Reapply "[Clang][CWG1815] Support lifetime extension of temporary created by aggregate initialization using a default member initializer" (#108039)

The PR reapply #97308. 

- Implement [CWG1815](https://wg21.link/CWG1815): Support lifetime
extension of temporary created by aggregate initialization using a
default member initializer.

- Fix crash that introduced in
#97308. In
`InitListChecker::FillInEmptyInitForField`, when we enter
rebuild-default-init context, we copy all the contents of the parent
context to the current context, which will cause the `MaybeODRUseExprs`
to be lost. But we don't need to copy the entire context, only the
`DelayedDefaultInitializationContext` was required, which is used to
build `SourceLocExpr`, etc.

---------

Signed-off-by: yronglin <[email protected]>

Author: yronglin

clang/docs/ReleaseNotes.rst

@@ -108,6 +108,9 @@ C++ Language Changes
 - Allow single element access of GCC vector/ext_vector_type object to be
   constant expression. Supports the `V.xyzw` syntax and other tidbits
   as seen in OpenCL. Selecting multiple elements is left as a future work.
+- Implement `CWG1815 <https://wg21.link/CWG1815>`_. Support lifetime extension 
+  of temporary created by aggregate initialization using a default member
+  initializer.
 
 - Accept C++26 user-defined ``static_assert`` messages in C++11 as an extension.
 

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -10162,13 +10162,6 @@ def warn_dangling_pointer_assignment : Warning<
    "will be destroyed at the end of the full-expression">,
    InGroup<DanglingAssignment>;
 
-def warn_unsupported_lifetime_extension : Warning<
-  "lifetime extension of "
-  "%select{temporary|backing array of initializer list}0 created "
-  "by aggregate initialization using a default member initializer "
-  "is not yet supported; lifetime of %select{temporary|backing array}0 "
-  "will end at the end of the full-expression">, InGroup<Dangling>;
-
 // For non-floating point, expressions of the form x == x or x != x
 // should result in a warning, since these always evaluate to a constant.
 // Array comparisons have similar warnings

clang/include/clang/Sema/Sema.h

@@ -6403,6 +6403,9 @@ class Sema final : public SemaBase {
     /// example, in a for-range initializer).
     bool InLifetimeExtendingContext = false;
 
+    /// Whether we should rebuild CXXDefaultArgExpr and CXXDefaultInitExpr.
+    bool RebuildDefaultArgOrDefaultInit = false;
+
     // When evaluating immediate functions in the initializer of a default
     // argument or default member initializer, this is the declaration whose
     // default initializer is being evaluated and the location of the call
@@ -7810,9 +7813,11 @@ class Sema final : public SemaBase {
   }
 
   bool isInLifetimeExtendingContext() const {
-    assert(!ExprEvalContexts.empty() &&
-           "Must be in an expression evaluation context");
-    return ExprEvalContexts.back().InLifetimeExtendingContext;
+    return currentEvaluationContext().InLifetimeExtendingContext;
+  }
+
+  bool needsRebuildOfDefaultArgOrInit() const {
+    return currentEvaluationContext().RebuildDefaultArgOrDefaultInit;
   }
 
   bool isCheckingDefaultArgumentOrInitializer() const {
@@ -7854,18 +7859,6 @@ class Sema final : public SemaBase {
     return Res;
   }
 
-  /// keepInLifetimeExtendingContext - Pull down InLifetimeExtendingContext
-  /// flag from previous context.
-  void keepInLifetimeExtendingContext() {
-    if (ExprEvalContexts.size() > 2 &&
-        parentEvaluationContext().InLifetimeExtendingContext) {
-      auto &LastRecord = ExprEvalContexts.back();
-      auto &PrevRecord = parentEvaluationContext();
-      LastRecord.InLifetimeExtendingContext =
-          PrevRecord.InLifetimeExtendingContext;
-    }
-  }
-
   DefaultedComparisonKind getDefaultedComparisonKind(const FunctionDecl *FD) {
     return getDefaultedFunctionKind(FD).asComparison();
   }

clang/lib/Parse/ParseDecl.cpp

@@ -2509,8 +2509,9 @@ Parser::DeclGroupPtrTy Parser::ParseDeclGroup(ParsingDeclSpec &DS,
 
