[SemaCXX] Implement CWG2137 (list-initialization from objects of the same type)

clang/docs/ReleaseNotes.rst

@@ -229,6 +229,11 @@ C++2c Feature Support
 
 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

@@ -4200,7 +4200,7 @@ static OverloadingResult ResolveConstructorOverload(
 /// \param IsListInit     Is this list-initialization?
 /// \param IsInitListCopy Is this non-list-initialization resulting from a
 ///                       list-initialization from {x} where x is the same
-///                       type as the entity?
+///                       aggregate type as the entity?
 static void TryConstructorInitialization(Sema &S,
                                          const InitializedEntity &Entity,
                                          const InitializationKind &Kind,
@@ -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
@@ -4242,11 +4250,17 @@ static void TryConstructorInitialization(Sema &S,
   if (S.getLangOpts().CPlusPlus17 && !RequireActualConstructor &&
       UnwrappedArgs.size() == 1 && UnwrappedArgs[0]->isPRValue() &&
       S.Context.hasSameUnqualifiedType(UnwrappedArgs[0]->getType(), DestType)) {
-    // Convert qualifications if necessary.
-    Sequence.AddQualificationConversionStep(DestType, VK_PRValue);
-    if (ILE)
-      Sequence.RewrapReferenceInitList(DestType, ILE);
-    return;
+    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:
@@ -4572,9 +4592,9 @@ static void TryListInitialization(Sema &S,
     return;
   }
 
-  // C++11 [dcl.init.list]p3, per DR1467:
-  // - If T is a class type and the initializer list has a single element of
-  //   type cv U, where U is T or a class derived from T, the object is
+  // C++11 [dcl.init.list]p3, per DR1467 and DR2137:
+  // - If T is an aggregate class and the initializer list has a single element
+  //   of type cv U, where U is T or a class derived from T, the object is
   //   initialized from that element (by copy-initialization for
   //   copy-list-initialization, or by direct-initialization for
   //   direct-list-initialization).
@@ -4585,7 +4605,7 @@ static void TryListInitialization(Sema &S,
   // - Otherwise, if T is an aggregate, [...] (continue below).
   if (S.getLangOpts().CPlusPlus11 && InitList->getNumInits() == 1 &&
       !IsDesignatedInit) {
-    if (DestType->isRecordType()) {
+    if (DestType->isRecordType() && DestType->isAggregateType()) {
       QualType InitType = InitList->getInit(0)->getType();
       if (S.Context.hasSameUnqualifiedType(InitType, DestType) ||
           S.IsDerivedFrom(InitList->getBeginLoc(), InitType, DestType)) {

clang/lib/Sema/SemaOverload.cpp

@@ -1568,19 +1568,37 @@ TryUserDefinedConversion(Sema &S, Expr *From, QualType ToType,
     //   called for those cases.
     if (CXXConstructorDecl *Constructor
           = dyn_cast<CXXConstructorDecl>(ICS.UserDefined.ConversionFunction)) {
-      QualType FromCanon
-        = S.Context.getCanonicalType(From->getType().getUnqualifiedType());
+      QualType FromType;
+      SourceLocation FromLoc;
+      // C++11 [over.ics.list]p6, per DR2137:
+      // C++17 [over.ics.list]p6:
+      //   If C is not an initializer-list constructor and the initializer list
+      //   has a single element of type cv U, where U is X or a class derived
+      //   from X, the implicit conversion sequence has Exact Match rank if U is
+      //   X, or Conversion rank if U is derived from X.
+      if (const auto *InitList = dyn_cast<InitListExpr>(From);
+          InitList && InitList->getNumInits() == 1 &&
+          !S.isInitListConstructor(Constructor)) {
+        const Expr *SingleInit = InitList->getInit(0);
+        FromType = SingleInit->getType();
+        FromLoc = SingleInit->getBeginLoc();
+      } else {
+        FromType = From->getType();
+        FromLoc = From->getBeginLoc();
+      }
+      QualType FromCanon =
+          S.Context.getCanonicalType(FromType.getUnqualifiedType());
       QualType ToCanon
         = S.Context.getCanonicalType(ToType).getUnqualifiedType();
       if (Constructor->isCopyConstructor() &&
           (FromCanon == ToCanon ||
-           S.IsDerivedFrom(From->getBeginLoc(), FromCanon, ToCanon))) {
+           S.IsDerivedFrom(FromLoc, FromCanon, ToCanon))) {
         // Turn this into a "standard" conversion sequence, so that it
         // gets ranked with standard conversion sequences.
         DeclAccessPair Found = ICS.UserDefined.FoundConversionFunction;
         ICS.setStandard();
         ICS.Standard.setAsIdentityConversion();
-        ICS.Standard.setFromType(From->getType());
+        ICS.Standard.setFromType(FromType);
         ICS.Standard.setAllToTypes(ToType);
         ICS.Standard.CopyConstructor = Constructor;
         ICS.Standard.FoundCopyConstructor = Found;
@@ -5306,18 +5324,18 @@ TryListConversion(Sema &S, InitListExpr *From, QualType ToType,
       IsDesignatedInit)
     return Result;
 
