[Clang] Implement P0963R3 "Structured binding declaration as a condition"

clang/docs/LanguageExtensions.rst

@@ -1604,53 +1604,54 @@ More information could be found `here <https://clang.llvm.org/docs/Modules.html>
 Language Extensions Back-ported to Previous Standards
 =====================================================
 
-============================================ ================================ ============= =============
-Feature                                      Feature Test Macro               Introduced In Backported To
-============================================ ================================ ============= =============
-variadic templates                           __cpp_variadic_templates         C++11         C++03
-Alias templates                              __cpp_alias_templates            C++11         C++03
-Non-static data member initializers          __cpp_nsdmi                      C++11         C++03
-Range-based ``for`` loop                     __cpp_range_based_for            C++11         C++03
-RValue references                            __cpp_rvalue_references          C++11         C++03
-Attributes                                   __cpp_attributes                 C++11         C++03
-Lambdas                                      __cpp_lambdas                    C++11         C++03
-Generalized lambda captures                  __cpp_init_captures              C++14         C++03
-Generic lambda expressions                   __cpp_generic_lambdas            C++14         C++03
-variable templates                           __cpp_variable_templates         C++14         C++03
-Binary literals                              __cpp_binary_literals            C++14         C++03
-Relaxed constexpr                            __cpp_constexpr                  C++14         C++11
-Static assert with no message                __cpp_static_assert >= 201411L   C++17         C++11
-Pack expansion in generalized lambda-capture __cpp_init_captures              C++17         C++03
-``if constexpr``                             __cpp_if_constexpr               C++17         C++11
-fold expressions                             __cpp_fold_expressions           C++17         C++03
-Lambda capture of \*this by value            __cpp_capture_star_this          C++17         C++03
-Attributes on enums                          __cpp_enumerator_attributes      C++17         C++03
-Guaranteed copy elision                      __cpp_guaranteed_copy_elision    C++17         C++03
-Hexadecimal floating literals                __cpp_hex_float                  C++17         C++03
-``inline`` variables                         __cpp_inline_variables           C++17         C++03
-Attributes on namespaces                     __cpp_namespace_attributes       C++17         C++11
-Structured bindings                          __cpp_structured_bindings        C++17         C++03
-template template arguments                  __cpp_template_template_args     C++17         C++03
-Familiar template syntax for generic lambdas __cpp_generic_lambdas            C++20         C++03
-``static operator[]``                        __cpp_multidimensional_subscript C++20         C++03
-Designated initializers                      __cpp_designated_initializers    C++20         C++03
-Conditional ``explicit``                     __cpp_conditional_explicit       C++20         C++03
-``using enum``                               __cpp_using_enum                 C++20         C++03
-``if consteval``                             __cpp_if_consteval               C++23         C++20
-``static operator()``                        __cpp_static_call_operator       C++23         C++03
-Attributes on Lambda-Expressions                                              C++23         C++11
-Attributes on Structured Bindings            __cpp_structured_bindings        C++26         C++03
-Packs in Structured Bindings                 __cpp_structured_bindings        C++26         C++03
-Static assert with user-generated message    __cpp_static_assert >= 202306L   C++26         C++11
-Pack Indexing                                __cpp_pack_indexing              C++26         C++03
-``= delete ("should have a reason");``       __cpp_deleted_function           C++26         C++03
-Variadic Friends                             __cpp_variadic_friend            C++26         C++03
--------------------------------------------- -------------------------------- ------------- -------------
-Designated initializers (N494)                                                C99           C89
-Array & element qualification (N2607)                                         C23           C89
-Attributes (N2335)                                                            C23           C89
-``#embed`` (N3017)                                                            C23           C89, C++