       // P2718R0 - Lifetime extension in range-based for loops.
       if (getLangOpts().CPlusPlus23) {
-        auto &LastRecord = Actions.ExprEvalContexts.back();
+        auto &LastRecord = Actions.currentEvaluationContext();
         LastRecord.InLifetimeExtendingContext = true;
+        LastRecord.RebuildDefaultArgOrDefaultInit = true;
       }
 
       if (getLangOpts().OpenMP)

clang/lib/Sema/CheckExprLifetime.cpp

@@ -896,11 +896,6 @@ static void visitLocalsRetainedByInitializer(IndirectLocalPath &Path,
 enum PathLifetimeKind {
   /// Lifetime-extend along this path.
   Extend,
-  /// We should lifetime-extend, but we don't because (due to technical
-  /// limitations) we can't. This happens for default member initializers,
-  /// which we don't clone for every use, so we don't have a unique
-  /// MaterializeTemporaryExpr to update.
-  ShouldExtend,
   /// Do not lifetime extend along this path.
   NoExtend
 };
@@ -912,7 +907,7 @@ shouldLifetimeExtendThroughPath(const IndirectLocalPath &Path) {
   PathLifetimeKind Kind = PathLifetimeKind::Extend;
   for (auto Elem : Path) {
     if (Elem.Kind == IndirectLocalPathEntry::DefaultInit)
-      Kind = PathLifetimeKind::ShouldExtend;
+      return PathLifetimeKind::Extend;
     else if (Elem.Kind != IndirectLocalPathEntry::LambdaCaptureInit)
       return PathLifetimeKind::NoExtend;
   }
@@ -1058,17 +1053,6 @@ static void checkExprLifetimeImpl(Sema &SemaRef,
         // Also visit the temporaries lifetime-extended by this initializer.
         return true;
 
-      case PathLifetimeKind::ShouldExtend:
-        // We're supposed to lifetime-extend the temporary along this path (per
-        // the resolution of DR1815), but we don't support that yet.
-        //
-        // FIXME: Properly handle this situation. Perhaps the easiest approach
-        // would be to clone the initializer expression on each use that would
-        // lifetime extend its temporaries.
-        SemaRef.Diag(DiagLoc, diag::warn_unsupported_lifetime_extension)
-            << RK << DiagRange;
-        break;
-
       case PathLifetimeKind::NoExtend:
         // If the path goes through the initialization of a variable or field,
         // it can't possibly reach a temporary created in this full-expression.

clang/lib/Sema/SemaExpr.cpp

@@ -5429,6 +5429,8 @@ struct EnsureImmediateInvocationInDefaultArgs
   EnsureImmediateInvocationInDefaultArgs(Sema &SemaRef)
       : TreeTransform(SemaRef) {}
 
+  bool AlwaysRebuild() { return true; }
+
   // Lambda can only have immediate invocations in the default
   // args of their parameters, which is transformed upon calling the closure.
   // The body is not a subexpression, so we have nothing to do.
@@ -5470,7 +5472,7 @@ ExprResult Sema::BuildCXXDefaultArgExpr(SourceLocation CallLoc,
   assert(Param->hasDefaultArg() && "can't build nonexistent default arg");
 
   bool NestedDefaultChecking = isCheckingDefaultArgumentOrInitializer();
-  bool InLifetimeExtendingContext = isInLifetimeExtendingContext();
+  bool NeedRebuild = needsRebuildOfDefaultArgOrInit();
   std::optional<ExpressionEvaluationContextRecord::InitializationContext>
       InitializationContext =
           OutermostDeclarationWithDelayedImmediateInvocations();
@@ -5506,13 +5508,15 @@ ExprResult Sema::BuildCXXDefaultArgExpr(SourceLocation CallLoc,
 
