[Clang] [C++26] Expansion Statements (Part 2: Parsing and Parser Tests)

clang/include/clang/Basic/DiagnosticCommonKinds.td

@@ -22,6 +22,10 @@ def select_constexpr_spec_kind : TextSubstitution<
 def fatal_too_many_errors
   : Error<"too many errors emitted, stopping now">, DefaultFatal;
 
+// TODO: Remove this.
+def err_expansion_statements_todo : Error<
+  "TODO (expansion statements)">;
+
 def warn_stack_exhausted : Warning<
   "stack nearly exhausted; compilation time may suffer, and "
   "crashes due to stack overflow are likely">,

clang/include/clang/Basic/DiagnosticParseKinds.td

@@ -16,6 +16,10 @@ let CategoryName = "Parse Issue" in {
 defm enum_fixed_underlying_type : CXX11Compat<
   "enumeration types with a fixed underlying type are",
   /*ext_warn=*/false>;
+
+// C++26 compatibility with C++23.
+defm expansion_statements : CXX26Compat<
+  "expansion statements are">;
 }
 
 def err_asm_qualifier_ignored : Error<
@@ -419,9 +423,10 @@ def warn_cxx98_compat_for_range : Warning<
   "range-based for loop is incompatible with C++98">,
   InGroup<CXX98Compat>, DefaultIgnore;
 def err_for_range_identifier : Error<
-  "range-based for loop requires type for loop variable">;
+  "%select{range-based for loop|expansion statement}0 requires "
+  "type for %select{loop|expansion}0 variable">;
 def err_for_range_expected_decl : Error<
-  "for range declaration must declare a variable">;
+  "%select{for range|expansion statement}0 declaration must declare a variable">;
 def err_argument_required_after_attribute : Error<
   "argument required after attribute">;
 def err_missing_param : Error<"expected parameter declarator">;
@@ -448,6 +453,14 @@ def err_unspecified_size_with_static : Error<
   "'static' may not be used without an array size">;
 def err_expected_parentheses_around_typename : Error<
   "expected parentheses around type name in %0 expression">;
+def err_expansion_stmt_requires_range : Error<
+  "expansion statement must be a range-based for loop">;
+def err_expansion_stmt_requires_cxx2c : Error<
+  "expansion statements are only supported in C++2c">;
+def err_for_template : Error<
+  "'for template' is invalid; use 'template for' instead">;
+def err_expansion_stmt_invalid_kw : Error<
+  "'template %0' is invalid">;
 
 def err_expected_case_before_expression: Error<
   "expected 'case' keyword before expression">;

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -2900,10 +2900,10 @@ def note_which_delegates_to : Note<"which delegates to">;
 
 // C++11 range-based for loop
 def err_for_range_decl_must_be_var : Error<
-  "for range declaration must declare a variable">;
+  "%select{for range|expansion statement}0 declaration must declare a variable">;
 def err_for_range_storage_class : Error<
-  "loop variable %0 may not be declared %select{'extern'|'static'|"
-  "'__private_extern__'|'auto'|'register'|'constexpr'|'thread_local'}1">;
+  "%select{loop|expansion}0 variable %1 may not be declared %select{'extern'|'static'|"
+  "'__private_extern__'|'auto'|'register'|'constexpr'|'thread_local'}2">;
 def err_type_defined_in_for_range : Error<
   "types may not be defined in a for range declaration">;
 def err_for_range_deduction_failure : Error<

clang/include/clang/Parse/Parser.h

@@ -1700,11 +1700,13 @@ class Parser : public CodeCompletionHandler {
   }
 
   /// Information on a C++0x for-range-initializer found while parsing a
-  /// declaration which turns out to be a for-range-declaration.
+  /// declaration which turns out to be a for-range-declaration. Also used
+  /// for C++26's expansion statements.
   struct ForRangeInit {
     SourceLocation ColonLoc;
     ExprResult RangeExpr;
     SmallVector<MaterializeTemporaryExpr *, 8> LifetimeExtendTemps;
+    bool ExpansionStmt = false;
     bool ParsedForRangeDecl() { return !ColonLoc.isInvalid(); }
   };
   struct ForRangeInfo : ForRangeInit {
@@ -4186,7 +4188,8 @@ class Parser : public CodeCompletionHandler {
   bool ParseExpressionList(SmallVectorImpl<Expr *> &Exprs,
                            llvm::function_ref<void()> ExpressionStarts =
                                llvm::function_ref<void()>(),
-                           bool FailImmediatelyOnInvalidExpr = false);
+                           bool FailImmediatelyOnInvalidExpr = false,
+                           bool ParsingExpansionInitList = false);
 
   /// ParseSimpleExpressionList - A simple comma-separated list of expressions,
   /// used for misc language extensions.
@@ -5246,6 +5249,16 @@ class Parser : public CodeCompletionHandler {
   ///
   ExprResult ParseBraceInitializer();
 
