[C23] Implement N3018: The constexpr specifier for object definitions

clang/docs/ReleaseNotes.rst

@@ -138,6 +138,9 @@ C23 Feature Support
   macros typically exposed from ``<inttypes.h>``, such as ``PRIb8``.
   (`#81896: <https://github.com/llvm/llvm-project/issues/81896>`_).
 
+- Clang now supports `N3018 The constexpr specifier for object definitions`
+  <https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3018.htm>`_.
+
 Non-comprehensive list of changes in this release
 -------------------------------------------------
 

clang/include/clang/AST/Expr.h

@@ -1875,6 +1875,17 @@ class StringLiteral final
     llvm_unreachable("Unsupported character width!");
   }
 
+  // Get code unit but preserve sign info.
+  int64_t getCodeUnitS(size_t I, uint64_t BitWidth) const {
+    int64_t V = getCodeUnit(I);
+    if (isOrdinary() || isWide()) {
+      unsigned Width = getCharByteWidth() * BitWidth;
+      llvm::APInt AInt(Width, (uint64_t)V);
+      V = AInt.getSExtValue();
+    }
+    return V;
+  }
+
   unsigned getByteLength() const { return getCharByteWidth() * getLength(); }
   unsigned getLength() const { return *getTrailingObjects<unsigned>(); }
   unsigned getCharByteWidth() const { return StringLiteralBits.CharByteWidth; }

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -2946,6 +2946,18 @@ def warn_private_extern : Warning<
 def note_private_extern : Note<
   "use __attribute__((visibility(\"hidden\"))) attribute instead">;
 
+// C23 constexpr
+def err_c23_constexpr_not_variable : Error<
+  "'constexpr' can only be used in variable declarations">;
+def err_c23_constexpr_invalid_type : Error<
+  "constexpr variable cannot have type %0">;
+def err_c23_constexpr_init_not_representable : Error<
+  "constexpr initializer evaluates to %0 which is not exactly representable in type %1">;
+def err_c23_constexpr_init_type_mismatch : Error<
+  "constexpr initializer for type %0 is of type %1">;
+def err_c23_constexpr_pointer_not_null : Error<
+  "constexpr pointer initializer is not null">;
+
 // C++ Concepts
 def err_concept_decls_may_only_appear_in_global_namespace_scope : Error<
   "concept declarations may only appear in global or namespace scope">;

clang/include/clang/Basic/TokenKinds.def

@@ -393,7 +393,7 @@ CXX11_KEYWORD(alignas               , KEYC23)
 CXX11_UNARY_EXPR_OR_TYPE_TRAIT(alignof, AlignOf, KEYC23)
 CXX11_KEYWORD(char16_t              , KEYNOMS18)
 CXX11_KEYWORD(char32_t              , KEYNOMS18)
-CXX11_KEYWORD(constexpr             , 0)
+CXX11_KEYWORD(constexpr             , KEYC23)
 CXX11_KEYWORD(decltype              , 0)
 CXX11_KEYWORD(noexcept              , 0)
 CXX11_KEYWORD(nullptr               , KEYC23)

clang/lib/AST/Decl.cpp

@@ -2465,7 +2465,7 @@ bool VarDecl::mightBeUsableInConstantExpressions(const ASTContext &C) const {
 
   // OpenCL permits const integral variables to be used in constant
   // expressions, like in C++98.
-  if (!Lang.CPlusPlus && !Lang.OpenCL)
+  if (!Lang.CPlusPlus && !Lang.OpenCL && !Lang.C23)
     return false;
 
   // Function parameters are never usable in constant expressions.
@@ -2487,14 +2487,19 @@ bool VarDecl::mightBeUsableInConstantExpressions(const ASTContext &C) const {
   if (!getType().isConstant(C) || getType().isVolatileQualified())
     return false;
 
-  // In C++, const, non-volatile variables of integral or enumeration types
-  // can be used in constant expressions.
-  if (getType()->isIntegralOrEnumerationType())
+  // In C++, but not in C, const, non-volatile variables of integral or
+  // enumeration types can be used in constant expressions.
+  if (getType()->isIntegralOrEnumerationType() && !Lang.C23)
     return true;
 