     // Rewrite the call argument that was created from the corresponding
     // parameter's default argument.
-    if (V.HasImmediateCalls || InLifetimeExtendingContext) {
+    if (V.HasImmediateCalls ||
+        (NeedRebuild && isa_and_present<ExprWithCleanups>(Param->getInit()))) {
       if (V.HasImmediateCalls)
         ExprEvalContexts.back().DelayedDefaultInitializationContext = {
             CallLoc, Param, CurContext};
       // Pass down lifetime extending flag, and collect temporaries in
       // CreateMaterializeTemporaryExpr when we rewrite the call argument.
-      keepInLifetimeExtendingContext();
+      currentEvaluationContext().InLifetimeExtendingContext =
+          parentEvaluationContext().InLifetimeExtendingContext;
       EnsureImmediateInvocationInDefaultArgs Immediate(*this);
       ExprResult Res;
       runWithSufficientStackSpace(CallLoc, [&] {
@@ -5558,7 +5562,7 @@ ExprResult Sema::BuildCXXDefaultInitExpr(SourceLocation Loc, FieldDecl *Field) {
   Expr *Init = nullptr;
 
   bool NestedDefaultChecking = isCheckingDefaultArgumentOrInitializer();
-
+  bool NeedRebuild = needsRebuildOfDefaultArgOrInit();
   EnterExpressionEvaluationContext EvalContext(
       *this, ExpressionEvaluationContext::PotentiallyEvaluated, Field);
 
@@ -5593,12 +5597,27 @@ ExprResult Sema::BuildCXXDefaultInitExpr(SourceLocation Loc, FieldDecl *Field) {
   ImmediateCallVisitor V(getASTContext());
   if (!NestedDefaultChecking)
     V.TraverseDecl(Field);
-  if (V.HasImmediateCalls) {
+
+  // CWG1815
+  // Support lifetime extension of temporary created by aggregate
+  // initialization using a default member initializer. We should rebuild
+  // the initializer in a lifetime extension context if the initializer
+  // expression is an ExprWithCleanups. Then make sure the normal lifetime
+  // extension code recurses into the default initializer and does lifetime
+  // extension when warranted.
+  bool ContainsAnyTemporaries =
+      isa_and_present<ExprWithCleanups>(Field->getInClassInitializer());
+  if (Field->getInClassInitializer() &&
+      !Field->getInClassInitializer()->containsErrors() &&
+      (V.HasImmediateCalls || (NeedRebuild && ContainsAnyTemporaries))) {
     ExprEvalContexts.back().DelayedDefaultInitializationContext = {Loc, Field,
                                                                    CurContext};
     ExprEvalContexts.back().IsCurrentlyCheckingDefaultArgumentOrInitializer =
         NestedDefaultChecking;
-
+    // Pass down lifetime extending flag, and collect temporaries in
+    // CreateMaterializeTemporaryExpr when we rewrite the call argument.
+    currentEvaluationContext().InLifetimeExtendingContext =
+        parentEvaluationContext().InLifetimeExtendingContext;
     EnsureImmediateInvocationInDefaultArgs Immediate(*this);
     ExprResult Res;
     runWithSufficientStackSpace(Loc, [&] {
@@ -17675,11 +17694,10 @@ void Sema::PopExpressionEvaluationContext() {
 
