[C++20] add Basic consteval specifier

Summary:
this revision adds Lexing, Parsing and Basic Semantic for the consteval specifier as specified by http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p1073r3.html

with this patch, the consteval specifier is treated as constexpr but can only be applied to function declaration.

Changes:
 - add the consteval keyword.
 - add parsing of consteval specifier for normal declarations and lambdas expressions.
 - add the whether a declaration is constexpr is now represented by and enum everywhere except for variable because they can't be consteval.
 - adapt diagnostic about constexpr to print constexpr or consteval depending on the case.
 - add tests for basic semantic.

Reviewers: rsmith, martong, shafik

Reviewed By: rsmith

Subscribers: eraman, efriedma, rnkovacs, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D61790

llvm-svn: 363362

clang/include/clang/AST/Decl.h

@@ -1861,7 +1861,7 @@ class FunctionDecl : public DeclaratorDecl,
   FunctionDecl(Kind DK, ASTContext &C, DeclContext *DC, SourceLocation StartLoc,
                const DeclarationNameInfo &NameInfo, QualType T,
                TypeSourceInfo *TInfo, StorageClass S, bool isInlineSpecified,
-               bool isConstexprSpecified);
+               ConstexprSpecKind ConstexprKind);
 
   using redeclarable_base = Redeclarable<FunctionDecl>;
 
@@ -1891,29 +1891,24 @@ class FunctionDecl : public DeclaratorDecl,
   using redeclarable_base::getMostRecentDecl;
   using redeclarable_base::isFirstDecl;
 
-  static FunctionDecl *Create(ASTContext &C, DeclContext *DC,
-                              SourceLocation StartLoc, SourceLocation NLoc,
-                              DeclarationName N, QualType T,
-                              TypeSourceInfo *TInfo,
-                              StorageClass SC,
-                              bool isInlineSpecified = false,
-                              bool hasWrittenPrototype = true,
-                              bool isConstexprSpecified = false) {
+  static FunctionDecl *
+  Create(ASTContext &C, DeclContext *DC, SourceLocation StartLoc,
+         SourceLocation NLoc, DeclarationName N, QualType T,
+         TypeSourceInfo *TInfo, StorageClass SC, bool isInlineSpecified = false,
+         bool hasWrittenPrototype = true,
+         ConstexprSpecKind ConstexprKind = CSK_unspecified) {
     DeclarationNameInfo NameInfo(N, NLoc);
-    return FunctionDecl::Create(C, DC, StartLoc, NameInfo, T, TInfo,
-                                SC,
+    return FunctionDecl::Create(C, DC, StartLoc, NameInfo, T, TInfo, SC,
                                 isInlineSpecified, hasWrittenPrototype,
-                                isConstexprSpecified);
+                                ConstexprKind);
   }
 
   static FunctionDecl *Create(ASTContext &C, DeclContext *DC,
                               SourceLocation StartLoc,
-                              const DeclarationNameInfo &NameInfo,
-                              QualType T, TypeSourceInfo *TInfo,
-                              StorageClass SC,
-                              bool isInlineSpecified,
-                              bool hasWrittenPrototype,
-                              bool isConstexprSpecified = false);
+                              const DeclarationNameInfo &NameInfo, QualType T,
+                              TypeSourceInfo *TInfo, StorageClass SC,
+                              bool isInlineSpecified, bool hasWrittenPrototype,
+                              ConstexprSpecKind ConstexprKind);
 
   static FunctionDecl *CreateDeserialized(ASTContext &C, unsigned ID);
 
@@ -2110,8 +2105,21 @@ class FunctionDecl : public DeclaratorDecl,
   }
 
   /// Whether this is a (C++11) constexpr function or constexpr constructor.
-  bool isConstexpr() const { return FunctionDeclBits.IsConstexpr; }
-  void setConstexpr(bool IC) { FunctionDeclBits.IsConstexpr = IC; }
+  bool isConstexpr() const {
+    return FunctionDeclBits.ConstexprKind != CSK_unspecified;
+  }
+  void setConstexprKind(ConstexprSpecKind CSK) {
+    FunctionDeclBits.ConstexprKind = CSK;
+  }
+  ConstexprSpecKind getConstexprKind() const {
+    return static_cast<ConstexprSpecKind>(FunctionDeclBits.ConstexprKind);
+  }
+  bool isConstexprSpecified() const {
+    return FunctionDeclBits.ConstexprKind == CSK_constexpr;
+  }
+  bool isConsteval() const {
+    return FunctionDeclBits.ConstexprKind == CSK_consteval;
+  }
 