+  /// ParseExpansionInitList - Called when the initializer of an expansion
+  /// statement starts with an open brace.
+  ///
+  /// \verbatim
+  ///       expansion-init-list: [C++26 [stmt.expand]]
+  ///          '{' expression-list ','[opt] '}'
+  ///          '{' '}'
+  /// \endverbatim
+  ExprResult ParseExpansionInitList();
+
   struct DesignatorCompletionInfo {
     SmallVectorImpl<Expr *> &InitExprs;
     QualType PreferredBaseType;
@@ -7452,8 +7465,12 @@ class Parser : public CodeCompletionHandler {
   /// [C++0x]   expression
   /// [C++0x]   braced-init-list            [TODO]
   /// \endverbatim
-  StmtResult ParseForStatement(SourceLocation *TrailingElseLoc,
-                               LabelDecl *PrecedingLabel);
+  StmtResult ParseForStatement(
+      SourceLocation *TrailingElseLoc, LabelDecl *PrecedingLabel,
+      CXXExpansionStmtDecl *CXXExpansionStmtDeclaration = nullptr);
+
+  void ParseForRangeInitializerAfterColon(ForRangeInit &FRI,
+                                          ParsingDeclSpec *VarDeclSpec);
 
   /// ParseGotoStatement
   /// \verbatim
@@ -7500,6 +7517,23 @@ class Parser : public CodeCompletionHandler {
 
   StmtResult ParseBreakOrContinueStatement(bool IsContinue);
 
+  /// ParseExpansionStatement - Parse a C++26 expansion
+  /// statement ('template for').
+  ///
+  /// \verbatim
+  ///     expansion-statement:
+  ///       'template' 'for' '(' init-statement[opt]
+  ///           for-range-declaration ':' expansion-initializer ')'
+  ///           compound-statement
+  ///
+  ///     expansion-initializer:
+  ///       expression
+  ///       expansion-init-list
+  /// \endverbatim
+  StmtResult ParseExpansionStatement(SourceLocation *TrailingElseLoc,
+                                     LabelDecl *PrecedingLabel,
+                                     SourceLocation TemplateLoc);
+
   StmtResult ParsePragmaLoopHint(StmtVector &Stmts, ParsedStmtContext StmtCtx,
                                  SourceLocation *TrailingElseLoc,
                                  ParsedAttributes &Attrs,

clang/include/clang/Sema/Sema.h

@@ -893,6 +893,7 @@ class Sema final : public SemaBase {
   // 33. Types (SemaType.cpp)
   // 34. FixIt Helpers (SemaFixItUtils.cpp)
   // 35. Function Effects (SemaFunctionEffects.cpp)
+  // 36. C++ Expansion Statements (SemaExpand.cpp)
 
   /// \name Semantic Analysis
   /// Implementations are in Sema.cpp
@@ -4097,7 +4098,7 @@ class Sema final : public SemaBase {
   /// complete.
   void ActOnInitializerError(Decl *Dcl);
 
-  void ActOnCXXForRangeDecl(Decl *D);
+  void ActOnCXXForRangeDecl(Decl *D, bool InExpansionStmt);
   StmtResult ActOnCXXForRangeIdentifier(Scope *S, SourceLocation IdentLoc,
                                         IdentifierInfo *Ident,
                                         ParsedAttributes &Attrs);
@@ -15613,6 +15614,36 @@ class Sema final : public SemaBase {
   void performFunctionEffectAnalysis(TranslationUnitDecl *TU);
 
   ///@}
+
+  //
+  //
+  // -------------------------------------------------------------------------
+  //
+  //
+
+  /// \name Expansion Statements
+  /// Implementations are in SemaExpand.cpp
+  ///@{
+public:
+  CXXExpansionStmtDecl *ActOnCXXExpansionStmtDecl(unsigned TemplateDepth,
+                                                  SourceLocation TemplateKWLoc);
+
+  CXXExpansionStmtDecl *
+  BuildCXXExpansionStmtDecl(DeclContext *Ctx, SourceLocation TemplateKWLoc,
+                            NonTypeTemplateParmDecl *NTTP);
+
+  ExprResult ActOnCXXExpansionInitList(MultiExprArg SubExprs,
+                                       SourceLocation LBraceLoc,
+                                       SourceLocation RBraceLoc);
+
+  StmtResult ActOnCXXExpansionStmtPattern(
+      CXXExpansionStmtDecl *ESD, Stmt *Init, Stmt *ExpansionVarStmt,
+      Expr *ExpansionInitializer, SourceLocation LParenLoc,
+      SourceLocation ColonLoc, SourceLocation RParenLoc,
+      ArrayRef<MaterializeTemporaryExpr *> LifetimeExtendTemps);
+
+  StmtResult FinishCXXExpansionStmt(Stmt *Expansion, Stmt *Body);
+  ///@}
 };
 