   // Append the collected materialized temporaries into previous context before
   // exit if the previous also is a lifetime extending context.
-  auto &PrevRecord = parentEvaluationContext();
   if (getLangOpts().CPlusPlus23 && Rec.InLifetimeExtendingContext &&
-      PrevRecord.InLifetimeExtendingContext &&
+      parentEvaluationContext().InLifetimeExtendingContext &&
       !Rec.ForRangeLifetimeExtendTemps.empty()) {
-    PrevRecord.ForRangeLifetimeExtendTemps.append(
+    parentEvaluationContext().ForRangeLifetimeExtendTemps.append(
         Rec.ForRangeLifetimeExtendTemps);
   }
 

clang/lib/Sema/SemaExprCXX.cpp

@@ -1540,9 +1540,6 @@ Sema::BuildCXXTypeConstructExpr(TypeSourceInfo *TInfo,
                                 bool ListInitialization) {
   QualType Ty = TInfo->getType();
   SourceLocation TyBeginLoc = TInfo->getTypeLoc().getBeginLoc();
-
-  assert((!ListInitialization || Exprs.size() == 1) &&
-         "List initialization must have exactly one expression.");
   SourceRange FullRange = SourceRange(TyBeginLoc, RParenOrBraceLoc);
 
   InitializedEntity Entity =

clang/lib/Sema/SemaInit.cpp

@@ -750,8 +750,21 @@ void InitListChecker::FillInEmptyInitForField(unsigned Init, FieldDecl *Field,
     if (Field->hasInClassInitializer()) {
       if (VerifyOnly)
         return;
-
-      ExprResult DIE = SemaRef.BuildCXXDefaultInitExpr(Loc, Field);
+      ExprResult DIE;
+      {
+        // Enter a default initializer rebuild context, then we can support
+        // lifetime extension of temporary created by aggregate initialization
+        // using a default member initializer.
+        // CWG1815 (https://wg21.link/CWG1815).
+        EnterExpressionEvaluationContext RebuildDefaultInit(
+            SemaRef, Sema::ExpressionEvaluationContext::PotentiallyEvaluated);
+        SemaRef.currentEvaluationContext().RebuildDefaultArgOrDefaultInit =
+            true;
+        SemaRef.currentEvaluationContext().DelayedDefaultInitializationContext =
+            SemaRef.parentEvaluationContext()
+                .DelayedDefaultInitializationContext;
+        DIE = SemaRef.BuildCXXDefaultInitExpr(Loc, Field);
+      }
       if (DIE.isInvalid()) {
         hadError = true;
         return;
@@ -7521,10 +7534,8 @@ Sema::CreateMaterializeTemporaryExpr(QualType T, Expr *Temporary,
   // are done in both CreateMaterializeTemporaryExpr and MaybeBindToTemporary,
   // but there may be a chance to merge them.
   Cleanup.setExprNeedsCleanups(false);
-  if (isInLifetimeExtendingContext()) {
-    auto &Record = ExprEvalContexts.back();
-    Record.ForRangeLifetimeExtendTemps.push_back(MTE);
-  }
+  if (isInLifetimeExtendingContext())
+    currentEvaluationContext().ForRangeLifetimeExtendTemps.push_back(MTE);
   return MTE;
 }
 

clang/lib/Sema/SemaTemplateInstantiateDecl.cpp

@@ -5481,7 +5481,10 @@ void Sema::InstantiateVariableInitializer(
     EnterExpressionEvaluationContext Evaluated(
         *this, Sema::ExpressionEvaluationContext::PotentiallyEvaluated, Var);
 
-    keepInLifetimeExtendingContext();
+    currentEvaluationContext().InLifetimeExtendingContext =
+        parentEvaluationContext().InLifetimeExtendingContext;
+    currentEvaluationContext().RebuildDefaultArgOrDefaultInit =
+        parentEvaluationContext().RebuildDefaultArgOrDefaultInit;
     // Instantiate the initializer.
     ExprResult Init;
 

clang/lib/Sema/TreeTransform.h

@@ -4254,7 +4254,10 @@ ExprResult TreeTransform<Derived>::TransformInitializer(Expr *Init,
       getSema(), EnterExpressionEvaluationContext::InitList,
       Construct->isListInitialization());
 