   /// Whether the instantiation of this function is pending.
   /// This bit is set when the decision to instantiate this function is made

clang/include/clang/AST/DeclBase.h

@@ -1500,7 +1500,9 @@ class DeclContext {
     uint64_t IsExplicitlyDefaulted : 1;
     uint64_t HasImplicitReturnZero : 1;
     uint64_t IsLateTemplateParsed : 1;
-    uint64_t IsConstexpr : 1;
+
+    /// Kind of contexpr specifier as defined by ConstexprSpecKind.
+    uint64_t ConstexprKind : 2;
     uint64_t InstantiationIsPending : 1;
 
     /// Indicates if the function uses __try.
@@ -1528,7 +1530,7 @@ class DeclContext {
   };
 
   /// Number of non-inherited bits in FunctionDeclBitfields.
-  enum { NumFunctionDeclBits = 24 };
+  enum { NumFunctionDeclBits = 25 };
 
   /// Stores the bits used by CXXConstructorDecl. If modified
   /// NumCXXConstructorDeclBits and the accessor
@@ -1545,7 +1547,7 @@ class DeclContext {
     /// exactly 64 bits and thus the width of NumCtorInitializers
     /// will need to be shrunk if some bit is added to NumDeclContextBitfields,
     /// NumFunctionDeclBitfields or CXXConstructorDeclBitfields.
-    uint64_t NumCtorInitializers : 24;
+    uint64_t NumCtorInitializers : 23;
     uint64_t IsInheritingConstructor : 1;
 
     /// Whether this constructor has a trail-allocated explicit specifier.

clang/include/clang/AST/DeclCXX.h

@@ -2057,7 +2057,7 @@ class CXXDeductionGuideDecl : public FunctionDecl {
                         const DeclarationNameInfo &NameInfo, QualType T,
                         TypeSourceInfo *TInfo, SourceLocation EndLocation)
       : FunctionDecl(CXXDeductionGuide, C, DC, StartLoc, NameInfo, T, TInfo,
-                     SC_None, false, false),
+                     SC_None, false, CSK_unspecified),
         ExplicitSpec(ES) {
     if (EndLocation.isValid())
       setRangeEnd(EndLocation);
@@ -2112,23 +2112,21 @@ class CXXMethodDecl : public FunctionDecl {
 protected:
   CXXMethodDecl(Kind DK, ASTContext &C, CXXRecordDecl *RD,
                 SourceLocation StartLoc, const DeclarationNameInfo &NameInfo,
-                QualType T, TypeSourceInfo *TInfo,
-                StorageClass SC, bool isInline,
-                bool isConstexpr, SourceLocation EndLocation)
-    : FunctionDecl(DK, C, RD, StartLoc, NameInfo, T, TInfo,
-                   SC, isInline, isConstexpr) {
+                QualType T, TypeSourceInfo *TInfo, StorageClass SC,
+                bool isInline, ConstexprSpecKind ConstexprKind,
+                SourceLocation EndLocation)
+      : FunctionDecl(DK, C, RD, StartLoc, NameInfo, T, TInfo, SC, isInline,
+                     ConstexprKind) {
     if (EndLocation.isValid())
       setRangeEnd(EndLocation);
   }
 
 public:
   static CXXMethodDecl *Create(ASTContext &C, CXXRecordDecl *RD,
                                SourceLocation StartLoc,
-                               const DeclarationNameInfo &NameInfo,
-                               QualType T, TypeSourceInfo *TInfo,
-                               StorageClass SC,
-                               bool isInline,
-                               bool isConstexpr,
+                               const DeclarationNameInfo &NameInfo, QualType T,
+                               TypeSourceInfo *TInfo, StorageClass SC,
+                               bool isInline, ConstexprSpecKind ConstexprKind,
                                SourceLocation EndLocation);
 
