[libc++] Implement LWG 3158

Summary:
LWG 3158 marks the allocator_arg_t constructor of std::tuple as
conditionnally explicit based on whether the default constructors
of the tuple's members are explicitly default constructible.

Reviewers: EricWF, mclow.lists

Subscribers: christof, jkorous, dexonsmith, libcxx-commits

Tags: #libc

Differential Revision: https://reviews.llvm.org/D65232

llvm-svn: 372778

2 files changed, 34 insertions, 1 deletions

diff --git a/libcxx/test/std/utilities/tuple/tuple.tuple/tuple.cnstr/alloc.fail.cpp b/libcxx/test/std/utilities/tuple/tuple.tuple/tuple.cnstr/alloc.fail.cpp
new file mode 100644
index 00000000000..abb8073a6a5
--- /dev/null
+++ b/libcxx/test/std/utilities/tuple/tuple.tuple/tuple.cnstr/alloc.fail.cpp
@@ -0,0 +1,33 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+
+// <tuple>
+
+// template <class... Types> class tuple;
+
+// template <class Alloc>
+//   explicit(see-below) tuple(allocator_arg_t, const Alloc& a);
+
+// Make sure we get the explicit-ness of the constructor right.
+// This is LWG 3158.
+
+#include <tuple>
+#include <memory>
+
+
+struct ExplicitDefault { explicit ExplicitDefault() { } };
+
+std::tuple<ExplicitDefault> explicit_default_test() {
+    return {std::allocator_arg, std::allocator<int>()}; // expected-error {{chosen constructor is explicit in copy-initialization}}
+}
+
+int main(int, char**) {
+    return 0;
+}
diff --git a/libcxx/test/std/utilities/tuple/tuple.tuple/tuple.cnstr/alloc.pass.cpp b/libcxx/test/std/utilities/tuple/tuple.tuple/tuple.cnstr/alloc.pass.cpp
index b0f957e3a37..13511fa5148 100644
--- a/libcxx/test/std/utilities/tuple/tuple.tuple/tuple.cnstr/alloc.pass.cpp
+++ b/libcxx/test/std/utilities/tuple/tuple.tuple/tuple.cnstr/alloc.pass.cpp
@@ -13,7 +13,7 @@
 // template <class... Types> class tuple;
 
 // template <class Alloc>
-//   tuple(allocator_arg_t, const Alloc& a);
+//   explicit(see-below) tuple(allocator_arg_t, const Alloc& a);
 
 // NOTE: this constructor does not currently support tags derived from
 // allocator_arg_t because libc++ has to deduce the parameter as a template