 DeductionFailureInfo

clang/lib/Parse/ParseDecl.cpp

@@ -2304,43 +2304,18 @@ Parser::DeclGroupPtrTy Parser::ParseDeclGroup(ParsingDeclSpec &DS,
   // Handle the Objective-C for-in loop variable similarly, although we
   // don't need to parse the container in advance.
   if (FRI && (Tok.is(tok::colon) || isTokIdentifier_in())) {
-    bool IsForRangeLoop = false;
+    bool IsForRangeLoopOrExpansionStmt = false;
     if (TryConsumeToken(tok::colon, FRI->ColonLoc)) {
-      IsForRangeLoop = true;
-      EnterExpressionEvaluationContext ForRangeInitContext(
-          Actions, Sema::ExpressionEvaluationContext::PotentiallyEvaluated,
-          /*LambdaContextDecl=*/nullptr,
-          Sema::ExpressionEvaluationContextRecord::EK_Other,
-          getLangOpts().CPlusPlus23);
-
-      // P2718R0 - Lifetime extension in range-based for loops.
-      if (getLangOpts().CPlusPlus23) {
-        auto &LastRecord = Actions.currentEvaluationContext();
-        LastRecord.InLifetimeExtendingContext = true;
-        LastRecord.RebuildDefaultArgOrDefaultInit = true;
-      }
-
-      if (getLangOpts().OpenMP)
+      IsForRangeLoopOrExpansionStmt = true;
+      if (getLangOpts().OpenMP && !FRI->ExpansionStmt)
         Actions.OpenMP().startOpenMPCXXRangeFor();
-      if (Tok.is(tok::l_brace))
-        FRI->RangeExpr = ParseBraceInitializer();
-      else
-        FRI->RangeExpr = ParseExpression();
-
-      // Before c++23, ForRangeLifetimeExtendTemps should be empty.
-      assert(
-          getLangOpts().CPlusPlus23 ||
-          Actions.ExprEvalContexts.back().ForRangeLifetimeExtendTemps.empty());
 
-      // Move the collected materialized temporaries into ForRangeInit before
-      // ForRangeInitContext exit.
-      FRI->LifetimeExtendTemps = std::move(
-          Actions.ExprEvalContexts.back().ForRangeLifetimeExtendTemps);
+      ParseForRangeInitializerAfterColon(*FRI, &DS);
     }
 
     Decl *ThisDecl = Actions.ActOnDeclarator(getCurScope(), D);
-    if (IsForRangeLoop) {
-      Actions.ActOnCXXForRangeDecl(ThisDecl);
+    if (IsForRangeLoopOrExpansionStmt) {
+      Actions.ActOnCXXForRangeDecl(ThisDecl, FRI->ExpansionStmt);
     } else {
       // Obj-C for loop
       if (auto *VD = dyn_cast_or_null<VarDecl>(ThisDecl))

clang/lib/Parse/ParseExpr.cpp

@@ -3166,7 +3166,8 @@ void Parser::injectEmbedTokens() {
 
 bool Parser::ParseExpressionList(SmallVectorImpl<Expr *> &Exprs,
                                  llvm::function_ref<void()> ExpressionStarts,
-                                 bool FailImmediatelyOnInvalidExpr) {
+                                 bool FailImmediatelyOnInvalidExpr,
+                                 bool ParsingExpansionInitList) {
   bool SawError = false;
   while (true) {
     if (ExpressionStarts)
@@ -3195,7 +3196,11 @@ bool Parser::ParseExpressionList(SmallVectorImpl<Expr *> &Exprs,
       SawError = true;
       if (FailImmediatelyOnInvalidExpr)
         break;
-      SkipUntil(tok::comma, tok::r_paren, StopAtSemi | StopBeforeMatch);
+
+      // We expect '}' rather than ')' at the end of an expansion-init-list.
+      SkipUntil(tok::comma,
+                ParsingExpansionInitList ? tok::r_brace : tok::r_paren,
+                StopAtSemi | StopBeforeMatch);
     } else {
       Exprs.push_back(Expr.get());
     }
@@ -3205,6 +3210,11 @@ bool Parser::ParseExpressionList(SmallVectorImpl<Expr *> &Exprs,
     // Move to the next argument, remember where the comma was.
     Token Comma = Tok;
     ConsumeToken();
+
+    // CWG 3061: Trailing commas are allowed in expansion-init-lists.
+    if (ParsingExpansionInitList && Tok.is(tok::r_brace))
+      break;
+
     checkPotentialAngleBracketDelimiter(Comma);
   }
   return SawError;

clang/lib/Parse/ParseInit.cpp

@@ -516,6 +516,25 @@ ExprResult Parser::ParseBraceInitializer() {
   return ExprError(); // an error occurred.
 }
 
+ExprResult Parser::ParseExpansionInitList() {
+  BalancedDelimiterTracker T(*this, tok::l_brace);
+  T.consumeOpen();
+
+  ExprVector InitExprs;
+
+  if (!Tok.is(tok::r_brace) &&
+      ParseExpressionList(InitExprs, /*ExpressionStarts=*/{},
+                          /*FailImmediatelyOnInvalidExpr=*/false,
+                          /*ParsingExpansionInitList=*/true)) {
+    T.consumeClose();
+    return ExprError();
+  }
+
+  T.consumeClose();
+  return Actions.ActOnCXXExpansionInitList(InitExprs, T.getOpenLocation(),
+                                           T.getCloseLocation());
+}
+
 bool Parser::ParseMicrosoftIfExistsBraceInitializer(ExprVector &InitExprs,
                                                     bool &InitExprsOk) {
   bool trailingComma = false;