[pair] Mark constructors as conditionally noexcept

Summary:
std::tuple marks its constructors as noexcept when the corresponding
memberwise constructors are noexcept too -- this commit improves std::pair
so that it behaves the same.

Note:
I did not add support in the explicit and non-explicit `pair(_Tuple&& __p)`
constructors because those are non-standard extensions, and supporting them
properly is tedious (we have to copy the rvalue-referenceness of the deduced
_Tuple&& onto the result of tuple_element).

<rdar://problem/29537079>

Reviewers: mclow.lists, EricWF

Subscribers: christof, llvm-commits

Differential Revision: https://reviews.llvm.org/D48669

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@348824 91177308-0d34-0410-b5e6-96231b3b80d8

test/libcxx/utilities/utility/pairs/pairs.pair/default.pass.cpp

@@ -0,0 +1,30 @@
+//===----------------------------------------------------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is dual licensed under the MIT and the University of Illinois Open
+// Source Licenses. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+// <utility>
+
+// template <class T1, class T2> struct pair
+
+// constexpr pair();
+
+
+#include <utility>
+#include <type_traits>
+
+#include "archetypes.hpp"
+
+
+int main() {
+    using NonThrowingDefault = NonThrowingTypes::DefaultOnly;
+    using ThrowingDefault = NonTrivialTypes::DefaultOnly;
+    static_assert(!std::is_nothrow_default_constructible<std::pair<ThrowingDefault, ThrowingDefault>>::value, "");
+    static_assert(!std::is_nothrow_default_constructible<std::pair<NonThrowingDefault, ThrowingDefault>>::value, "");
+    static_assert(!std::is_nothrow_default_constructible<std::pair<ThrowingDefault, NonThrowingDefault>>::value, "");
+    static_assert( std::is_nothrow_default_constructible<std::pair<NonThrowingDefault, NonThrowingDefault>>::value, "");
+}