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[Clang] Extend lifetime of temporaries in mem-default-init for P2718R0 (#86960)
Depends on [CWG1815](#108039). Fixes #85613. In [[Clang] Implement P2718R0 "Lifetime extension in range-based for loops"](#76361), we've not implement the lifetime extensions for the temporaries which in `CXXDefaultInitExpr`. As the confirmation in #85613, we should extend lifetime for that. To avoid modifying current CodeGen rules, in a lifetime extension context, the cleanup of `CXXDefaultInitExpr` was ignored. --------- Signed-off-by: yronglin <[email protected]>
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‎clang/docs/ReleaseNotes.rst

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@@ -171,6 +171,9 @@ C++23 Feature Support
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^^^^^^^^^^^^^^^^^^^^^
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- Removed the restriction to literal types in constexpr functions in C++23 mode.
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- Extend lifetime of temporaries in mem-default-init for P2718R0. Clang now fully
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supported `P2718R0 Lifetime extension in range-based for loops <https://wg21.link/P2718R0>`_.
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C++20 Feature Support
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^^^^^^^^^^^^^^^^^^^^^
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‎clang/lib/Sema/SemaExpr.cpp

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@@ -5649,6 +5649,8 @@ ExprResult Sema::BuildCXXDefaultInitExpr(SourceLocation Loc, FieldDecl *Field) {
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runWithSufficientStackSpace(Loc, [&] {
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MarkDeclarationsReferencedInExpr(E, /*SkipLocalVariables=*/false);
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});
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if (isInLifetimeExtendingContext())
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DiscardCleanupsInEvaluationContext();
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// C++11 [class.base.init]p7:
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// The initialization of each base and member constitutes a
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// full-expression.

‎clang/lib/Sema/SemaInit.cpp

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@@ -763,6 +763,8 @@ void InitListChecker::FillInEmptyInitForField(unsigned Init, FieldDecl *Field,
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SemaRef.currentEvaluationContext().DelayedDefaultInitializationContext =
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SemaRef.parentEvaluationContext()
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.DelayedDefaultInitializationContext;
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SemaRef.currentEvaluationContext().InLifetimeExtendingContext =
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SemaRef.parentEvaluationContext().InLifetimeExtendingContext;
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DIE = SemaRef.BuildCXXDefaultInitExpr(Loc, Field);
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}
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if (DIE.isInvalid()) {