+  // C23 6.6p7: An identifier that is:
+  // ...
+  // - declared with storage-class specifier constexpr and has an object type,
+  // is a named constant, ... such a named constant is a constant expression
+  // with the type and value of the declared object.
   // Additionally, in C++11, non-volatile constexpr variables can be used in
   // constant expressions.
-  return Lang.CPlusPlus11 && isConstexpr();
+  return (Lang.CPlusPlus11 || Lang.C23) && isConstexpr();
 }
 
 bool VarDecl::isUsableInConstantExpressions(const ASTContext &Context) const {
@@ -2572,11 +2577,11 @@ APValue *VarDecl::evaluateValueImpl(SmallVectorImpl<PartialDiagnosticAt> &Notes,
   bool Result = Init->EvaluateAsInitializer(Eval->Evaluated, Ctx, this, Notes,
                                             IsConstantInitialization);
 
-  // In C++, this isn't a constant initializer if we produced notes. In that
+  // In C++/C23, this isn't a constant initializer if we produced notes. In that
   // case, we can't keep the result, because it may only be correct under the
   // assumption that the initializer is a constant context.
-  if (IsConstantInitialization && Ctx.getLangOpts().CPlusPlus &&
-      !Notes.empty())
+  if (IsConstantInitialization &&
+      (Ctx.getLangOpts().CPlusPlus || Ctx.getLangOpts().C23) && !Notes.empty())
     Result = false;
 
   // Ensure the computed APValue is cleaned up later if evaluation succeeded,
@@ -2634,7 +2639,9 @@ bool VarDecl::checkForConstantInitialization(
   // std::is_constant_evaluated()).
   assert(!Eval->WasEvaluated &&
          "already evaluated var value before checking for constant init");
-  assert(getASTContext().getLangOpts().CPlusPlus && "only meaningful in C++");
+  assert((getASTContext().getLangOpts().CPlusPlus ||
+          getASTContext().getLangOpts().C23) &&
+         "only meaningful in C++/C23");
 
   assert(!getInit()->isValueDependent());
 

clang/lib/AST/ExprConstant.cpp

@@ -4131,6 +4131,10 @@ static CompleteObject findCompleteObject(EvalInfo &Info, const Expr *E,
     }
 
     bool IsConstant = BaseType.isConstant(Info.Ctx);
+    bool ConstexprVar = false;
+    if (const auto *VD = dyn_cast_if_present<VarDecl>(
+            Info.EvaluatingDecl.dyn_cast<const ValueDecl *>()))
+      ConstexprVar = VD->isConstexpr();
 
     // Unless we're looking at a local variable or argument in a constexpr call,
     // the variable we're reading must be const.
@@ -4150,6 +4154,9 @@ static CompleteObject findCompleteObject(EvalInfo &Info, const Expr *E,
         return CompleteObject();
       } else if (VD->isConstexpr()) {
         // OK, we can read this variable.
+      } else if (Info.getLangOpts().C23 && ConstexprVar) {
+        Info.FFDiag(E);
+        return CompleteObject();
       } else if (BaseType->isIntegralOrEnumerationType()) {
         if (!IsConstant) {
           if (!IsAccess)
@@ -15822,7 +15829,8 @@ bool Expr::EvaluateAsInitializer(APValue &Value, const ASTContext &Ctx,
   EStatus.Diag = &Notes;
 
   EvalInfo Info(Ctx, EStatus,
-                (IsConstantInitialization && Ctx.getLangOpts().CPlusPlus)
+                (IsConstantInitialization &&
+                 (Ctx.getLangOpts().CPlusPlus || Ctx.getLangOpts().C23))
                     ? EvalInfo::EM_ConstantExpression
                     : EvalInfo::EM_ConstantFold);
   Info.setEvaluatingDecl(VD, Value);

clang/lib/Parse/ParseDecl.cpp

@@ -4265,6 +4265,8 @@ void Parser::ParseDeclarationSpecifiers(
 
     // constexpr, consteval, constinit specifiers
     case tok::kw_constexpr:
+      if (getLangOpts().C23)
+        Diag(Tok, diag::warn_c23_compat_keyword) << Tok.getName();
       isInvalid = DS.SetConstexprSpec(ConstexprSpecKind::Constexpr, Loc,
                                       PrevSpec, DiagID);
       break;

