[SemaCXX] Allow some copy elision of T{ T_prvalue } if an initializer…

…-list constructor isn't used (CWG2311)

Differential Revision: https://reviews.llvm.org/D156032

clang/docs/ReleaseNotes.rst

@@ -231,6 +231,9 @@ Resolutions to C++ Defect Reports
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 - Implemented `CWG2137 <https://wg21.link/CWG2137>`_ which allows
   list-initialization from objects of the same type.
+- Implemented `CWG2311 <https://wg21.link/CWG2311>`_: given a prvalue ``e`` of object type
+  ``T``, ``T{e}`` will try to resolve an initializer list constructor and will use it if successful (CWG2137).
+  Otherwise, if there is no initializer list constructor, the copy will be elided as if it was ``T(e)``.
 
 - Implemented `CWG2598 <https://wg21.link/CWG2598>`_ and `CWG2096 <https://wg21.link/CWG2096>`_,
   making unions (that have either no members or at least one literal member) literal types.

clang/lib/Sema/SemaInit.cpp

@@ -4230,6 +4230,14 @@ static void TryConstructorInitialization(Sema &S,
         Entity.getKind() !=
             InitializedEntity::EK_LambdaToBlockConversionBlockElement);
 
+  bool CopyElisionPossible = false;
+  auto ElideConstructor = [&] {
+    // Convert qualifications if necessary.
+    Sequence.AddQualificationConversionStep(DestType, VK_PRValue);
+    if (ILE)
+      Sequence.RewrapReferenceInitList(DestType, ILE);
+  };
+
   // C++17 [dcl.init]p17:
   //     - If the initializer expression is a prvalue and the cv-unqualified
   //       version of the source type is the same class as the class of the
@@ -4240,13 +4248,19 @@ static void TryConstructorInitialization(Sema &S,
   // ObjC++: Lambda captured by the block in the lambda to block conversion
   // should avoid copy elision.
   if (S.getLangOpts().CPlusPlus17 && !RequireActualConstructor &&
-      Args.size() == 1 && Args[0]->isPRValue() &&
-      S.Context.hasSameUnqualifiedType(Args[0]->getType(), DestType)) {
-    // Convert qualifications if necessary.
-    Sequence.AddQualificationConversionStep(DestType, VK_PRValue);
-    if (ILE)
-      Sequence.RewrapReferenceInitList(DestType, ILE);
-    return;
+      UnwrappedArgs.size() == 1 && UnwrappedArgs[0]->isPRValue() &&
+      S.Context.hasSameUnqualifiedType(UnwrappedArgs[0]->getType(), DestType)) {
+    if (ILE && !DestType->isAggregateType()) {
+      // CWG2311: T{ prvalue_of_type_T } is not eligible for copy elision
+      // Make this an elision if this won't call an initializer-list
+      // constructor. (Always on an aggregate type or check constructors first.)
+      assert(!IsInitListCopy &&
+             "IsInitListCopy only possible with aggregate types");
+      CopyElisionPossible = true;
+    } else {
+      ElideConstructor();
+      return;
+    }
   }
 
   const RecordType *DestRecordType = DestType->getAs<RecordType>();
@@ -4291,6 +4305,12 @@ static void TryConstructorInitialization(Sema &S,
           S, Kind.getLocation(), Args, CandidateSet, DestType, Ctors, Best,
           CopyInitialization, AllowExplicit,
           /*OnlyListConstructors=*/true, IsListInit, RequireActualConstructor);
+
+    if (CopyElisionPossible && Result == OR_No_Viable_Function) {
+      // No initializer list candidate
+      ElideConstructor();
+      return;
+    }
   }
 
   // C++11 [over.match.list]p1:

clang/test/CXX/drs/dr23xx.cpp

@@ -6,6 +6,16 @@
 // RUN: %clang_cc1 -std=c++23 %s -verify=expected,since-cxx11,since-cxx14,since-cxx17,since-cxx20 -fexceptions -fcxx-exceptions -pedantic-errors 2>&1 | FileCheck %s
 // RUN: %clang_cc1 -std=c++2c %s -verify=expected,since-cxx11,since-cxx14,since-cxx17,since-cxx20 -fexceptions -fcxx-exceptions -pedantic-errors 2>&1 | FileCheck %s
 