-============================================ ================================ ============= =============
+============================================= ================================ ============= =============
+Feature                                       Feature Test Macro               Introduced In Backported To
+============================================= ================================ ============= =============
+variadic templates                            __cpp_variadic_templates         C++11         C++03
+Alias templates                               __cpp_alias_templates            C++11         C++03
+Non-static data member initializers           __cpp_nsdmi                      C++11         C++03
+Range-based ``for`` loop                      __cpp_range_based_for            C++11         C++03
+RValue references                             __cpp_rvalue_references          C++11         C++03
+Attributes                                    __cpp_attributes                 C++11         C++03
+Lambdas                                       __cpp_lambdas                    C++11         C++03
+Generalized lambda captures                   __cpp_init_captures              C++14         C++03
+Generic lambda expressions                    __cpp_generic_lambdas            C++14         C++03
+variable templates                            __cpp_variable_templates         C++14         C++03
+Binary literals                               __cpp_binary_literals            C++14         C++03
+Relaxed constexpr                             __cpp_constexpr                  C++14         C++11
+Static assert with no message                 __cpp_static_assert >= 201411L   C++17         C++11
+Pack expansion in generalized lambda-capture  __cpp_init_captures              C++17         C++03
+``if constexpr``                              __cpp_if_constexpr               C++17         C++11
+fold expressions                              __cpp_fold_expressions           C++17         C++03
+Lambda capture of \*this by value             __cpp_capture_star_this          C++17         C++03
+Attributes on enums                           __cpp_enumerator_attributes      C++17         C++03
+Guaranteed copy elision                       __cpp_guaranteed_copy_elision    C++17         C++03
+Hexadecimal floating literals                 __cpp_hex_float                  C++17         C++03
+``inline`` variables                          __cpp_inline_variables           C++17         C++03
+Attributes on namespaces                      __cpp_namespace_attributes       C++17         C++11
+Structured bindings                           __cpp_structured_bindings        C++17         C++03
+template template arguments                   __cpp_template_template_args     C++17         C++03
+Familiar template syntax for generic lambdas  __cpp_generic_lambdas            C++20         C++03
+``static operator[]``                         __cpp_multidimensional_subscript C++20         C++03
+Designated initializers                       __cpp_designated_initializers    C++20         C++03
+Conditional ``explicit``                      __cpp_conditional_explicit       C++20         C++03
+``using enum``                                __cpp_using_enum                 C++20         C++03
+``if consteval``                              __cpp_if_consteval               C++23         C++20
+``static operator()``                         __cpp_static_call_operator       C++23         C++03
+Attributes on Lambda-Expressions                                               C++23         C++11
+Attributes on Structured Bindings             __cpp_structured_bindings        C++26         C++03
+Packs in Structured Bindings                  __cpp_structured_bindings        C++26         C++03
+Structured binding declaration as a condition __cpp_structured_bindings        C++26         C++98
+Static assert with user-generated message     __cpp_static_assert >= 202306L   C++26         C++11
+Pack Indexing                                 __cpp_pack_indexing              C++26         C++03
+``= delete ("should have a reason");``        __cpp_deleted_function           C++26         C++03
+Variadic Friends                              __cpp_variadic_friend            C++26         C++03
+--------------------------------------------- -------------------------------- ------------- -------------
+Designated initializers (N494)                                                 C99           C89
+Array & element qualification (N2607)                                          C23           C89
+Attributes (N2335)                                                             C23           C89
+``#embed`` (N3017)                                                             C23           C89, C++
+============================================= ================================ ============= =============
 
 Builtin type aliases
 ====================

clang/docs/ReleaseNotes.rst

@@ -76,6 +76,8 @@ C++2c Feature Support
 
 - Implemented `P1061R10 Structured Bindings can introduce a Pack <https://wg21.link/P1061R10>`_.
 
+- Implemented `P0963R3 Structured binding declaration as a condition <https://wg21.link/P0963R3>`_.
+
 C++23 Feature Support
 ^^^^^^^^^^^^^^^^^^^^^
 

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -529,8 +529,12 @@ def warn_cxx14_compat_decomp_decl : Warning<
 def ext_decomp_decl : ExtWarn<
   "decomposition declarations are a C++17 extension">, InGroup<CXX17>;
 def ext_decomp_decl_cond : ExtWarn<
-  "ISO C++17 does not permit structured binding declaration in a condition">,
-  InGroup<DiagGroup<"binding-in-condition">>;
+  "structured binding declaration in a condition is a C++2c extenstion">,
+  InGroup<CXX26>;
+def warn_cxx26_decomp_decl_cond : Warning<
+  "structured binding declaration in a condition is incompatible with "
+  "C++ standards before C++2c">,
+  InGroup<CXXPre26Compat>, DefaultIgnore;
 def err_decomp_decl_spec : Error<
   "decomposition declaration cannot be declared "
   "%plural{1:'%1'|:with '%1' specifiers}0">;

clang/lib/AST/ByteCode/Compiler.cpp

@@ -5051,7 +5051,7 @@ template <class Emitter> bool Compiler<Emitter>::visitStmt(const Stmt *S) {
   case Stmt::CompoundStmtClass:
     return visitCompoundStmt(cast<CompoundStmt>(S));
   case Stmt::DeclStmtClass:
-    return visitDeclStmt(cast<DeclStmt>(S));
+    return visitDeclStmt(cast<DeclStmt>(S), /*EvaluateConditionDecl=*/true);
   case Stmt::ReturnStmtClass:
     return visitReturnStmt(cast<ReturnStmt>(S));
   case Stmt::IfStmtClass:
@@ -5105,7 +5105,18 @@ bool Compiler<Emitter>::visitCompoundStmt(const CompoundStmt *S) {
 }
 
 template <class Emitter>
-bool Compiler<Emitter>::visitDeclStmt(const DeclStmt *DS) {
+bool Compiler<Emitter>::maybeEmitDeferredVarInit(const VarDecl *VD) {
+  if (auto *DD = dyn_cast_if_present<DecompositionDecl>(VD)) {
+    for (auto *BD : DD->bindings())
+      if (auto *KD = BD->getHoldingVar(); KD && !this->visitVarDecl(KD))
+        return false;
+  }
+  return true;
+}
+
+template <class Emitter>
+bool Compiler<Emitter>::visitDeclStmt(const DeclStmt *DS,
+                                      bool EvaluateConditionDecl) {
   for (const auto *D : DS->decls()) {
     if (isa<StaticAssertDecl, TagDecl, TypedefNameDecl, BaseUsingDecl,
             FunctionDecl, NamespaceAliasDecl, UsingDirectiveDecl>(D))
@@ -5118,13 +5129,8 @@ bool Compiler<Emitter>::visitDeclStmt(const DeclStmt *DS) {
       return false;
 
     // Register decomposition decl holding vars.
-    if (const auto *DD = dyn_cast<DecompositionDecl>(VD)) {
-      for (auto *BD : DD->bindings())
-        if (auto *KD = BD->getHoldingVar()) {
-          if (!this->visitVarDecl(KD))
-            return false;
-        }
-    }
+    if (EvaluateConditionDecl && !this->maybeEmitDeferredVarInit(VD))
+      return false;
   }
 
   return true;
@@ -5189,6 +5195,9 @@ template <class Emitter> bool Compiler<Emitter>::visitIfStmt(const IfStmt *IS) {
       return false;
   }
 
+  if (!this->maybeEmitDeferredVarInit(IS->getConditionVariable()))
+    return false;
+
   if (const Stmt *Else = IS->getElse()) {
     LabelTy LabelElse = this->getLabel();
     LabelTy LabelEnd = this->getLabel();
@@ -5249,6 +5258,10 @@ bool Compiler<Emitter>::visitWhileStmt(const WhileStmt *S) {
 
     if (!this->visitBool(Cond))
       return false;
+
+    if (!this->maybeEmitDeferredVarInit(S->getConditionVariable()))
+      return false;
+
     if (!this->jumpFalse(EndLabel))
       return false;
 
@@ -5330,6 +5343,9 @@ bool Compiler<Emitter>::visitForStmt(const ForStmt *S) {
         return false;
     }
 
+    if (!this->maybeEmitDeferredVarInit(S->getConditionVariable()))
+      return false;
+
     if (Body && !this->visitStmt(Body))
       return false;
 
@@ -5452,6 +5468,9 @@ bool Compiler<Emitter>::visitSwitchStmt(const SwitchStmt *S) {
   if (!this->emitSetLocal(CondT, CondVar, S))
     return false;
 
+  if (!this->maybeEmitDeferredVarInit(S->getConditionVariable()))
+    return false;
+
   CaseMap CaseLabels;
   // Create labels and comparison ops for all case statements.
   for (const SwitchCase *SC = S->getSwitchCaseList(); SC;

clang/lib/AST/ByteCode/Compiler.h

@@ -213,7 +213,7 @@ class Compiler : public ConstStmtVisitor<Compiler<Emitter>, bool>,
 
   // Statements.
   bool visitCompoundStmt(const CompoundStmt *S);
-  bool visitDeclStmt(const DeclStmt *DS);
+  bool visitDeclStmt(const DeclStmt *DS, bool EvaluateConditionDecl = false);
   bool visitReturnStmt(const ReturnStmt *RS);
   bool visitIfStmt(const IfStmt *IS);
   bool visitWhileStmt(const WhileStmt *S);
@@ -386,6 +386,7 @@ class Compiler : public ConstStmtVisitor<Compiler<Emitter>, bool>,
   bool compileUnionAssignmentOperator(const CXXMethodDecl *MD);
 
   bool checkLiteralType(const Expr *E);
+  bool maybeEmitDeferredVarInit(const VarDecl *VD);
 
 protected:
   /// Variable to storage mapping.

clang/lib/AST/ExprConstant.cpp

@@ -5218,20 +5218,41 @@ static bool EvaluateVarDecl(EvalInfo &Info, const VarDecl *VD) {
   return true;
 }
 
-static bool EvaluateDecl(EvalInfo &Info, const Decl *D) {
-  bool OK = true;
+static bool EvaluateDecompositionDeclInit(EvalInfo &Info,
+                                          const DecompositionDecl *DD);
 
+static bool EvaluateDecl(EvalInfo &Info, const Decl *D,
+                         bool EvaluateConditionDecl = false) {
+  bool OK = true;
   if (const VarDecl *VD = dyn_cast<VarDecl>(D))
     OK &= EvaluateVarDecl(Info, VD);
 
-  if (const DecompositionDecl *DD = dyn_cast<DecompositionDecl>(D))
-    for (auto *BD : DD->flat_bindings())
-      if (auto *VD = BD->getHoldingVar())
-        OK &= EvaluateDecl(Info, VD);
+  if (const DecompositionDecl *DD = dyn_cast<DecompositionDecl>(D);
+      EvaluateConditionDecl && DD)
+    OK &= EvaluateDecompositionDeclInit(Info, DD);
+
+  return OK;
+}
+
+static bool EvaluateDecompositionDeclInit(EvalInfo &Info,
+                                          const DecompositionDecl *DD) {
+  bool OK = true;
+  for (auto *BD : DD->flat_bindings())
+    if (auto *VD = BD->getHoldingVar())
+      OK &= EvaluateDecl(Info, VD, /*EvaluateConditionDecl=*/true);
 
   return OK;
 }
 
+static bool MaybeEvaluateDeferredVarDeclInit(EvalInfo &Info,
+                                             const VarDecl *VD) {
+  if (auto *DD = dyn_cast_if_present<DecompositionDecl>(VD)) {
+    if (!EvaluateDecompositionDeclInit(Info, DD))
+      return false;
+  }
+  return true;
+}
+
 static bool EvaluateDependentExpr(const Expr *E, EvalInfo &Info) {
   assert(E->isValueDependent());
   if (Info.noteSideEffect())
@@ -5251,6 +5272,8 @@ static bool EvaluateCond(EvalInfo &Info, const VarDecl *CondDecl,
     return false;
   if (!EvaluateAsBooleanCondition(Cond, Result, Info))
     return false;
+  if (!MaybeEvaluateDeferredVarDeclInit(Info, CondDecl))
+    return false;
   return Scope.destroy();
 }
 
@@ -5335,6 +5358,9 @@ static EvalStmtResult EvaluateSwitch(StmtResult &Result, EvalInfo &Info,
     if (!EvaluateInteger(SS->getCond(), Value, Info))
       return ESR_Failed;
 
+    if (!MaybeEvaluateDeferredVarDeclInit(Info, SS->getConditionVariable()))
+      return ESR_Failed;
+
     if (!CondScope.destroy())
       return ESR_Failed;
   }
@@ -5559,7 +5585,8 @@ static EvalStmtResult EvaluateStmt(StmtResult &Result, EvalInfo &Info,
         return ESR_Failed;
       // Each declaration initialization is its own full-expression.
       FullExpressionRAII Scope(Info);
-      if (!EvaluateDecl(Info, D) && !Info.noteFailure())
+      if (!EvaluateDecl(Info, D, /*EvaluateConditionDecl=*/true) &&
+          !Info.noteFailure())
         return ESR_Failed;
       if (!Scope.destroy())
         return ESR_Failed;

clang/lib/CodeGen/CGDecl.cpp

@@ -48,7 +48,7 @@ using namespace CodeGen;
 static_assert(clang::Sema::MaximumAlignment <= llvm::Value::MaximumAlignment,
               "Clang max alignment greater than what LLVM supports?");
 
-void CodeGenFunction::EmitDecl(const Decl &D) {
+void CodeGenFunction::EmitDecl(const Decl &D, bool EvaluateConditionDecl) {
   switch (D.getKind()) {
   case Decl::BuiltinTemplate:
   case Decl::TranslationUnit:
@@ -152,7 +152,7 @@ void CodeGenFunction::EmitDecl(const Decl &D) {
     return;
   case Decl::UsingPack:
     for (auto *Using : cast<UsingPackDecl>(D).expansions())
-      EmitDecl(*Using);
+      EmitDecl(*Using, /*EvaluateConditionDecl=*/EvaluateConditionDecl);
     return;
   case Decl::UsingDirective: // using namespace X; [C++]
     if (CGDebugInfo *DI = getDebugInfo())
@@ -164,10 +164,8 @@ void CodeGenFunction::EmitDecl(const Decl &D) {
     assert(VD.isLocalVarDecl() &&
            "Should not see file-scope variables inside a function!");
     EmitVarDecl(VD);
-    if (auto *DD = dyn_cast<DecompositionDecl>(&VD))
-      for (auto *B : DD->flat_bindings())
-        if (auto *HD = B->getHoldingVar())
-          EmitVarDecl(*HD);
+    if (EvaluateConditionDecl)
+      MaybeEmitDeferredVarDeclInit(&VD);
 
     return;
   }
@@ -2057,6 +2055,14 @@ void CodeGenFunction::EmitAutoVarInit(const AutoVarEmission &emission) {
                         /*IsAutoInit=*/false);
 }
 
+void CodeGenFunction::MaybeEmitDeferredVarDeclInit(const VarDecl *VD) {
+  if (auto *DD = dyn_cast_if_present<DecompositionDecl>(VD)) {
+    for (auto *B : DD->flat_bindings())
+      if (auto *HD = B->getHoldingVar())
+        EmitVarDecl(*HD);
+  }
+}
+
 /// Emit an expression as an initializer for an object (variable, field, etc.)
 /// at the given location.  The expression is not necessarily the normal
 /// initializer for the object, and the address is not necessarily