clang/lib/Sema/DeclSpec.cpp

@@ -1377,6 +1377,20 @@ void DeclSpec::Finish(Sema &S, const PrintingPolicy &Policy) {
       ThreadStorageClassSpec = TSCS_unspecified;
       ThreadStorageClassSpecLoc = SourceLocation();
     }
+    if (S.getLangOpts().C23 &&
+        getConstexprSpecifier() == ConstexprSpecKind::Constexpr) {
+      S.Diag(ConstexprLoc, diag::err_invalid_decl_spec_combination)
+          << DeclSpec::getSpecifierName(getThreadStorageClassSpec())
+          << SourceRange(getThreadStorageClassSpecLoc());
+    }
+  }
+
+  if (S.getLangOpts().C23 &&
+      getConstexprSpecifier() == ConstexprSpecKind::Constexpr &&
+      StorageClassSpec == SCS_extern) {
+    S.Diag(ConstexprLoc, diag::err_invalid_decl_spec_combination)
+        << DeclSpec::getSpecifierName(getStorageClassSpec())
+        << SourceRange(getStorageClassSpecLoc());
   }
 
   // If no type specifier was provided and we're parsing a language where

clang/lib/Sema/SemaDecl.cpp

@@ -5144,6 +5144,8 @@ Decl *Sema::ParsedFreeStandingDeclSpec(Scope *S, AccessSpecifier AS,
       Diag(DS.getConstexprSpecLoc(), diag::err_constexpr_tag)
           << GetDiagnosticTypeSpecifierID(DS)
           << static_cast<int>(DS.getConstexprSpecifier());
+    else if (getLangOpts().C23)
+      Diag(DS.getConstexprSpecLoc(), diag::err_c23_constexpr_not_variable);
     else
       Diag(DS.getConstexprSpecLoc(), diag::err_constexpr_wrong_decl_kind)
           << static_cast<int>(DS.getConstexprSpecifier());
@@ -8646,6 +8648,38 @@ static bool checkForConflictWithNonVisibleExternC(Sema &S, const T *ND,
   return false;
 }
 
+static bool CheckC23ConstexprVarType(Sema &SemaRef, SourceLocation VarLoc,
+                                     QualType T) {
+  QualType CanonT = SemaRef.Context.getCanonicalType(T);
+  // C23 6.7.1p5: An object declared with storage-class specifier constexpr or
+  // any of its members, even recursively, shall not have an atomic type, or a
+  // variably modified type, or a type that is volatile or restrict qualified.
+  if (CanonT->isVariablyModifiedType()) {
+    SemaRef.Diag(VarLoc, diag::err_c23_constexpr_invalid_type) << T;
+    return true;
+  }
+
+  // Arrays are qualified by their element type, so get the base type (this
+  // works on non-arrays as well).
+  CanonT = SemaRef.Context.getBaseElementType(CanonT);
+
+  if (CanonT->isAtomicType() || CanonT.isVolatileQualified() ||
+      CanonT.isRestrictQualified()) {
+    SemaRef.Diag(VarLoc, diag::err_c23_constexpr_invalid_type) << T;
+    return true;
+  }
+
+  if (CanonT->isRecordType()) {
+    const RecordDecl *RD = CanonT->getAsRecordDecl();
+    if (llvm::any_of(RD->fields(), [&SemaRef, VarLoc](const FieldDecl *F) {
+          return CheckC23ConstexprVarType(SemaRef, VarLoc, F->getType());
+        }))
+      return true;
+  }
+
+  return false;
+}
+
 void Sema::CheckVariableDeclarationType(VarDecl *NewVD) {
   // If the decl is already known invalid, don't check it.
   if (NewVD->isInvalidDecl())
@@ -8896,6 +8930,12 @@ void Sema::CheckVariableDeclarationType(VarDecl *NewVD) {
     return;
   }
 
+  if (getLangOpts().C23 && NewVD->isConstexpr() &&
+      CheckC23ConstexprVarType(*this, NewVD->getLocation(), T)) {
+    NewVD->setInvalidDecl();
+    return;
+  }
+
   if (NewVD->isConstexpr() && !T->isDependentType() &&
       RequireLiteralType(NewVD->getLocation(), T,
                          diag::err_constexpr_var_non_literal)) {
@@ -9278,6 +9318,22 @@ static FunctionDecl *CreateNewFunctionDecl(Sema &SemaRef, Declarator &D,
   FunctionDecl *NewFD = nullptr;
   bool isInline = D.getDeclSpec().isInlineSpecified();
 