-  getSema().keepInLifetimeExtendingContext();
+  getSema().currentEvaluationContext().InLifetimeExtendingContext =
+      getSema().parentEvaluationContext().InLifetimeExtendingContext;
+  getSema().currentEvaluationContext().RebuildDefaultArgOrDefaultInit =
+      getSema().parentEvaluationContext().RebuildDefaultArgOrDefaultInit;
   SmallVector<Expr*, 8> NewArgs;
   bool ArgChanged = false;
   if (getDerived().TransformExprs(Construct->getArgs(), Construct->getNumArgs(),
@@ -8924,8 +8927,9 @@ TreeTransform<Derived>::TransformCXXForRangeStmt(CXXForRangeStmt *S) {
 
   // P2718R0 - Lifetime extension in range-based for loops.
   if (getSema().getLangOpts().CPlusPlus23) {
-    auto &LastRecord = getSema().ExprEvalContexts.back();
+    auto &LastRecord = getSema().currentEvaluationContext();
     LastRecord.InLifetimeExtendingContext = true;
+    LastRecord.RebuildDefaultArgOrDefaultInit = true;
   }
   StmtResult Init =
       S->getInit() ? getDerived().TransformStmt(S->getInit()) : StmtResult();
@@ -14443,6 +14447,13 @@ TreeTransform<Derived>::TransformCXXTemporaryObjectExpr(
     if (TransformExprs(E->getArgs(), E->getNumArgs(), true, Args,
                        &ArgumentChanged))
       return ExprError();
+
+    if (E->isListInitialization() && !E->isStdInitListInitialization()) {
+      ExprResult Res = RebuildInitList(E->getBeginLoc(), Args, E->getEndLoc());
+      if (Res.isInvalid())
+        return ExprError();
+      Args = {Res.get()};
+    }
   }
 
   if (!getDerived().AlwaysRebuild() &&
@@ -14454,12 +14465,9 @@ TreeTransform<Derived>::TransformCXXTemporaryObjectExpr(
     return SemaRef.MaybeBindToTemporary(E);
   }
 
-  // FIXME: We should just pass E->isListInitialization(), but we're not
-  // prepared to handle list-initialization without a child InitListExpr.
   SourceLocation LParenLoc = T->getTypeLoc().getEndLoc();
   return getDerived().RebuildCXXTemporaryObjectExpr(
-      T, LParenLoc, Args, E->getEndLoc(),
-      /*ListInitialization=*/LParenLoc.isInvalid());
+      T, LParenLoc, Args, E->getEndLoc(), E->isListInitialization());
 }
 
 template<typename Derived>

clang/test/AST/ast-dump-default-init-json.cpp

@@ -789,10 +789,10 @@ void test() {
 // CHECK-NEXT:                  "valueCategory": "lvalue",
 // CHECK-NEXT:                  "extendingDecl": {
 // CHECK-NEXT:                   "id": "0x{{.*}}",
-// CHECK-NEXT:                   "kind": "FieldDecl",
-// CHECK-NEXT:                   "name": "a",
+// CHECK-NEXT:                   "kind": "VarDecl",
+// CHECK-NEXT:                   "name": "b",
 // CHECK-NEXT:                   "type": {
-// CHECK-NEXT:                    "qualType": "const A &"
+// CHECK-NEXT:                    "qualType": "B"
 // CHECK-NEXT:                   }
 // CHECK-NEXT:                  },
 // CHECK-NEXT:                  "storageDuration": "automatic",

clang/test/AST/ast-dump-default-init.cpp

@@ -13,7 +13,7 @@ void test() {
 }
 // CHECK: -CXXDefaultInitExpr 0x{{[^ ]*}} <{{.*}}> 'const A' lvalue has rewritten init
 // CHECK-NEXT:  `-ExprWithCleanups 0x{{[^ ]*}} <{{.*}}> 'const A' lvalue
-// CHECK-NEXT:    `-MaterializeTemporaryExpr 0x{{[^ ]*}} <{{.*}}> 'const A' lvalue extended by Field 0x{{[^ ]*}} 'a' 'const A &'
+// CHECK-NEXT:    `-MaterializeTemporaryExpr 0x{{[^ ]*}} <{{.*}}> 'const A' lvalue extended by Var 0x{{[^ ]*}} 'b' 'B'
 // CHECK-NEXT:      `-ImplicitCastExpr 0x{{[^ ]*}} <{{.*}}> 'const A' <NoOp>
 // CHECK-NEXT:        `-CXXFunctionalCastExpr 0x{{[^ ]*}} <{{.*}}> 'A' functional cast to A <NoOp>
 // CHECK-NEXT:          `-InitListExpr 0x{{[^ ]*}} <{{.*}}> 'A'

