[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 (CXXConstructorDecl *Constructor =
     //   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) [...].
     if (getASTContext().getLangOpts().CPlusPlus11
             ? Constructor->isUserProvided()
             : !Constructor->isImplicit())
       data().Aggregate = false;
+    // C++1z [dcl.init.aggr]p1:
+    //  - no user-provided, explicit, or inherited constructors,
+    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;

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@-4 {{here}}
+#endif
+}