[Clang][CWG1815] Support lifetime extension of temporary created by aggregate initialization using a default member initializer (#87933)

This PR complete [DR1815](https://wg21.link/CWG1815) under the guidance
of `FIXME` comments. And reuse `CXXDefaultInitExpr` rewrite machinery to
clone the initializer expression on each use that would lifetime extend
its temporaries.

---------

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

Author: yronglin

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -10035,12 +10035,6 @@ def warn_new_dangling_initializer_list : Warning<
   "the allocated initializer list}0 "
   "will be destroyed at the end of the full-expression">,
   InGroup<DanglingInitializerList>;
-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.

clang/lib/Sema/SemaExpr.cpp

@@ -5777,10 +5777,9 @@ ExprResult Sema::BuildCXXDefaultArgExpr(SourceLocation CallLoc,
         Res = Immediate.TransformInitializer(Param->getInit(),
                                              /*NotCopy=*/false);
       });
-      if (Res.isInvalid())
-        return ExprError();
-      Res = ConvertParamDefaultArgument(Param, Res.get(),
-                                        Res.get()->getBeginLoc());
+      if (Res.isUsable())
+        Res = ConvertParamDefaultArgument(Param, Res.get(),
+                                          Res.get()->getBeginLoc());
       if (Res.isInvalid())
         return ExprError();
       Init = Res.get();
@@ -5816,7 +5815,7 @@ ExprResult Sema::BuildCXXDefaultInitExpr(SourceLocation Loc, FieldDecl *Field) {
   Expr *Init = nullptr;
 
   bool NestedDefaultChecking = isCheckingDefaultArgumentOrInitializer();
-
+  bool InLifetimeExtendingContext = isInLifetimeExtendingContext();
   EnterExpressionEvaluationContext EvalContext(
       *this, ExpressionEvaluationContext::PotentiallyEvaluated, Field);
 
@@ -5851,19 +5850,35 @@ 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 always rebuild
+  // the initializer if it contains any temporaries (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 (V.HasImmediateCalls || InLifetimeExtendingContext ||
+      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.
+    keepInLifetimeExtendingContext();
     EnsureImmediateInvocationInDefaultArgs Immediate(*this);
     ExprResult Res;
+
+    // Rebuild CXXDefaultInitExpr might cause diagnostics.
+    SFINAETrap Trap(*this);
     runWithSufficientStackSpace(Loc, [&] {
       Res = Immediate.TransformInitializer(Field->getInClassInitializer(),
                                            /*CXXDirectInit=*/false);
     });
-    if (!Res.isInvalid())
+    if (Res.isUsable())
       Res = ConvertMemberDefaultInitExpression(Field, Res.get(), Loc);
     if (Res.isInvalid()) {
       Field->setInvalidDecl();

clang/lib/Sema/SemaInit.cpp

@@ -8065,11 +8065,6 @@ 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
 };
@@ -8081,7 +8076,7 @@ shouldLifetimeExtendThroughPath(const IndirectLocalPath &Path) {
   PathLifetimeKind Kind = PathLifetimeKind::Extend;
   for (auto Elem : Path) {
     if (Elem.Kind == IndirectLocalPathEntry::DefaultInit)
-      Kind = PathLifetimeKind::ShouldExtend;
+      Kind = PathLifetimeKind::Extend;
     else if (Elem.Kind != IndirectLocalPathEntry::LambdaCaptureInit)
       return PathLifetimeKind::NoExtend;
   }
@@ -8201,18 +8196,6 @@ void Sema::checkInitializerLifetime(const InitializedEntity &Entity,
                               ExtendingEntity->allocateManglingNumber());
         // 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.
-        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/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

