Merge commit 6e4930c67508 from llvm-project (by Alexander Kornienko):

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

  This reverts commit 924701311aa79180e86ad8ce43d253f27d25ec7d. Causes compilation
  errors on valid code, see
  llvm/llvm-project#77768 (comment).

In particular, this fixes bogus "call to constructor of 'SomeType' is
ambiguous" errors.

PR:		276104
MFC after:	1 month

(cherry picked from commit ddbac70)

Author: DimitryAndric

contrib/llvm-project/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
-///                       aggregate type as the entity?
+///                       type as the entity?
 static void TryConstructorInitialization(Sema &S,
                                          const InitializedEntity &Entity,
                                          const InitializationKind &Kind,
@@ -4230,14 +4230,6 @@ 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
@@ -4250,17 +4242,11 @@ static void TryConstructorInitialization(Sema &S,
   if (S.getLangOpts().CPlusPlus17 && !RequireActualConstructor &&
       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;
-    }
+    // Convert qualifications if necessary.
+    Sequence.AddQualificationConversionStep(DestType, VK_PRValue);
+    if (ILE)
+      Sequence.RewrapReferenceInitList(DestType, ILE);
+    return;
   }
 
   const RecordType *DestRecordType = DestType->getAs<RecordType>();
@@ -4305,12 +4291,6 @@ 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:
@@ -4592,9 +4572,9 @@ static void TryListInitialization(Sema &S,
     return;
   }
 
-  // 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
+  // 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
   //   initialized from that element (by copy-initialization for
   //   copy-list-initialization, or by direct-initialization for
   //   direct-list-initialization).
@@ -4605,7 +4585,7 @@ static void TryListInitialization(Sema &S,
   // - Otherwise, if T is an aggregate, [...] (continue below).
   if (S.getLangOpts().CPlusPlus11 && InitList->getNumInits() == 1 &&
       !IsDesignatedInit) {
-    if (DestType->isRecordType() && DestType->isAggregateType()) {
+    if (DestType->isRecordType()) {
       QualType InitType = InitList->getInit(0)->getType();
       if (S.Context.hasSameUnqualifiedType(InitType, DestType) ||
           S.IsDerivedFrom(InitList->getBeginLoc(), InitType, DestType)) {

contrib/llvm-project/clang/lib/Sema/SemaOverload.cpp

@@ -1568,37 +1568,19 @@ TryUserDefinedConversion(Sema &S, Expr *From, QualType ToType,
     //   called for those cases.
     if (CXXConstructorDecl *Constructor
           = dyn_cast<CXXConstructorDecl>(ICS.UserDefined.ConversionFunction)) {
-      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 FromCanon
+        = S.Context.getCanonicalType(From->getType().getUnqualifiedType());
       QualType ToCanon
         = S.Context.getCanonicalType(ToType).getUnqualifiedType();
       if (Constructor->isCopyConstructor() &&
           (FromCanon == ToCanon ||
-           S.IsDerivedFrom(FromLoc, FromCanon, ToCanon))) {
+           S.IsDerivedFrom(From->getBeginLoc(), 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(FromType);
+        ICS.Standard.setFromType(From->getType());
         ICS.Standard.setAllToTypes(ToType);
         ICS.Standard.CopyConstructor = Constructor;
         ICS.Standard.FoundCopyConstructor = Found;
@@ -5324,18 +5306,18 @@ TryListConversion(Sema &S, InitListExpr *From, QualType ToType,
       IsDesignatedInit)
     return Result;
 
-  // 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.
+  // 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.
   //
   //   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() && ToType->isAggregateType()) {
+    if (ToType->isRecordType()) {
       QualType InitType = From->getInit(0)->getType();
       if (S.Context.hasSameUnqualifiedType(InitType, ToType) ||
           S.IsDerivedFrom(From->getBeginLoc(), InitType, ToType))