‎clang/test/AST/ast-dump-for-range-lifetime.cpp

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@@ -449,4 +449,63 @@ void test13() {
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for (auto e : dg<A>().r().g().r().g().r().g())
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bar(e);
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}
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extern "C" void exit(int);
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struct A14 {
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int arr[1];
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~A14() noexcept(false) { throw 42; }
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};
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struct B14 {
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int x;
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const A14 &a = A14{{0}};
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const int *begin() { return a.arr; }
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const int *end() { return &a.arr[1]; }
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};
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void test14() {
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// The ExprWithCleanups in CXXDefaultInitExpr will be ignored.
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// CHECK: FunctionDecl {{.*}} test14 'void ()'
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// CHECK: -CXXForRangeStmt {{.*}}
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// CHECK-NEXT: |-<<<NULL>>>
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// CHECK-NEXT: |-DeclStmt {{.*}}
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// CHECK-NEXT: | `-VarDecl {{.*}} implicit used __range1 'const int (&)[1]' cinit
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// CHECK-NEXT: | `-ExprWithCleanups {{.*}} 'const int[1]' lvalue
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// CHECK-NEXT: | `-MemberExpr {{.*}} 'const int[1]' lvalue .arr {{.*}}
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// CHECK-NEXT: | `-MemberExpr {{.*}} 'const A14':'const P2718R0::A14' lvalue .a {{.*}}
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// CHECK-NEXT: | `-MaterializeTemporaryExpr {{.*}} 'B14':'P2718R0::B14' xvalue extended by Var {{.*}} '__range1' 'const int (&)[1]'
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// CHECK-NEXT: | `-CXXFunctionalCastExpr {{.*}} 'B14':'P2718R0::B14' functional cast to B14 <NoOp>
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// CHECK-NEXT: | `-InitListExpr {{.*}} 'B14':'P2718R0::B14'
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// CHECK-NEXT: | |-IntegerLiteral {{.*}} 'int' 0
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// CHECK-NEXT: | `-CXXDefaultInitExpr {{.*}} 'const A14':'const P2718R0::A14' lvalue has rewritten init
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// CHECK-NEXT: | `-MaterializeTemporaryExpr {{.*}} 'const A14':'const P2718R0::A14' lvalue extended by Var {{.*}} '__range1' 'const int (&)[1]'
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// CHECK-NEXT: | `-ImplicitCastExpr {{.*}} 'const A14':'const P2718R0::A14' <NoOp>
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// CHECK-NEXT: | `-CXXFunctionalCastExpr {{.*}} 'A14':'P2718R0::A14' functional cast to A14 <NoOp>
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// CHECK-NEXT: | `-CXXBindTemporaryExpr {{.*}} 'A14':'P2718R0::A14' (CXXTemporary {{.*}})
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// CHECK-NEXT: | `-InitListExpr {{.*}} 'A14':'P2718R0::A14'
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// CHECK-NEXT: | `-InitListExpr {{.*}} 'int[1]'
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// CHECK-NEXT: | `-IntegerLiteral {{.*}} 'int' 0
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for (auto &&x : B14{0}.a.arr) { exit(0); }
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// CHECK: -CXXForRangeStmt {{.*}}
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// CHECK-NEXT: |-<<<NULL>>>
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// CHECK-NEXT: |-DeclStmt {{.*}}
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// CHECK-NEXT: | `-VarDecl {{.*}} col:19 implicit used __range1 'B14 &&' cinit
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// CHECK-NEXT: | `-ExprWithCleanups {{.*}} 'B14':'P2718R0::B14' xvalue
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// CHECK-NEXT: | `-MaterializeTemporaryExpr {{.*}} 'B14':'P2718R0::B14' xvalue extended by Var {{.*}} '__range1' 'B14 &&'
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// CHECK-NEXT: | `-CXXFunctionalCastExpr {{.*}} 'B14':'P2718R0::B14' functional cast to B14 <NoOp>
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// CHECK-NEXT: | `-InitListExpr {{.*}} 'B14':'P2718R0::B14'
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// CHECK-NEXT: | |-IntegerLiteral {{.*}} 'int' 0
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// CHECK-NEXT: | `-CXXDefaultInitExpr {{.*}} 'const A14':'const P2718R0::A14' lvalue has rewritten init
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// CHECK-NEXT: | `-MaterializeTemporaryExpr {{.*}} 'const A14':'const P2718R0::A14' lvalue extended by Var {{.*}} '__range1' 'B14 &&'
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// CHECK-NEXT: | `-ImplicitCastExpr {{.*}} 'const A14':'const P2718R0::A14' <NoOp>
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// CHECK-NEXT: | `-CXXFunctionalCastExpr {{.*}} 'A14':'P2718R0::A14' functional cast to A14 <NoOp>
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// CHECK-NEXT: | `-CXXBindTemporaryExpr {{.*}} 'A14':'P2718R0::A14' (CXXTemporary {{.*}})
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// CHECK-NEXT: | `-InitListExpr {{.*}} 'A14':'P2718R0::A14'
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// CHECK-NEXT: | `-InitListExpr {{.*}} 'int[1]'
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// CHECK-NEXT: | `-IntegerLiteral {{.*}} 'int' 0
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for (auto &&x : B14{0}) { exit(0); }
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}
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} // namespace P2718R0