+namespace std {
+  __extension__ typedef __SIZE_TYPE__ size_t;
+
+  template<typename E> struct initializer_list {
+    const E *p; size_t n;
+    initializer_list(const E *p, size_t n);
+    initializer_list();
+  };
+}
+
 #if __cplusplus >= 201103L
 namespace dr2303 { // dr2303: 12
 template <typename... T>
@@ -47,6 +57,81 @@ void g() {
 } //namespace dr2303
 #endif
 
+namespace dr2311 {  // dr2311: 18 open
+#if __cplusplus >= 201707L
+template<typename T>
+void test() {
+  // Ensure none of these try to call a move constructor.
+  T a = T{T(0)};
+  T b{T(0)};
+  auto c{T(0)};
+  T d = {T(0)};
+  auto e = {T(0)};
+#if __cplusplus >= 202302L
+  auto f = auto{T(0)};
+#endif
+  void(*fn)(T);
+  fn({T(0)});
+}
+
+struct NonMovable {
+  NonMovable(int);
+  NonMovable(NonMovable&&) = delete;
+};
+struct NonMovableNonApplicableIList {
+  NonMovableNonApplicableIList(int);
+  NonMovableNonApplicableIList(NonMovableNonApplicableIList&&) = delete;
+  NonMovableNonApplicableIList(std::initializer_list<int>);
+};
+struct ExplicitMovable {
+  ExplicitMovable(int);
+  explicit ExplicitMovable(ExplicitMovable&&);
+};
+struct ExplicitNonMovable {
+  ExplicitNonMovable(int);
+  explicit ExplicitNonMovable(ExplicitNonMovable&&) = delete;
+};
+struct ExplicitNonMovableNonApplicableIList {
+  ExplicitNonMovableNonApplicableIList(int);
+  explicit ExplicitNonMovableNonApplicableIList(ExplicitNonMovableNonApplicableIList&&) = delete;
+  ExplicitNonMovableNonApplicableIList(std::initializer_list<int>);
+};
+struct CopyOnly {
+  CopyOnly(int);
+  CopyOnly(const CopyOnly&);
+  CopyOnly(CopyOnly&&) = delete;
+};
+struct ExplicitCopyOnly {
+  ExplicitCopyOnly(int);
+  explicit ExplicitCopyOnly(const ExplicitCopyOnly&);
+  explicit ExplicitCopyOnly(ExplicitCopyOnly&&) = delete;
+};
+
+template void test<NonMovable>();
+template void test<NonMovableNonApplicableIList>();
+template void test<ExplicitMovable>();
+template void test<ExplicitNonMovable>();
+template void test<ExplicitNonMovableNonApplicableIList>();
+template void test<CopyOnly>();
+template void test<ExplicitCopyOnly>();
+
+struct any {
+    template<typename T>
+    any(T&&);
+};
+
+template<typename T>
+struct X {
+    X();
+    X(T) = delete; // #dr2311-X
+};
+
+X<std::initializer_list<any>> x{ X<std::initializer_list<any>>() };
+// since-cxx17-error@-1 {{call to deleted constructor of 'X<std::initializer_list<any>>'}}
+//   since-cxx17-note@#dr2311-X {{'X' has been explicitly marked deleted here}}
+#endif
+}
+
 // dr2331: na
 // dr2335 is in dr2335.cxx
 

clang/www/cxx_dr_status.html

@@ -13674,7 +13674,7 @@ <h2 id="cxxdr">C++ defect report implementation status</h2>
     <td><a href="https://cplusplus.github.io/CWG/issues/2311.html">2311</a></td>
     <td>open</td>
     <td>Missed case for guaranteed copy elision</td>
-    <td align="center">Not resolved</td>
+    <td class="unreleased" align="center">Clang 18</td>
   </tr>
   <tr id="2312">
     <td><a href="https://cplusplus.github.io/CWG/issues/2312.html">2312</a></td>