   static CXXMethodDecl *CreateDeserialized(ASTContext &C, unsigned ID);
@@ -2575,7 +2573,7 @@ class CXXConstructorDecl final
   CXXConstructorDecl(ASTContext &C, CXXRecordDecl *RD, SourceLocation StartLoc,
                      const DeclarationNameInfo &NameInfo, QualType T,
                      TypeSourceInfo *TInfo, ExplicitSpecifier ES, bool isInline,
-                     bool isImplicitlyDeclared, bool isConstexpr,
+                     bool isImplicitlyDeclared, ConstexprSpecKind ConstexprKind,
                      InheritedConstructor Inherited);
 
   void anchor() override;
@@ -2628,7 +2626,7 @@ class CXXConstructorDecl final
   Create(ASTContext &C, CXXRecordDecl *RD, SourceLocation StartLoc,
          const DeclarationNameInfo &NameInfo, QualType T, TypeSourceInfo *TInfo,
          ExplicitSpecifier ES, bool isInline, bool isImplicitlyDeclared,
-         bool isConstexpr,
+         ConstexprSpecKind ConstexprKind,
          InheritedConstructor Inherited = InheritedConstructor());
 
   ExplicitSpecifier getExplicitSpecifier() {
@@ -2834,12 +2832,11 @@ class CXXDestructorDecl : public CXXMethodDecl {
   Expr *OperatorDeleteThisArg = nullptr;
 
   CXXDestructorDecl(ASTContext &C, CXXRecordDecl *RD, SourceLocation StartLoc,
-                    const DeclarationNameInfo &NameInfo,
-                    QualType T, TypeSourceInfo *TInfo,
-                    bool isInline, bool isImplicitlyDeclared)
-    : CXXMethodDecl(CXXDestructor, C, RD, StartLoc, NameInfo, T, TInfo,
-                    SC_None, isInline, /*isConstexpr=*/false, SourceLocation())
-  {
+                    const DeclarationNameInfo &NameInfo, QualType T,
+                    TypeSourceInfo *TInfo, bool isInline,
+                    bool isImplicitlyDeclared)
+      : CXXMethodDecl(CXXDestructor, C, RD, StartLoc, NameInfo, T, TInfo,
+                      SC_None, isInline, CSK_unspecified, SourceLocation()) {
     setImplicit(isImplicitlyDeclared);
   }
 
@@ -2890,9 +2887,9 @@ class CXXConversionDecl : public CXXMethodDecl {
   CXXConversionDecl(ASTContext &C, CXXRecordDecl *RD, SourceLocation StartLoc,
                     const DeclarationNameInfo &NameInfo, QualType T,
                     TypeSourceInfo *TInfo, bool isInline, ExplicitSpecifier ES,
-                    bool isConstexpr, SourceLocation EndLocation)
+                    ConstexprSpecKind ConstexprKind, SourceLocation EndLocation)
       : CXXMethodDecl(CXXConversion, C, RD, StartLoc, NameInfo, T, TInfo,
-                      SC_None, isInline, isConstexpr, EndLocation),
+                      SC_None, isInline, ConstexprKind, EndLocation),
         ExplicitSpec(ES) {}
   void anchor() override;
 
@@ -2907,7 +2904,7 @@ class CXXConversionDecl : public CXXMethodDecl {
   static CXXConversionDecl *
   Create(ASTContext &C, CXXRecordDecl *RD, SourceLocation StartLoc,
          const DeclarationNameInfo &NameInfo, QualType T, TypeSourceInfo *TInfo,
-         bool isInline, ExplicitSpecifier ES, bool isConstexpr,
+         bool isInline, ExplicitSpecifier ES, ConstexprSpecKind ConstexprKind,
          SourceLocation EndLocation);
   static CXXConversionDecl *CreateDeserialized(ASTContext &C, unsigned ID);
 

clang/include/clang/Basic/DiagnosticCommonKinds.td

@@ -172,6 +172,9 @@ def ext_cxx11_longlong : Extension<
 def warn_cxx98_compat_longlong : Warning<
   "'long long' is incompatible with C++98">,
   InGroup<CXX98CompatPedantic>, DefaultIgnore;
+def warn_cxx20_compat_consteval : Warning<
+  "consteval is incompatible with C++ standards before C++20">,
+  InGroup<CXX2aCompat>, DefaultIgnore;
 def err_integer_literal_too_large : Error<
   "integer literal is too large to be represented in any %select{signed |}0"
   "integer type">;

clang/include/clang/Basic/DiagnosticParseKinds.td

@@ -360,7 +360,7 @@ def err_typename_invalid_storageclass : Error<
 def err_typename_invalid_functionspec : Error<
   "type name does not allow function specifier to be specified">;
 def err_typename_invalid_constexpr : Error<
-  "type name does not allow constexpr specifier to be specified">;
+  "type name does not allow %select{constexpr|consteval}0 specifier to be specified">;
 def err_typename_identifiers_only : Error<
   "typename is allowed for identifiers only">;
 