-  // Per DR1467:
-  //   If the parameter type is a class X and the initializer list has a single
-  //   element of type cv U, where U is X or a class derived from X, the
-  //   implicit conversion sequence is the one required to convert the element
-  //   to the parameter type.
+  // Per DR1467 and DR2137:
+  //   If the parameter type is an aggregate class X and the initializer list
+  //   has a single element of type cv U, where U is X or a class derived from
+  //   X, the implicit conversion sequence is the one required to convert the
+  //   element to the parameter type.
   //
   //   Otherwise, if the parameter type is a character array [... ]
   //   and the initializer list has a single element that is an
   //   appropriately-typed string literal (8.5.2 [dcl.init.string]), the
   //   implicit conversion sequence is the identity conversion.
   if (From->getNumInits() == 1 && !IsDesignatedInit) {
-    if (ToType->isRecordType()) {
+    if (ToType->isRecordType() && ToType->isAggregateType()) {
       QualType InitType = From->getInit(0)->getType();
       if (S.Context.hasSameUnqualifiedType(InitType, ToType) ||
           S.IsDerivedFrom(From->getBeginLoc(), InitType, ToType))

clang/test/CXX/drs/dr14xx.cpp

@@ -488,16 +488,6 @@ namespace dr1467 {  // dr1467: 3.7 c++11
     }
   } // nonaggregate
 
-  namespace SelfInitIsNotListInit {
-    struct S {
-      S();
-      explicit S(S &);
-      S(const S &);
-    };
-    S s1;
-    S s2 = {s1}; // ok, not list-initialization so we pick the non-explicit constructor
-  }
-
   struct NestedInit { int a, b, c; };
   NestedInit ni[1] = {{NestedInit{1, 2, 3}}};
 

clang/test/CXX/drs/dr21xx.cpp

@@ -11,6 +11,16 @@
 // cxx98-error@-1 {{variadic macros are a C99 feature}}
 #endif
 
+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();
+  };
+}
+
 namespace dr2100 { // dr2100: 12
   template<const int *P, bool = true> struct X {};
   template<typename T> struct A {
@@ -132,6 +142,41 @@ namespace dr2126 { // dr2126: 12
 #endif
 }
 
+namespace dr2137 { // dr2137: 18
+#if __cplusplus >= 201103L
+  struct Q {
+    Q();
+    Q(Q&&);
+    Q(std::initializer_list<Q>) = delete; // #dr2137-Qcons
+  };
+
+  Q x = Q { Q() };
+  // since-cxx11-error@-1 {{call to deleted constructor of 'Q'}}
+  //   since-cxx11-note@#dr2137-Qcons {{'Q' has been explicitly marked deleted here}}
+
+  int f(Q); // #dr2137-f
+  int y = f({ Q() });
+  // since-cxx11-error@-1 {{call to deleted constructor of 'Q'}}
+  //   since-cxx11-note@#dr2137-Qcons {{'Q' has been explicitly marked deleted here}}
+  //   since-cxx11-note@#dr2137-f {{passing argument to parameter here}}
+
+  struct U {
+    U();
+    U(const U&);
+  };
+
+  struct Derived : U {
+    Derived();
+    Derived(const Derived&);
+  } d;
+
+  int g(Derived);
+  int g(U(&&)[1]) = delete;
+
+  int z = g({ d });
+#endif
+}
+
 namespace dr2140 { // dr2140: 9
 #if __cplusplus >= 201103L
   union U { int a; decltype(nullptr) b; };

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

@@ -12630,7 +12630,7 @@ <h2 id="cxxdr">C++ defect report implementation status</h2>
     <td><a href="https://cplusplus.github.io/CWG/issues/2137.html">2137</a></td>
     <td>CD4</td>
     <td>List-initialization from object of same type</td>
-    <td class="unknown" align="center">Unknown</td>
+    <td class="unreleased" align="center">Clang 18</td>
   </tr>
   <tr id="2138">
     <td><a href="https://cplusplus.github.io/CWG/issues/2138.html">2138</a></td>
@@ -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>

libcxx/test/std/utilities/utility/pairs/pairs.pair/ctor.pair_U_V_move.pass.cpp

@@ -121,7 +121,26 @@ int main(int, char**)
         test_pair_rv<CopyOnly, CopyOnly&>();
         test_pair_rv<CopyOnly, CopyOnly&&>();
 
-        test_pair_rv<ExplicitTypes::CopyOnly, ExplicitTypes::CopyOnly>();
+        /* For ExplicitTypes::CopyOnly, two of the viable candidates for initializing from a non-const xvalue are:
+         *   pair(const pair&);  // (defaulted copy constructor)
+         *   template<class U1, class U2> explicit pair(const pair<U1, U2>&&); [U1 = ExplicitTypes::CopyOnly, U2 = int]
+         * This results in diverging behavior for test_convertible which uses copy-list-initialization
+         * Prior to CWG2137, this would have selected the first (non-explicit) ctor as explicit ctors would not be considered
+         * Afterwards, it should select the second since it is a better match, and then failed because it is explicit
+         *
+         * This may change with future defect reports, and some compilers only have partial support for CWG2137,
+         * so use std::is_convertible directly to avoid a copy-list-initialization
+         */
+        {
+          using P1  = std::pair<ExplicitTypes::CopyOnly, int>;
+          using P2  = std::pair<int, ExplicitTypes::CopyOnly>;
+          using UP1 = std::pair<ExplicitTypes::CopyOnly, int>&&;
+          using UP2 = std::pair<int, ExplicitTypes::CopyOnly>&&;
+          static_assert(std::is_constructible<P1, UP1>::value, "");
+          static_assert(std::is_convertible<P1, UP1>::value, "");
+          static_assert(std::is_constructible<P2, UP2>::value, "");
+          static_assert(std::is_convertible<P2, UP2>::value, "");
+        }
         test_pair_rv<ExplicitTypes::CopyOnly, ExplicitTypes::CopyOnly&, true, false>();
         test_pair_rv<ExplicitTypes::CopyOnly, ExplicitTypes::CopyOnly&&, true, false>();