[Sema] Don't perform aggregate initialization for types with explicit constructors

[EricWF]

Summary

The C++17 rules for aggregate initialization changed to disallow types with explicit constructors [dcl.init.aggr]p1. This patch implements that new rule.

[rsmith]

Please also add a test to test/CXX/drs/dr15xx.cpp for core issue 1518, which this paper was resolving.

lib/AST/DeclCXX.cpp

564

Do we correctly handle the "or inherited" part? I'd also like to find out whether core intended for this issue to be treated as a DR or not (if so, this should apply all the way back to C++11).

[EricWF]

• Disallow classes with inherited constructors
• Add tests under test/CXX/drs/dr15xx.cpp

I still only implemented this change in C++1z, please let me know if you want me to backport the DR.

[EricWF]

@rsmith ping. This is kind-of blocking making libc++'s tag types not constructible from {}.

[rsmith]

lib/AST/DeclCXX.cpp

564

According to the current issues list, this issue is in DRWP status, so this change should be applied retroactively to C++11 and C++14 as well.

[EricWF]

OK, I've applied the fix to C++11 and C++14. Although the inheriting-constructor part still only matters in C++1z since it requires having base classes.

I also fixed aggregate initialization for types with non-aggregate base classes. For example:

struct A { A(int); };
struct B : A {};
B b = {42}; // OK in C++1z

[EricWF]

Add tests that explicit default constructors are still allowed outside of copy-initialization.

lib/AST/DeclCXX.cpp

if (Constructor->isConstexpr() && !Constructor->isCopyOrMoveConstructor())
       data().HasConstexprNonCopyMoveConstructor = true;
 
     // C++ [dcl.init.aggr]p1:
     //   An aggregate is an array or a class with no user-declared
     //   constructors [...].
     // C++11 [dcl.init.aggr]p1:
     //   An aggregate is an array or a class with no user-provided
     //   constructors [...].
-    // C++11 [dcl.init.aggr]p1:
-    //   An aggregate is an array or a class with no user-provided
-    //   constructors (including those inherited from a base class) [...].
+    // C++1z [dcl.init.aggr]p1:
+    //  An aggregate is an array or a class with no user-provided, explicit, or
+    //  inherited constructors
     if (getASTContext().getLangOpts().CPlusPlus11
             ? Constructor->isUserProvided()
             : !Constructor->isImplicit())
       data().Aggregate = false;
+    if (getASTContext().getLangOpts().CPlusPlus1z && Constructor->isExplicit())
+      data().Aggregate = false;
   }
 
   // Handle destructors.
   if (CXXDestructorDecl *DD = dyn_cast<CXXDestructorDecl>(D)) {
     SMKind |= SMF_Destructor;
 
     if (DD->isUserProvided())
       data().HasIrrelevantDestructor = false;
if (Shadow->getDeclName().getNameKind()
           == DeclarationName::CXXConversionFunctionName) {
       ASTContext &Ctx = getASTContext();
       data().Conversions.get(Ctx).addDecl(Ctx, Shadow, Shadow->getAccess());
     }
   }
 
   if (UsingDecl *Using = dyn_cast<UsingDecl>(D)) {
     if (Using->getDeclName().getNameKind() ==
-        DeclarationName::CXXConstructorName)
+        DeclarationName::CXXConstructorName) {
       data().HasInheritedConstructor = true;
+      // C++1z [dcl.init.aggr]p1:
+      //  An aggregate is [...] a class [...] with no inherited constructors
+      if (getASTContext().getLangOpts().CPlusPlus1z)
+        data().Aggregate = false;
+    }
 
     if (Using->getDeclName().getCXXOverloadedOperator() == OO_Equal)
       data().HasInheritedAssignment = true;
   }
 }
 
 void CXXRecordDecl::finishedDefaultedOrDeletedMember(CXXMethodDecl *D) {
   assert(!D->isImplicit() && !D->isUserProvided());

test/CXX/drs/dr15xx.cpp

#if __cplusplus >= 201103L
     void(Wrap<nullptr_t>() < Wrap<int*>());
     void(Wrap<nullptr_t>() > Wrap<int*>());
     void(Wrap<nullptr_t>() <= Wrap<int*>());
     void(Wrap<nullptr_t>() >= Wrap<int*>());
 #endif
   }
 }
 
+namespace dr1518 { // dr1518: 4.0
+#if __cplusplus >= 201103L
+struct Z0 { // expected-note 0+ {{candidate}}
+  explicit Z0() = default; // expected-note 0+ {{here}}
+};
+struct Z { // expected-note 0+ {{candidate}}
+  explicit Z(); // expected-note 0+ {{here}}
+  explicit Z(int);
+  explicit Z(int, int); // expected-note 0+ {{here}}
+};
+template <class T> int Eat(T); // expected-note 0+ {{candidate}}
+Z0 a;
+Z0 b{};
+Z0 c = {};
+int i = Eat<Z0>({});
+#if __cplusplus > 201402L
+// expected-error@-3 {{explicit in copy-initialization}}
+// expected-error@-3 {{no matching function for call to 'Eat'}}
+#endif
+Z c2 = {}; // expected-error {{explicit in copy-initialization}}
+int i2 = Eat<Z>({}); // expected-error {{no matching function for call to 'Eat'}}
+Z a1 = 1; // expected-error {{no viable conversion}}
+Z a3 = Z(1);
+Z a2(1);
+Z *p = new Z(1);
+Z a4 = (Z)1;
+Z a5 = static_cast<Z>(1);
+Z a6 = {4, 3}; // expected-error {{explicit in copy-initialization}}
+
+struct UserProvidedBaseCtor { // expected-note 0+ {{candidate}}
+  UserProvidedBaseCtor() {}
+};
+struct DoesntInheritCtor : UserProvidedBaseCtor { // expected-note 0+ {{candidate}}
+  int x;
+};
+DoesntInheritCtor I{{}, 42};
+#if __cplusplus <= 201402L
+// expected-error@-2 {{no matching constructor}}
+#endif
+
+struct BaseCtor { BaseCtor() = default; }; // expected-note 0+ {{candidate}}
+struct InheritsCtor : BaseCtor { // expected-note 1+ {{candidate}}
+  using BaseCtor::BaseCtor;      // expected-note 2 {{inherited here}}
+  int x;
+};
+InheritsCtor II = {{}, 42}; // expected-error {{no matching constructor}}
+
+#endif                      // __cplusplus >= 201103L
+}
+
 namespace dr1550 { // dr1550: yes
   int f(bool b, int n) {
     return (b ? (throw 0) : n) + (b ? n : (throw 0));
   }
 }
 
 namespace dr1560 { // dr1560: 3.5
   void f(bool b, int n) {

test/SemaCXX/aggregate-initialization.cpp

#endif
   B h() { return {A{}}; }
 #if __cplusplus <= 201402L
   // expected-error@-2 {{no matching constructor}}
   // expected-note@-24 3{{candidate}}
 #endif
   // expected-error@-5 {{protected constructor}}
   // expected-note@-30 {{here}}
 }
+
+namespace diff_cpp1z_dcl_init_aggr_example {
+struct ExplicitDefault {
+  explicit ExplicitDefault() = default;
+};
+ExplicitDefault d = {};
+#if __cplusplus > 201402L
+// expected-error@-2 {{explicit in copy-initialization}}
+// expected-note@-5 {{here}}
+#endif
+}