[c++20] P1143R2: Add support for the C++20 'constinit' keyword.

This is mostly the same as the
[[clang::require_constant_initialization]] attribute, but has a couple
of additional syntactic and semantic restrictions.

In passing, I added a warning for the attribute form being added after
we have already seen the initialization of the variable (but before we
see the definition); that case previously slipped between the cracks and
the attribute was silently ignored.

llvm-svn: 370972

Author: zygoloid

clang/include/clang/AST/Decl.h

@@ -1226,6 +1226,14 @@ class VarDecl : public DeclaratorDecl, public Redeclarable<VarDecl> {
 
   void setInit(Expr *I);
 
+  /// Get the initializing declaration of this variable, if any. This is
+  /// usually the definition, except that for a static data member it can be
+  /// the in-class declaration.
+  VarDecl *getInitializingDeclaration();
+  const VarDecl *getInitializingDeclaration() const {
+    return const_cast<VarDecl *>(this)->getInitializingDeclaration();
+  }
+
   /// Determine whether this variable's value might be usable in a
   /// constant expression, according to the relevant language standard.
   /// This only checks properties of the declaration, and does not check

clang/include/clang/Basic/Attr.td

@@ -911,6 +911,17 @@ def Const : InheritableAttr {
   let Documentation = [Undocumented];
 }
 
+def ConstInit : InheritableAttr {
+  // This attribute does not have a C [[]] spelling because it requires the
+  // CPlusPlus language option.
+  let Spellings = [Keyword<"constinit">,
+                   Clang<"require_constant_initialization", 0>];
+  let Subjects = SubjectList<[GlobalVar], ErrorDiag>;
+  let Accessors = [Accessor<"isConstinit", [Keyword<"constinit">]>];
+  let Documentation = [RequireConstantInitDocs];
+  let LangOpts = [CPlusPlus];
+}
+
 def Constructor : InheritableAttr {
   let Spellings = [GCC<"constructor">];
   let Args = [DefaultIntArgument<"Priority", 65535>];
@@ -1935,15 +1946,6 @@ def ReqdWorkGroupSize : InheritableAttr {
   let Documentation = [Undocumented];
 }
 
-def RequireConstantInit : InheritableAttr {
-  // This attribute does not have a C [[]] spelling because it requires the
-  // CPlusPlus language option.
-  let Spellings = [Clang<"require_constant_initialization", 0>];
-  let Subjects = SubjectList<[GlobalVar], ErrorDiag>;
-  let Documentation = [RequireConstantInitDocs];
-  let LangOpts = [CPlusPlus];
-}
-
 def WorkGroupSizeHint :  InheritableAttr {
   // Does not have a [[]] spelling because it is an OpenCL-related attribute.
   let Spellings = [GNU<"work_group_size_hint">];

clang/include/clang/Basic/DiagnosticCommonKinds.td

@@ -12,6 +12,11 @@
 
 let Component = "Common" in {
 
+// Substitutions.
+
+def select_constexpr_spec_kind : TextSubstitution<
+  "%select{<ERROR>|constexpr|consteval|constinit}0">;
+
 // Basic.
 
 def fatal_too_many_errors
@@ -112,6 +117,10 @@ def err_attribute_not_type_attr : Error<
   "%0 attribute cannot be applied to types">;
 def err_enum_template : Error<"enumeration cannot be a template">;
 
+def warn_cxx20_compat_consteval : Warning<
+  "'consteval' specifier is incompatible with C++ standards before C++20">,
+  InGroup<CXX2aCompat>, DefaultIgnore;
+
 }
 
 let CategoryName = "Nullability Issue" in {
@@ -177,9 +186,6 @@ 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

@@ -358,7 +358,8 @@ 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 %select{constexpr|consteval}0 specifier to be specified">;
+  "type name does not allow %sub{select_constexpr_spec_kind}0 specifier "
+  "to be specified">;
 def err_typename_identifiers_only : Error<
   "typename is allowed for identifiers only">;
 

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -2343,18 +2343,18 @@ def warn_cxx14_compat_constexpr_not_const : Warning<
   "in C++14; add 'const' to avoid a change in behavior">,
   InGroup<DiagGroup<"constexpr-not-const">>;
 def err_invalid_constexpr : Error<
-  "%select{function parameter|typedef|non-static data member}0 "
-  "cannot be %select{constexpr|consteval}1">;
+  "%select{function parameter|typedef}0 "
+  "cannot be %sub{select_constexpr_spec_kind}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 %select{constexpr|consteval}1">;
+  "cannot be marked %sub{select_constexpr_spec_kind}1">;
 def err_constexpr_dtor : Error<
-  "destructor cannot be marked %select{constexpr|consteval}0">;
+  "destructor cannot be marked %sub{select_constexpr_spec_kind}0">;
 def err_constexpr_wrong_decl_kind : Error<
-  "%select{constexpr|consteval}0 can only be used "
-  "in %select{variable and |}0function declarations">;
+  "%sub{select_constexpr_spec_kind}0 can only be used "
+  "in %select{|variable and function|function|variable}0 declarations">;
 def err_invalid_constexpr_var_decl : Error<
   "constexpr variable declaration must be a definition">;
 def err_constexpr_static_mem_var_requires_init : Error<
@@ -7514,10 +7514,30 @@ def note_inequality_comparison_to_or_assign : Note<
 def err_incomplete_type_used_in_type_trait_expr : Error<
   "incomplete type %0 used in type trait expression">;
 
