[gcc r11-4575] libstdc++: Fix the default constructor of ranges::__detail::__box

[Patrick Palka <ppalka@gcc.gnu.org>]

https://gcc.gnu.org/g:f3ced6772e2257bdef4e6f4dbdd0bdcf34b5bb3f

commit r11-4575-gf3ced6772e2257bdef4e6f4dbdd0bdcf34b5bb3f
Author: Patrick Palka <ppalka@redhat.com>
Date:   Fri Oct 30 12:33:13 2020 -0400

    libstdc++: Fix the default constructor of ranges::__detail::__box
    
    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
    that are already semiregular) does not.
    
    This patch fixes this issue, and additionally makes the corresponding in
    place constructor explicit (as in the primary template).
    
    libstdc++-v3/ChangeLog:
    
            * include/std/ranges (__detail::__box): For the partial
            specialization used by types that are already semiregular,
            make the default constructor value-initialize the underlying
            object instead of default-initializing it.  Make its in place
            constructor explicit.
            * testsuite/std/ranges/adaptors/detail/semiregular_box.cc:
            Augment test.

Diff:
---
 libstdc++-v3/include/std/ranges                                |  4 ++--
 .../testsuite/std/ranges/adaptors/detail/semiregular_box.cc    | 10 ++++++++++
 2 files changed, 12 insertions(+), 2 deletions(-)

diff --git a/libstdc++-v3/include/std/ranges b/libstdc++-v3/include/std/ranges
index a3e5354848a..610083167d8 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 = _Tp();
 
       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/adaptors/detail/semiregular_box.cc b/libstdc++-v3/testsuite/std/ranges/adaptors/detail/semiregular_box.cc
index 392acff3eb6..748137f7e33 100644
--- a/libstdc++-v3/testsuite/std/ranges/adaptors/detail/semiregular_box.cc
+++ b/libstdc++-v3/testsuite/std/ranges/adaptors/detail/semiregular_box.cc
@@ -71,3 +71,13 @@ static_assert(requires (__box<U> a) {
   a.operator->();
   a.has_value();
 });
+
+constexpr bool
+test01()
+{
+  // Verify the default constructor value-initializes the underlying object.
+  __box<int> x;
+  __glibcxx_assert(*x == 0);
+  return true;
+}
+static_assert(test01());