[SemaCXX] Implement CWG2137 (list-initialization from objects of the same type) (#77768)

Closes #77638, #24186

Rebased from <https://reviews.llvm.org/D156032>, see there for more
information.

Implements wording change in [CWG2137](https://wg21.link/CWG2137) in the
first commit.

This also implements an approach to [CWG2311](https://wg21.link/CWG2311)
in the second commit, because too much code that relies on `T{ T_prvalue}` 
being an elision would break. Because that issue is still open and
the CWG issue doesn't provide wording to fix the issue, there may be
different behaviours on other compilers.

1 files changed, 30 insertions, 10 deletions

diff --git a/clang/lib/Sema/SemaInit.cpp b/clang/lib/Sema/SemaInit.cpp
index 18440a6..3170f41 100644
--- a/clang/lib/Sema/SemaInit.cpp
+++ b/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)) {