+  ConstexprSpecKind ConstexprKind = D.getDeclSpec().getConstexprSpecifier();
+  if (ConstexprKind == ConstexprSpecKind::Constinit ||
+      (SemaRef.getLangOpts().C23 &&
+       ConstexprKind == ConstexprSpecKind::Constexpr)) {
+
+    if (SemaRef.getLangOpts().C23)
+      SemaRef.Diag(D.getDeclSpec().getConstexprSpecLoc(),
+                   diag::err_c23_constexpr_not_variable);
+    else
+      SemaRef.Diag(D.getDeclSpec().getConstexprSpecLoc(),
+                   diag::err_constexpr_wrong_decl_kind)
+          << static_cast<int>(ConstexprKind);
+    ConstexprKind = ConstexprSpecKind::Unspecified;
+    D.getMutableDeclSpec().ClearConstexprSpec();
+  }
+
   if (!SemaRef.getLangOpts().CPlusPlus) {
     // Determine whether the function was written with a prototype. This is
     // true when:
@@ -9311,15 +9367,6 @@ static FunctionDecl *CreateNewFunctionDecl(Sema &SemaRef, Declarator &D,
   }
 
   ExplicitSpecifier ExplicitSpecifier = D.getDeclSpec().getExplicitSpecifier();
-
-  ConstexprSpecKind ConstexprKind = D.getDeclSpec().getConstexprSpecifier();
-  if (ConstexprKind == ConstexprSpecKind::Constinit) {
-    SemaRef.Diag(D.getDeclSpec().getConstexprSpecLoc(),
-                 diag::err_constexpr_wrong_decl_kind)
-        << static_cast<int>(ConstexprKind);
-    ConstexprKind = ConstexprSpecKind::Unspecified;
-    D.getMutableDeclSpec().ClearConstexprSpec();
-  }
   Expr *TrailingRequiresClause = D.getTrailingRequiresClause();
 
   SemaRef.CheckExplicitObjectMemberFunction(DC, D, Name, R);
@@ -13906,7 +13953,9 @@ void Sema::AddInitializerToDecl(Decl *RealDecl, Expr *Init, bool DirectInit) {
       VDecl->setStorageClass(SC_Extern);
 
     // C99 6.7.8p4. All file scoped initializers need to be constant.
-    if (!getLangOpts().CPlusPlus && !VDecl->isInvalidDecl())
+    // Avoid duplicate diagnostics for constexpr variables.
+    if (!getLangOpts().CPlusPlus && !VDecl->isInvalidDecl() &&
+        !VDecl->isConstexpr())
       CheckForConstantInitializer(Init, DclT);
   }
 
@@ -14517,17 +14566,21 @@ void Sema::CheckCompleteVariableDeclaration(VarDecl *var) {
   QualType baseType = Context.getBaseElementType(type);
   bool HasConstInit = true;
 
+  if (getLangOpts().C23 && var->isConstexpr() && !Init)
+    Diag(var->getLocation(), diag::err_constexpr_var_requires_const_init)
+        << var;
+
   // Check whether the initializer is sufficiently constant.
-  if (getLangOpts().CPlusPlus && !type->isDependentType() && Init &&
-      !Init->isValueDependent() &&
+  if ((getLangOpts().CPlusPlus || (getLangOpts().C23 && var->isConstexpr())) &&
+      !type->isDependentType() && Init && !Init->isValueDependent() &&
       (GlobalStorage || var->isConstexpr() ||
        var->mightBeUsableInConstantExpressions(Context))) {
     // If this variable might have a constant initializer or might be usable in
     // constant expressions, check whether or not it actually is now.  We can't
     // do this lazily, because the result might depend on things that change
     // later, such as which constexpr functions happen to be defined.
     SmallVector<PartialDiagnosticAt, 8> Notes;
-    if (!getLangOpts().CPlusPlus11) {
+    if (!getLangOpts().CPlusPlus11 && !getLangOpts().C23) {
       // Prior to C++11, in contexts where a constant initializer is required,
       // the set of valid constant initializers is described by syntactic rules
       // in [expr.const]p2-6.