PR44540: Prefer an inherited default constructor over an initializer

list constructor when initializing from {}.

We would previously pick between calling an initializer list constructor
and calling a default constructor unstably in this situation, depending
on whether the inherited default constructor had already been used
elsewhere in the program.

Author: zygoloid

clang/lib/AST/DeclCXX.cpp

@@ -738,49 +738,55 @@ void CXXRecordDecl::addedMember(Decl *D) {
 
   // Handle constructors.
   if (const auto *Constructor = dyn_cast<CXXConstructorDecl>(D)) {
-    if (!Constructor->isImplicit()) {
-      // Note that we have a user-declared constructor.
-      data().UserDeclaredConstructor = true;
+    if (Constructor->isInheritingConstructor()) {
+      // Ignore constructor shadow declarations. They are lazily created and
+      // so shouldn't affect any properties of the class.
+    } else {
+      if (!Constructor->isImplicit()) {
+        // Note that we have a user-declared constructor.
+        data().UserDeclaredConstructor = true;
+
+        // C++ [class]p4:
+        //   A POD-struct is an aggregate class [...]
+        // Since the POD bit is meant to be C++03 POD-ness, clear it even if
+        // the type is technically an aggregate in C++0x since it wouldn't be
+        // in 03.
+        data().PlainOldData = false;
+      }
 
-      // C++ [class]p4:
-      //   A POD-struct is an aggregate class [...]
-      // Since the POD bit is meant to be C++03 POD-ness, clear it even if the
-      // type is technically an aggregate in C++0x since it wouldn't be in 03.
-      data().PlainOldData = false;
-    }
+      if (Constructor->isDefaultConstructor()) {
+        SMKind |= SMF_DefaultConstructor;
 
-    if (Constructor->isDefaultConstructor()) {
-      SMKind |= SMF_DefaultConstructor;
+        if (Constructor->isUserProvided())
+          data().UserProvidedDefaultConstructor = true;
+        if (Constructor->isConstexpr())
+          data().HasConstexprDefaultConstructor = true;
+        if (Constructor->isDefaulted())
+          data().HasDefaultedDefaultConstructor = true;
+      }
 
-      if (Constructor->isUserProvided())
-        data().UserProvidedDefaultConstructor = true;
-      if (Constructor->isConstexpr())
-        data().HasConstexprDefaultConstructor = true;
-      if (Constructor->isDefaulted())
-        data().HasDefaultedDefaultConstructor = true;
-    }
+      if (!FunTmpl) {
+        unsigned Quals;
+        if (Constructor->isCopyConstructor(Quals)) {
+          SMKind |= SMF_CopyConstructor;
 
-    if (!FunTmpl) {
-      unsigned Quals;
-      if (Constructor->isCopyConstructor(Quals)) {
-        SMKind |= SMF_CopyConstructor;
+          if (Quals & Qualifiers::Const)
+            data().HasDeclaredCopyConstructorWithConstParam = true;
+        } else if (Constructor->isMoveConstructor())
+          SMKind |= SMF_MoveConstructor;
+      }
 
-        if (Quals & Qualifiers::Const)
-          data().HasDeclaredCopyConstructorWithConstParam = true;
-      } else if (Constructor->isMoveConstructor())
-        SMKind |= SMF_MoveConstructor;
+      // C++11 [dcl.init.aggr]p1: DR1518
+      //   An aggregate is an array or a class with no user-provided [or]
+      //   explicit [...] constructors
+      // C++20 [dcl.init.aggr]p1:
+      //   An aggregate is an array or a class with no user-declared [...]
+      //   constructors
+      if (getASTContext().getLangOpts().CPlusPlus2a
+              ? !Constructor->isImplicit()
+              : (Constructor->isUserProvided() || Constructor->isExplicit()))
+        data().Aggregate = false;
     }
-
-    // C++11 [dcl.init.aggr]p1: DR1518
-    //   An aggregate is an array or a class with no user-provided [or]
-    //   explicit [...] constructors
-    // C++20 [dcl.init.aggr]p1:
-    //   An aggregate is an array or a class with no user-declared [...]
-    //   constructors
-    if (getASTContext().getLangOpts().CPlusPlus2a
-            ? !Constructor->isImplicit()
-            : (Constructor->isUserProvided() || Constructor->isExplicit()))
-      data().Aggregate = false;
   }
 
   // Handle constructors, including those inherited from base classes.

clang/lib/Sema/SemaInit.cpp

@@ -4059,7 +4059,7 @@ static void TryConstructorInitialization(Sema &S,
 
     // If the initializer list has no elements and T has a default constructor,
     // the first phase is omitted.
-    if (!(UnwrappedArgs.empty() && DestRecordDecl->hasDefaultConstructor()))
+    if (!(UnwrappedArgs.empty() && S.LookupDefaultConstructor(DestRecordDecl)))
       Result = ResolveConstructorOverload(S, Kind.getLocation(), Args,
                                           CandidateSet, DestType, Ctors, Best,
                                           CopyInitialization, AllowExplicit,
@@ -4343,7 +4343,7 @@ static void TryListInitialization(Sema &S,
       //     value-initialized.
       if (InitList->getNumInits() == 0) {
         CXXRecordDecl *RD = DestType->getAsCXXRecordDecl();
-        if (RD->hasDefaultConstructor()) {
+        if (S.LookupDefaultConstructor(RD)) {
           TryValueInitialization(S, Entity, Kind, Sequence, InitList);
           return;
         }

clang/test/CXX/dcl.decl/dcl.init/dcl.init.list/p3.cpp

@@ -86,6 +86,20 @@ namespace bullet6 {
   const int& i1 = { 1 };
   const int& i2 = { 1.1 };  // expected-error {{type 'double' cannot be narrowed to 'int' in initializer list}} expected-note {{silence}} expected-warning {{implicit conversion}}
   const int (&iar)[2] = { 1, 2 };
+
+  // We interpret "class type with a default constructor" as including the case
+  // where a default constructor is inherited.
+  struct X {
+    X();
+    X(std::initializer_list<int>) = delete;
+  };
+  struct Y : X {
+    using X::X;
+    Y(int);
+  };
+  Y y1{};
+  void use() { Y y; }
+  Y y2{};
 }
 
 namespace bullet7 {