@@ -483,8 +483,6 @@ 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

clang/test/CXX/drs/cwg18xx.cpp

@@ -206,19 +206,28 @@ namespace cwg1814 { // cwg1814: yes
 #endif
 }
 
-namespace cwg1815 { // cwg1815: no
+namespace cwg1815 { // cwg1815: yes
 #if __cplusplus >= 201402L
-  // FIXME: needs codegen test
-  struct A { int &&r = 0; }; // #cwg1815-A 
+  struct A { int &&r = 0; };
   A a = {};
-  // 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}} FIXME
-  //   since-cxx14-note@#cwg1815-A {{initializing field 'r' with default member initializer}}
 
   struct B { int &&r = 0; }; // #cwg1815-B
   // since-cxx14-error@-1 {{reference member 'r' binds to a temporary object whose lifetime would be shorter than the lifetime of the constructed object}}
   //   since-cxx14-note@#cwg1815-B {{initializing field 'r' with default member initializer}}
   //   since-cxx14-note@#cwg1815-b {{in implicit default constructor for 'cwg1815::B' first required here}}
   B b; // #cwg1815-b
+
+#if __cplusplus >= 201703L
+  struct C { const int &r = 0; };
+  constexpr C c = {}; // OK, since cwg1815
+  static_assert(c.r == 0);
+
+  constexpr int f() {
+    A a = {}; // OK, since cwg1815
+    return a.r;
+  }
+  static_assert(f() == 0);
+#endif
 #endif
 }
 

clang/test/CXX/special/class.temporary/p6.cpp

@@ -269,6 +269,40 @@ void init_capture_init_list() {
   // CHECK: }
 }
 
+void check_dr1815() { // dr1815: yes
+#if __cplusplus >= 201402L
+
+  struct A {
+    int &&r = 0;
+    ~A() {}
+  };
+
+  struct B {
+    A &&a = A{};
+    ~B() {}
+  };
+  B a = {};
+  
+  // CHECK: call {{.*}}block_scope_begin_function
+  extern void block_scope_begin_function();
+  extern void block_scope_end_function();
+  block_scope_begin_function();
+  {
+    // CHECK: call void @_ZZ12check_dr1815vEN1BD1Ev
+    // CHECK: call void @_ZZ12check_dr1815vEN1AD1Ev
+    B b = {};
+  }
+  // CHECK: call {{.*}}block_scope_end_function
+  block_scope_end_function();
+
+  // CHECK: call {{.*}}some_other_function
+  extern void some_other_function();
+  some_other_function();
+  // CHECK: call void @_ZZ12check_dr1815vEN1BD1Ev
+  // CHECK: call void @_ZZ12check_dr1815vEN1AD1Ev
+#endif
+}
+
 namespace P2718R0 {
 namespace basic {
 template <typename E> using T2 = std::list<E>;

clang/test/SemaCXX/constexpr-default-arg.cpp

@@ -32,8 +32,8 @@ void test_default_arg2() {
 }
 
 // Check that multiple CXXDefaultInitExprs don't cause an assertion failure.
-struct A { int &&r = 0; }; // expected-note 2{{default member initializer}}
+struct A { int &&r = 0; };
 struct B { A x, y; };
-B b = {}; // expected-warning 2{{lifetime extension of temporary created by aggregate initialization using a default member initializer is not yet supported}}
+B b = {}; // expected-no-diagnostics
 
 }

clang/test/SemaCXX/eval-crashes.cpp

@@ -25,11 +25,9 @@ namespace pr33140_0b {
 }
 
 namespace pr33140_2 {
-  // FIXME: The declaration of 'b' below should lifetime-extend two int
-  // temporaries.
-  struct A { int &&r = 0; }; // expected-note 2{{initializing field 'r' with default member initializer}}
+  struct A { int &&r = 0; };
   struct B { A x, y; };
-  B b = {}; // expected-warning 2{{lifetime extension of temporary created by aggregate initialization using a default member initializer is not yet supported}}
+  B b = {};
 }
 
 namespace pr33140_3 {

clang/www/cxx_dr_status.html

@@ -10698,7 +10698,7 @@ <h2 id="cxxdr">C++ defect report implementation status</h2>
     <td><a href="https://cplusplus.github.io/CWG/issues/1815.html">1815</a></td>
     <td>CD4</td>
     <td>Lifetime extension in aggregate initialization</td>
-    <td class="none" align="center">No</td>
+    <td class="unreleased" align="center">Clang 19</td>
   </tr>
   <tr id="1816">
     <td><a href="https://cplusplus.github.io/CWG/issues/1816.html">1816</a></td>