[libc++] Take 2: Implement LWG 2510

Summary:
LWG2510 makes tag types like allocator_arg_t explicitly default
constructible instead of implicitly default constructible. It also
makes the constructors for std::pair and std::tuple conditionally
explicit based on the explicit-ness of the default constructibility
for the pair/tuple's elements.

This was previously committed as r372777 and reverted in r372832 due to
the commit breaking LLVM's build in C++14 mode. This issue has now been
addressed.

Reviewers: mclow.lists

Subscribers: christof, jkorous, dexonsmith, libcxx-commits

Tags: #libc

Differential Revision: https://reviews.llvm.org/D65161

llvm-svn: 372983

1 files changed, 7 insertions, 2 deletions

diff --git a/libcxx/test/std/utilities/tuple/tuple.tuple/tuple.cnstr/default.pass.cpp b/libcxx/test/std/utilities/tuple/tuple.tuple/tuple.cnstr/default.pass.cpp
index 46fd35ad4dd..c5b650d79a1 100644
--- a/libcxx/test/std/utilities/tuple/tuple.tuple/tuple.cnstr/default.pass.cpp
+++ b/libcxx/test/std/utilities/tuple/tuple.tuple/tuple.cnstr/default.pass.cpp
@@ -10,7 +10,7 @@
 
 // template <class... Types> class tuple;
 
-// constexpr tuple();
+// explicit(see-below) constexpr tuple();
 
 // UNSUPPORTED: c++98, c++03
 
@@ -107,6 +107,11 @@ int main(int, char**)
         IllFormedDefault v(0);
         std::tuple<IllFormedDefault> t(v);
     }
+    {
+        struct Base { };
+        struct Derived : Base { protected: Derived() = default; };
+        static_assert(!std::is_default_constructible<std::tuple<Derived, int> >::value, "");
+    }
 
-  return 0;
+    return 0;
 }