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

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@@ -463,6 +463,80 @@ template void default_arg_dependent_context2<int>();
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template void default_arg_dependent_context3<int>();
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} // namespace default_arg
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namespace default_init {
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template <class T>
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struct DepA {
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T arr[1];
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~DepA() {}
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};
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template <class T>
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struct DepB {
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int x;
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const DepA<T> &a = DepA<T>{{0}};
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~DepB() {}
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const int *begin() { return a.arr; }
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const int *end() { return &a.arr[1]; }
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};
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template <typename T>
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void default_init1_dependent() {
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// CHECK-CXX23: void @_ZN7P2718R012default_init23default_init1_dependentINS0_4DepBIiEEEEvv()
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// CHECK-CXX23-LABEL: for.cond.cleanup:
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// CHECK-CXX23-NEXT: call void @_ZN7P2718R012default_init4DepBIiED1Ev(
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// CHECK-CXX23-NEXT: call void @_ZN7P2718R012default_init4DepAIiED1Ev(
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for (auto &&x : T{0}) {}
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}
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template <typename T>
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void default_init2_dependent() {
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// CHECK-CXX23: void @_ZN7P2718R012default_init23default_init2_dependentINS0_4DepBIiEEEEvv()
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// CHECK-CXX23-LABEL: for.cond.cleanup:
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// CHECK-CXX23-NEXT: call void @_ZN7P2718R012default_init4DepBIiED1Ev(
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// CHECK-CXX23-NEXT: call void @_ZN7P2718R012default_init4DepAIiED1Ev(
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for (auto &&x : T{0}.a.arr) {}
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}
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template void default_init1_dependent<DepB<int>>();
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template void default_init2_dependent<DepB<int>>();
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} // namespace default_init
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// -- Examples from https://wg21.link/p2718r0
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extern void block_scope_begin_function();
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extern void block_scope_end_function();
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namespace std_examples {
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using T = std::list<int>;
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const T& f1(const T& t) { return t; }
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const T& f2(T t) { return t; }
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T g();
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void foo() {
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// CHECK-CXX23: define {{.*}} void @_ZN7P2718R012std_examples3fooEv()
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// CHECK-CXX23: call void @_ZN7P2718R026block_scope_begin_functionEv
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block_scope_begin_function();
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{
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// CHECK-CXX23-NEXT: call void @_ZN7P2718R012std_examples1gEv
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// CHECK-CXX23-NEXT: call {{.*}} @_ZN7P2718R012std_examples2f1ERKSt4listIiE
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// CHECK-CXX23: for.cond.cleanup:
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// CHECK-CXX23-NEXT: call void @_ZNSt4listIiED1Ev
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for (auto e : f1(g())) {} // OK, lifetime of return value of g() extended
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}
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// CHECK-CXX23: call void @_ZN7P2718R024block_scope_end_functionEv
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block_scope_end_function();
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// The lifetime of temporary returned by g() in this case will not be extended.
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// CHECK-CXX23: call void @_ZN7P2718R026block_scope_begin_functionEv
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block_scope_begin_function();
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{
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// CHECK-CXX23-NEXT: call void @_ZN7P2718R012std_examples1gEv
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// CHECK-CXX23-NEXT: call {{.*}} @_ZN7P2718R012std_examples2f2ESt4listIiE
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// CHECK-CXX23-NEXT: call void @_ZNSt4listIiED1Ev
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for (auto e : f2(g())) {} // undefined behavior
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}
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// CHECK-CXX23: call void @_ZN7P2718R024block_scope_end_functionEv
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block_scope_end_function();
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}
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} // namespace std_examples
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namespace basic {
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using T = std::list<int>;
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const T& f1(const T& t) { return t; }
@@ -579,5 +653,51 @@ void default_arg3() {
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for (auto e : C(0, C(0, C(0, C())))) {}
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}
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} // namespace default_arg
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} // namespace P2718R0
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namespace default_init {
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struct X {
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int x;
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~X() {}
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};
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struct Y {
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int y;
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const X &x = X{1};
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~Y() {}
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};
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struct A {
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int arr[1];
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const Y &y = Y{1};
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~A() {}
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};
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struct B {
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int x;
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const A &a = A{{0}};
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~B() {}
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const int *begin() { return a.arr; }
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const int *end() { return &a.arr[1]; }
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};
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void default_init1() {
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// CHECK-CXX23: void @_ZN7P2718R012default_init13default_init1Ev()
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// CHECK-CXX23-LABEL: for.cond.cleanup:
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// CHECK-CXX23-NEXT: call void @_ZN7P2718R012default_init1BD1Ev(
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// CHECK-CXX23-NEXT: call void @_ZN7P2718R012default_init1AD1Ev(
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// CHECK-CXX23-NEXT: call void @_ZN7P2718R012default_init1YD1Ev(
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// CHECK-CXX23-NEXT: call void @_ZN7P2718R012default_init1XD1Ev(
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for (auto &&x : B{0}) {}
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}
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void default_init2() {
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// CHECK-CXX23: void @_ZN7P2718R012default_init13default_init2Ev()
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// CHECK-CXX23-LABEL: for.cond.cleanup:
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// CHECK-CXX23-NEXT: call void @_ZN7P2718R012default_init1BD1Ev(
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// CHECK-CXX23-NEXT: call void @_ZN7P2718R012default_init1AD1Ev(
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// CHECK-CXX23-NEXT: call void @_ZN7P2718R012default_init1YD1Ev(
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// CHECK-CXX23-NEXT: call void @_ZN7P2718R012default_init1XD1Ev(
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for (auto &&x : B{0}.a.arr) {}
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}
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} // namespace default_init
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} // namespace P2718R0

‎clang/www/cxx_status.html

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@@ -475,14 +475,7 @@ <h2 id="cxx23">C++23 implementation status</h2>
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<tr>
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<td>Lifetime extension in range-based for loops</td>
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<td><a href="https://wg21.link/P2718R0">P2718R0</a></td>
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<td class="partial" align="center">
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<details>
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<summary>Clang 19 (Partial)</summary>
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The lifetime extension of temporaries bound to member references
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by default member initializers in aggregate initialization was
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not supported now.
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</details>
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</td>
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<td class="full" align="center">Clang 20</td>
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</tr>
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<!--Issaquah 2023 papers-->
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<tr>

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