clang/test/Analysis/lifetime-extended-regions.cpp

@@ -120,10 +120,11 @@ void aggregateWithReferences() {
   clang_analyzer_dump(viaReference);    // expected-warning-re {{&lifetime_extended_object{RefAggregate, viaReference, S{{[0-9]+}}} }}
   clang_analyzer_dump(viaReference.rx); // expected-warning-re {{&lifetime_extended_object{int, viaReference, S{{[0-9]+}}} }}
   clang_analyzer_dump(viaReference.ry); // expected-warning-re {{&lifetime_extended_object{Composite, viaReference, S{{[0-9]+}}} }}
-
-  // clang does not currently implement extending lifetime of object bound to reference members of aggregates,
-  // that are created from default member initializer (see `warn_unsupported_lifetime_extension` from `-Wdangling`)
-  RefAggregate defaultInitExtended{i}; // clang-bug does not extend `Composite`
+  
+  // FIXME: clang currently support extending lifetime of object bound to reference members of aggregates,
+  // that are created from default member initializer. But CFG and ExprEngine need to be updated to address this change.
+  // The following expect warning: {{&lifetime_extended_object{Composite, defaultInitExtended, S{{[0-9]+}}} }}
+  RefAggregate defaultInitExtended{i};
   clang_analyzer_dump(defaultInitExtended.ry); // expected-warning {{Unknown }}
 }
 

clang/test/CXX/drs/cwg16xx.cpp

@@ -449,6 +449,27 @@ namespace cwg1696 { // cwg1696: 7
       //   since-cxx14-note@-2 {{default member initializer declared here}}
     };
     A a{a, a};
+
+    struct A1 {
+      A1() : v(42) {}
+      // since-cxx14-error@-1 {{reference member 'v' binds to a temporary object whose lifetime would be shorter than the lifetime of the constructed object}}
+      // since-cxx14-note@#cwg1696-A1 {{reference member declared here}}
+      const int &v; // #cwg1696-A1
+    };
+
+    struct A2 {
+      A2() = default;
+      // since-cxx14-error@-1 {{reference member 'v' binds to a temporary object whose lifetime would be shorter than the lifetime of the constructed object}}
+      // since-cxx14-note-re@#cwg1696-A2-b {{in defaulted default constructor for {{.*}} first required here}}
+      // since-cxx14-note@#cwg1696-A2-a {{initializing field 'v' with default member initializer}}
+      A2(int v) : v(v) {}
+      // since-cxx14-warning@-1 {{binding reference member 'v' to stack allocated parameter 'v'}}
+      // since-cxx14-note@#cwg1696-A2-a {{reference member declared here}}
+      const int &v = 42;  // #cwg1696-A2-a
+    };
+    A2 a1;    // #cwg1696-A2-b
+    
+    A2 a2(1); // OK, unfortunately
 #endif
   }
 
@@ -483,8 +504,6 @@ namespace cwg1696 { // cwg1696: 7
     const A &a = A(); // #cwg1696-D1-a
   };
   D1 d1 = {}; // #cwg1696-d1
-  // since-cxx14-warning@-1 {{lifetime extension of temporary created by aggregate initialization using a default member initializer is not yet supported; lifetime of temporary will end at the end of the full-expression}}
-  //   since-cxx14-note@#cwg1696-D1-a {{initializing field 'a' with default member initializer}}
 
   struct D2 {
     const A &a = A(); // #cwg1696-D2-a