[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

5 files changed, 102 insertions, 2 deletions

diff --git a/libcxx/test/std/utilities/utility/pairs/pair.piecewise/piecewise_construct.pass.cpp b/libcxx/test/std/utilities/utility/pairs/pair.piecewise/piecewise_construct.pass.cpp
index d70d060deb0..903aa935583 100644
--- a/libcxx/test/std/utilities/utility/pairs/pair.piecewise/piecewise_construct.pass.cpp
+++ b/libcxx/test/std/utilities/utility/pairs/pair.piecewise/piecewise_construct.pass.cpp
@@ -12,7 +12,7 @@
 
 // template <class T1, class T2> struct pair
 
-// struct piecewise_construct_t { };
+// struct piecewise_construct_t { explicit piecewise_construct_t() = default; };
 // constexpr piecewise_construct_t piecewise_construct = piecewise_construct_t();
 
 #include <utility>
diff --git a/libcxx/test/std/utilities/utility/pairs/pair.piecewise/piecewise_construct_t.fail.cpp b/libcxx/test/std/utilities/utility/pairs/pair.piecewise/piecewise_construct_t.fail.cpp
new file mode 100644
index 00000000000..b8dd28a5764
--- /dev/null
+++ b/libcxx/test/std/utilities/utility/pairs/pair.piecewise/piecewise_construct_t.fail.cpp
@@ -0,0 +1,29 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++98, c++03
+
+// Before GCC 6, aggregate initialization kicks in.
+// See https://stackoverflow.com/q/41799015/627587.
+// UNSUPPORTED: gcc-5
+
+// <utility>
+
+// struct piecewise_construct_t { explicit piecewise_construct_t() = default; };
+// constexpr piecewise_construct_t piecewise_construct = piecewise_construct_t();
+
+// This test checks for LWG 2510.
+
+#include <utility>
+
+
+std::piecewise_construct_t f() { return {}; } // expected-error 1 {{chosen constructor is explicit in copy-initialization}}
+
+int main(int, char**) {
+    return 0;
+}
diff --git a/libcxx/test/std/utilities/utility/pairs/pair.piecewise/piecewise_construct_t.pass.cpp b/libcxx/test/std/utilities/utility/pairs/pair.piecewise/piecewise_construct_t.pass.cpp
new file mode 100644
index 00000000000..0fdba067487
--- /dev/null
+++ b/libcxx/test/std/utilities/utility/pairs/pair.piecewise/piecewise_construct_t.pass.cpp
@@ -0,0 +1,24 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++98, c++03
+
+// <utility>
+
+// struct piecewise_construct_t { explicit piecewise_construct_t() = default; };
+// constexpr piecewise_construct_t piecewise_construct = piecewise_construct_t();
+
+#include <utility>
+
+
+int main(int, char**) {
+    std::piecewise_construct_t x = std::piecewise_construct;
+    (void)x;
+
+    return 0;
+}
diff --git a/libcxx/test/std/utilities/utility/pairs/pairs.pair/default.explicit.fail.cpp b/libcxx/test/std/utilities/utility/pairs/pairs.pair/default.explicit.fail.cpp
new file mode 100644
index 00000000000..511f21ec301
--- /dev/null
+++ b/libcxx/test/std/utilities/utility/pairs/pairs.pair/default.explicit.fail.cpp
@@ -0,0 +1,42 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++98, c++03
+
+// Before GCC 6, aggregate initialization kicks in.
+// See https://stackoverflow.com/q/41799015/627587.
+// UNSUPPORTED: gcc-5
+
+// <utility>
+
+// template <class T1, class T2> struct pair
+
+// explicit(see-below) constexpr pair();
+
+// This test checks the conditional explicitness of std::pair's default
+// constructor as introduced by the resolution of LWG 2510.
+
+#include <utility>
+
+
+struct ImplicitlyDefaultConstructible {
+    ImplicitlyDefaultConstructible() = default;
+};
+
+struct ExplicitlyDefaultConstructible {
+    explicit ExplicitlyDefaultConstructible() = default;
+};
+
+std::pair<ImplicitlyDefaultConstructible, ExplicitlyDefaultConstructible> test1() { return {}; } // expected-error 1 {{chosen constructor is explicit in copy-initialization}}
+std::pair<ExplicitlyDefaultConstructible, ImplicitlyDefaultConstructible> test2() { return {}; } // expected-error 1 {{chosen constructor is explicit in copy-initialization}}
+std::pair<ExplicitlyDefaultConstructible, ExplicitlyDefaultConstructible> test3() { return {}; } // expected-error 1 {{chosen constructor is explicit in copy-initialization}}
+std::pair<ImplicitlyDefaultConstructible, ImplicitlyDefaultConstructible> test4() { return {}; }
+
+int main(int, char**) {
+    return 0;
+}
diff --git a/libcxx/test/std/utilities/utility/pairs/pairs.pair/default.pass.cpp b/libcxx/test/std/utilities/utility/pairs/pairs.pair/default.pass.cpp
index 049c2206088..dc05a2b4885 100644
--- a/libcxx/test/std/utilities/utility/pairs/pairs.pair/default.pass.cpp
+++ b/libcxx/test/std/utilities/utility/pairs/pairs.pair/default.pass.cpp
@@ -10,7 +10,7 @@
 
 // template <class T1, class T2> struct pair
 
-// constexpr pair();
+// explicit(see-below) constexpr pair();
 
 // NOTE: The SFINAE on the default constructor is tested in
 //       default-sfinae.pass.cpp
@@ -45,6 +45,11 @@ int main(int, char**)
         using P2 = std::pair<NoDefault, int>;
         static_assert(!std::is_default_constructible<P2>::value, "");
     }
+    {
+        struct Base { };
+        struct Derived : Base { protected: Derived() = default; };
+        static_assert(!std::is_default_constructible<std::pair<Derived, int> >::value, "");
+    }
 #endif
 
   return 0;