+// C++20 constinit and require_constant_initialization attribute
+def warn_cxx20_compat_constinit : Warning<
+  "'constinit' specifier is incompatible with C++ standards before C++20">,
+  InGroup<CXX2aCompat>, DefaultIgnore;
+def err_constinit_local_variable : Error<
+  "local variable cannot be declared 'constinit'">;
 def err_require_constant_init_failed : Error<
   "variable does not have a constant initializer">;
 def note_declared_required_constant_init_here : Note<
-  "required by 'require_constant_initialization' attribute here">;
+  "required by %select{'require_constant_initialization' attribute|"
+  "'constinit' specifier}0 here">;
+def ext_constinit_missing : ExtWarn<
+  "'constinit' specifier missing on initializing declaration of %0">,
+  InGroup<DiagGroup<"missing-constinit">>;
+def note_constinit_specified_here : Note<"variable declared constinit here">;
+def err_constinit_added_too_late : Error<
+  "'constinit' specifier added after initialization of variable">;
+def warn_require_const_init_added_too_late : Warning<
+  "'require_constant_initialization' attribute added after initialization "
+  "of variable">, InGroup<IgnoredAttributes>;
+def note_constinit_missing_here : Note<
+  "add the "
+  "%select{'require_constant_initialization' attribute|'constinit' specifier}0 "
+  "to the initializing declaration here">;
 
 def err_dimension_expr_not_constant_integer : Error<
   "dimension expression does not evaluate to a constant unsigned int">;

clang/include/clang/Basic/Specifiers.h

@@ -32,7 +32,8 @@ namespace clang {
   enum ConstexprSpecKind {
     CSK_unspecified,
     CSK_constexpr,
-    CSK_consteval
+    CSK_consteval,
+    CSK_constinit
   };
 
   /// Specifies the width of a type, e.g., short, long, or long long.

clang/include/clang/Basic/TokenKinds.def

@@ -389,6 +389,7 @@ MODULES_KEYWORD(import)
 // C++20 keywords.
 CXX2A_KEYWORD(char8_t               , CHAR8SUPPORT)
 CXX2A_KEYWORD(consteval             , 0)
+CXX2A_KEYWORD(constinit             , 0)
 
 // C11 Extension
 KEYWORD(_Float16                    , KEYALL)

clang/lib/AST/Decl.cpp

@@ -2220,6 +2220,22 @@ Stmt **VarDecl::getInitAddress() {
   return Init.getAddrOfPtr1();
 }
 
+VarDecl *VarDecl::getInitializingDeclaration() {
+  VarDecl *Def = nullptr;
+  for (auto I : redecls()) {
+    if (I->hasInit())
+      return I;
+
+    if (I->isThisDeclarationADefinition()) {
+      if (isStaticDataMember())
+        return I;
+      else
+        Def = I;
+    }
+  }
+  return Def;
+}
+
 bool VarDecl::isOutOfLine() const {
   if (Decl::isOutOfLine())
     return true;

clang/lib/Frontend/InitPreprocessor.cpp

@@ -541,8 +541,10 @@ static void InitializeCPlusPlusFeatureTestMacros(const LangOptions &LangOpts,
     Builder.defineMacro("__cpp_template_template_args", "201611L");
 
   // C++20 features.
-  if (LangOpts.CPlusPlus2a)
+  if (LangOpts.CPlusPlus2a) {
     Builder.defineMacro("__cpp_conditional_explicit", "201806L");
+    Builder.defineMacro("__cpp_constinit", "201907L");
+  }
   if (LangOpts.Char8)
     Builder.defineMacro("__cpp_char8_t", "201811L");
   Builder.defineMacro("__cpp_impl_destroying_delete", "201806L");

clang/lib/Parse/ParseDecl.cpp

@@ -2514,7 +2514,7 @@ void Parser::ParseSpecifierQualifierList(DeclSpec &DS, AccessSpecifier AS,
   // Issue diagnostic and remove constexpr specifier if present.
   if (DS.hasConstexprSpecifier() && DSC != DeclSpecContext::DSC_condition) {
     Diag(DS.getConstexprSpecLoc(), diag::err_typename_invalid_constexpr)
-        << (DS.getConstexprSpecifier() == CSK_consteval);
+        << DS.getConstexprSpecifier();
     DS.ClearConstexprSpec();
   }
 }
@@ -3653,15 +3653,16 @@ void Parser::ParseDeclarationSpecifiers(DeclSpec &DS,
       isInvalid = DS.setModulePrivateSpec(Loc, PrevSpec, DiagID);
       break;
 
-    // constexpr
+    // constexpr, consteval, constinit specifiers
     case tok::kw_constexpr:
       isInvalid = DS.SetConstexprSpec(CSK_constexpr, Loc, PrevSpec, DiagID);
       break;
-
-    // consteval
     case tok::kw_consteval:
       isInvalid = DS.SetConstexprSpec(CSK_consteval, Loc, PrevSpec, DiagID);
       break;
+    case tok::kw_constinit:
+      isInvalid = DS.SetConstexprSpec(CSK_constinit, Loc, PrevSpec, DiagID);
+      break;
 
     // type-specifier
     case tok::kw_short:
@@ -5080,8 +5081,9 @@ bool Parser::isDeclarationSpecifier(bool DisambiguatingWithExpression) {
   case tok::annot_decltype:
   case tok::kw_constexpr:
 
-    // C++20 consteval.
+    // C++20 consteval and constinit.
   case tok::kw_consteval:
+  case tok::kw_constinit:
 
     // C11 _Atomic
   case tok::kw__Atomic: