[Clang] Implement P0963R3 "Structured binding declaration as a condition" (#130228)

This implements the R2 semantics of P0963.

The R1 semantics, as outlined in the paper, were introduced in Clang 6.
In addition to that, the paper proposes swapping the evaluation order of
condition expressions and the initialization of binding declarations
(i.e. std::tuple-like decompositions).

Author: zyn0217

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

@@ -79,6 +79,8 @@ 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

@@ -5093,7 +5093,7 @@ 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:
@@ -5147,7 +5147,18 @@ 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))
@@ -5160,13 +5171,8 @@ 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;
@@ -5249,6 +5255,9 @@ 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();
@@ -5294,6 +5303,10 @@ 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;
 
@@ -5375,6 +5388,9 @@ bool Compiler<Emitter>::visitForStmt(const ForStmt *S) {
         return false;
     }
 
+    if (!this->maybeEmitDeferredVarInit(S->getConditionVariable()))
+      return false;
+
     if (Body && !this->visitStmt(Body))
       return false;
 
@@ -5497,6 +5513,9 @@ 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);
@@ -389,6 +389,7 @@ 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

@@ -5227,20 +5227,41 @@ 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())
@@ -5260,6 +5281,8 @@ 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();
 }
 
@@ -5344,6 +5367,9 @@ 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;
   }
@@ -5568,7 +5594,8 @@ 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;
   }
@@ -2059,6 +2057,14 @@ 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