType = S.Context.getBaseElementType(F->getType());
-    if (const RecordType *RecordTy = BaseType->getAs<RecordType>()) {
-      CXXRecordDecl *FieldRecDecl = cast<CXXRecordDecl>(RecordTy->getDecl());
-      if (!specialMemberIsConstexpr(S, FieldRecDecl, CSM,
-                                    BaseType.getCVRQualifiers(),
-                                    ConstArg && !F->isMutable()))
+  if (!S.getLangOpts().CPlusPlus23) {
+    for (const auto *F : ClassDecl->fields()) {
+      if (F->isInvalidDecl())
+        continue;
+      if (CSM == Sema::CXXDefaultConstructor && F->hasInClassInitializer())
+        continue;
+      QualType BaseType = S.Context.getBaseElementType(F->getType());
+      if (const RecordType *RecordTy = BaseType->getAs<RecordType>()) {
+        CXXRecordDecl *FieldRecDecl = cast<CXXRecordDecl>(RecordTy->getDecl());
+        if (!specialMemberIsConstexpr(S, FieldRecDecl, CSM,
+                                      BaseType.getCVRQualifiers(),
+                                      ConstArg && !F->isMutable()))
+          return false;
+      } else if (CSM == Sema::CXXDefaultConstructor) {
         return false;
-    } else if (CSM == Sema::CXXDefaultConstructor) {
-      return false;
+      }
     }
   }
 
@@ -7858,18 +7872,17 @@ bool Sema::CheckExplicitlyDefaultedSpecialMember(CXXMethodDecl *MD,
       MD->isConstexpr() && !Constexpr &&
       MD->getTemplatedKind() == FunctionDecl::TK_NonTemplate) {
         if (!MD->isConsteval() && RD->getNumVBases()) {
-          Diag(MD->getBeginLoc(), diag::err_incorrect_defaulted_constexpr_with_vb)
+          Diag(MD->getBeginLoc(),
+               diag::err_incorrect_defaulted_constexpr_with_vb)
               << CSM;
           for (const auto &I : RD->vbases())
             Diag(I.getBeginLoc(), diag::note_constexpr_virtual_base_here);
         } else {
-          Diag(MD->getBeginLoc(), MD->isConsteval()
-                                      ? diag::err_incorrect_defaulted_consteval
-                                      : diag::err_incorrect_defaulted_constexpr)
-              << CSM;
+          Diag(MD->getBeginLoc(), diag::err_incorrect_defaulted_constexpr)
+              << CSM << MD->isConsteval();
         }
-    // FIXME: Explain why the special member can't be constexpr.
-    HadError = true;
+        HadError = true;
+        // FIXME: Explain why the special member can't be constexpr.
   }
 
   if (First) {
@@ -9101,13 +9114,11 @@ bool Sema::CheckExplicitlyDefaultedComparison(Scope *S, FunctionDecl *FD,
   //    - if the function is a constructor or destructor, its class does not
   //      have any virtual base classes.
   if (FD->isConstexpr()) {
-    if (CheckConstexprReturnType(*this, FD, CheckConstexprKind::Diagnose) &&
+    if (!getLangOpts().CPlusPlus23 &&
+        CheckConstexprReturnType(*this, FD, CheckConstexprKind::Diagnose) &&
         CheckConstexprParameterTypes(*this, FD, CheckConstexprKind::Diagnose) &&
         !Info.Constexpr) {
-      Diag(FD->getBeginLoc(),
-           getLangOpts().CPlusPlus23
-               ? diag::warn_cxx23_compat_defaulted_comparison_constexpr_mismatch
-               : diag::ext_defaulted_comparison_constexpr_mismatch)
+      Diag(FD->getBeginLoc(), diag::err_defaulted_comparison_constexpr_mismatch)
           << FD->isImplicit() << (int)DCK << FD->isConsteval();
       DefaultedComparisonAnalyzer(*this, RD, FD, DCK,
                                   DefaultedComparisonAnalyzer::ExplainConstexpr)