[PATCH] libstdc++: Fix the default constructor of ranges::__detail::__box

[Patrick Palka <ppalka@redhat.com>]

The class template semiregular-box<T> of [range.semi.wrap] is specified
to value-initialize the underlying object whenever its type is default-
initializable.  Our primary template for __detail::__box respects this
requirement, but the recently added partial specialization (for types
which are already semiregular) does not.

This patch fixes this issue, and additionally makes the in place
constructor explicit (as in the primary template).

Tested on x86_64-pc-linux-gnu, does this look OK for trunk?

libstdc++-v3/ChangeLog:

	* include/std/ranges (__detail::__box): For the partial
	specialization for types that are already semiregular,
	make the default constructor value-initialize the underlying
	object instead of default-initializing it.  Make the
	corresponding in place constructor explicit.
	* testsuite/std/ranges/detail/semiregular_box.cc: New test.
---
 libstdc++-v3/include/std/ranges               |  4 +--
 .../std/ranges/detail/semiregular_box.cc      | 33 +++++++++++++++++++
 2 files changed, 35 insertions(+), 2 deletions(-)
 create mode 100644 libstdc++-v3/testsuite/std/ranges/detail/semiregular_box.cc

diff --git a/libstdc++-v3/include/std/ranges b/libstdc++-v3/include/std/ranges
index df02b03cada..59aac326309 100644
--- a/libstdc++-v3/include/std/ranges
+++ b/libstdc++-v3/include/std/ranges
@@ -141,7 +141,7 @@ namespace ranges
       struct __box<_Tp>
       {
       private:
-	[[no_unique_address]] _Tp _M_value;
+	[[no_unique_address]] _Tp _M_value = {};
 
       public:
 	__box() = default;
@@ -160,7 +160,7 @@ namespace ranges
 
 	template<typename... _Args>
 	  requires constructible_from<_Tp, _Args...>
-	  constexpr
+	  constexpr explicit
 	  __box(in_place_t, _Args&&... __args)
 	  noexcept(is_nothrow_constructible_v<_Tp, _Args...>)
 	  : _M_value{std::forward<_Args>(__args)...}
diff --git a/libstdc++-v3/testsuite/std/ranges/detail/semiregular_box.cc b/libstdc++-v3/testsuite/std/ranges/detail/semiregular_box.cc
new file mode 100644
index 00000000000..2a909d6ae50
--- /dev/null
+++ b/libstdc++-v3/testsuite/std/ranges/detail/semiregular_box.cc
@@ -0,0 +1,33 @@
+// Copyright (C) 2020 Free Software Foundation, Inc.
+//
+// This file is part of the GNU ISO C++ Library.  This library is free
+// software; you can redistribute it and/or modify it under the
+// terms of the GNU General Public License as published by the
+// Free Software Foundation; either version 3, or (at your option)
+// any later version.
+
+// This library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License along
+// with this library; see the file COPYING3.  If not see
+// <http://www.gnu.org/licenses/>.
+
+// { dg-options "-std=gnu++2a" }
+// { dg-do compile { target c++2a } }
+
+#include <ranges>
+
+using std::ranges::__detail::__box;
+
+constexpr bool
+test01()
+{
+  // Verify the default constructor value-initializes the underlying object.
+  __box<int> x;
+  __glibcxx_assert(*x == 0);
+  return true;
+}
+static_assert(test01());
-- 
2.29.0.rc0