@@ -875,9 +875,9 @@ def warn_cxx98_compat_lambda : Warning<
   InGroup<CXX98Compat>, DefaultIgnore;
 def err_lambda_missing_parens : Error<
   "lambda requires '()' before %select{'mutable'|return type|"
-  "attribute specifier|'constexpr'}0">;
+  "attribute specifier|'constexpr'|'consteval'}0">;
 def err_lambda_decl_specifier_repeated : Error<
-  "%select{'mutable'|'constexpr'}0 cannot appear multiple times in a lambda declarator">;
+  "%select{'mutable'|'constexpr'|'consteval'}0 cannot appear multiple times in a lambda declarator">;
 def err_lambda_capture_misplaced_ellipsis : Error<
   "ellipsis in pack %select{|init-}0capture must appear %select{after|before}0 "
   "the name of the capture">;

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -704,7 +704,7 @@ def ext_noreturn_main : ExtWarn<
   "'main' is not allowed to be declared _Noreturn">, InGroup<Main>;
 def note_main_remove_noreturn : Note<"remove '_Noreturn'">;
 def err_constexpr_main : Error<
-  "'main' is not allowed to be declared constexpr">;
+  "'main' is not allowed to be declared %select{constexpr|consteval}0">;
 def err_deleted_main : Error<"'main' is not allowed to be deleted">;
 def err_mainlike_template_decl : Error<"%0 cannot be a template">;
 def err_main_returns_nonint : Error<"'main' must return 'int'">;
@@ -2312,14 +2312,17 @@ def warn_cxx14_compat_constexpr_not_const : Warning<
   InGroup<DiagGroup<"constexpr-not-const">>;
 def err_invalid_constexpr : Error<
   "%select{function parameter|typedef|non-static data member}0 "
-  "cannot be constexpr">;
+  "cannot be %select{constexpr|consteval}1">;
 def err_invalid_constexpr_member : Error<"non-static data member cannot be "
   "constexpr%select{; did you intend to make it %select{const|static}0?|}1">;
 def err_constexpr_tag : Error<
-  "%select{class|struct|interface|union|enum}0 cannot be marked constexpr">;
-def err_constexpr_dtor : Error<"destructor cannot be marked constexpr">;
-def err_constexpr_no_declarators : Error<
-  "constexpr can only be used in variable and function declarations">;
+  "%select{class|struct|interface|union|enum}0 "
+  "cannot be marked %select{constexpr|consteval}1">;
+def err_constexpr_dtor : Error<
+  "destructor cannot be marked %select{constexpr|consteval}0">;
+def err_constexpr_wrong_decl_kind : Error<
+  "%select{constexpr|consteval}0 can only be used "
+  "in %select{variable and |}0function declarations">;
 def err_invalid_constexpr_var_decl : Error<
   "constexpr variable declaration must be a definition">;
 def err_constexpr_static_mem_var_requires_init : Error<
@@ -2329,8 +2332,8 @@ def err_constexpr_var_non_literal : Error<
 def err_constexpr_var_requires_const_init : Error<
   "constexpr variable %0 must be initialized by a constant expression">;
 def err_constexpr_redecl_mismatch : Error<
-  "%select{non-constexpr declaration of %0 follows constexpr declaration"
-  "|constexpr declaration of %0 follows non-constexpr declaration}1">;
+  "%select{non-constexpr|constexpr|consteval}1 declaration of %0"
+  " follows %select{non-constexpr|constexpr|consteval}2 declaration">;
 def err_constexpr_virtual : Error<"virtual function cannot be constexpr">;
 def warn_cxx17_compat_constexpr_virtual : Warning<
   "virtual constexpr functions are incompatible with "
@@ -2345,12 +2348,12 @@ def note_non_literal_virtual_base : Note<"%select{struct|interface|class}0 "
   "with virtual base %plural{1:class|:classes}1 is not a literal type">;
 def note_constexpr_virtual_base_here : Note<"virtual base class declared here">;
 def err_constexpr_non_literal_return : Error<
-  "constexpr function's return type %0 is not a literal type">;
+  "%select{constexpr|consteval}0 function's return type %1 is not a literal type">;
 def err_constexpr_non_literal_param : Error<
-  "constexpr %select{function|constructor}1's %ordinal0 parameter type %2 is "
+  "%select{constexpr|consteval}2 %select{function|constructor}1's %ordinal0 parameter type %3 is "
   "not a literal type">;
 def err_constexpr_body_invalid_stmt : Error<
-  "statement not allowed in constexpr %select{function|constructor}0">;
+  "statement not allowed in %select{constexpr|consteval}1 %select{function|constructor}0">;
 def ext_constexpr_body_invalid_stmt : ExtWarn<
   "use of this statement in a constexpr %select{function|constructor}0 "
   "is a C++14 extension">, InGroup<CXX14>;
@@ -2400,9 +2403,9 @@ def err_diagnose_if_invalid_diagnostic_type : Error<
   "invalid diagnostic type for 'diagnose_if'; use \"error\" or \"warning\" "
   "instead">;
 def err_constexpr_body_no_return : Error<
-  "no return statement in constexpr function">;
+  "no return statement in %select{constexpr|consteval}0 function">;
 def err_constexpr_return_missing_expr : Error<
-  "non-void constexpr function %0 should return a value">;
+  "non-void %select{constexpr|consteval}1 function %0 should return a value">;
 def warn_cxx11_compat_constexpr_body_no_return : Warning<
   "constexpr function with no return statements is incompatible with C++ "
   "standards before C++14">, InGroup<CXXPre14Compat>, DefaultIgnore;
@@ -7853,6 +7856,9 @@ def err_defaulted_copy_assign_not_ref : Error<
 def err_incorrect_defaulted_constexpr : Error<
   "defaulted definition of %sub{select_special_member_kind}0 "
   "is not constexpr">;
+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<DiagGroup<"defaulted-function-deleted">>;
@@ -9422,7 +9428,7 @@ def err_coroutine_invalid_func_context : Error<
   "'%1' cannot be used in %select{a constructor|a destructor"
   "|a copy assignment operator|a move assignment operator|the 'main' function"
   "|a constexpr function|a function with a deduced return type"
-  "|a varargs function}0">;
+  "|a varargs function|a consteval function}0">;
 def err_implied_coroutine_type_not_found : Error<
   "%0 type was not found; include <experimental/coroutine> before defining "
   "a coroutine">;
@@ -9660,7 +9666,7 @@ def err_multiversion_doesnt_support : Error<
   "attribute '%select{target|cpu_specific|cpu_dispatch}0' multiversioned functions do not "
   "yet support %select{function templates|virtual functions|"
   "deduced return types|constructors|destructors|deleted functions|"
-  "defaulted functions|constexpr functions}1">;
+  "defaulted functions|constexpr functions|consteval function}1">;
 def err_multiversion_not_allowed_on_main : Error<
   "'main' cannot be a multiversioned function">;
 def err_multiversion_not_supported : Error<

clang/include/clang/Basic/Specifiers.h

@@ -28,6 +28,13 @@ namespace clang {
     Unresolved,
   };
 
+  /// Define the kind of constexpr specifier.
+  enum ConstexprSpecKind {
+    CSK_unspecified,
+    CSK_constexpr,
+    CSK_consteval
+  };
+
   /// Specifies the width of a type, e.g., short, long, or long long.
   enum TypeSpecifierWidth {
     TSW_unspecified,

clang/include/clang/Basic/TokenKinds.def

@@ -384,8 +384,9 @@ COROUTINES_KEYWORD(co_yield)
 MODULES_KEYWORD(module)
 MODULES_KEYWORD(import)
 
-// C++ char8_t proposal
+// C++20 keywords.
 CXX2A_KEYWORD(char8_t               , CHAR8SUPPORT)
+CXX2A_KEYWORD(consteval             , 0)
 
 // C11 Extension
 KEYWORD(_Float16                    , KEYALL)