libstdc++: Make ranges::construct_at constexpr-friendly [PR95788]

This rewrites ranges::construct_at in terms of std::construct_at so
that we can piggyback on the compiler's existing support for
intercepting placement new within std::construct_at during constexpr
evaluation, instead of having to additionally teach the compiler about
ranges::construct_at.

While we're making changes to ranges::construct_at, this patch also
declares it conditionally noexcept and qualifies the calls to declval in
its requires-clause.

libstdc++-v3/ChangeLog:

	PR libstdc++/95788
	* include/bits/ranges_uninitialized.h:
	(__construct_at_fn::operator()): Rewrite in terms of
	std::construct_at.  Declare it conditionally noexcept.  Qualify
	calls to declval in its requires-clause.
	* testsuite/20_util/specialized_algorithms/construct_at/95788.cc:
	New test.

2 files changed, 48 insertions, 3 deletions

diff --git a/libstdc++-v3/include/bits/ranges_uninitialized.h b/libstdc++-v3/include/bits/ranges_uninitialized.h
index d758078..25e664d 100644
--- a/libstdc++-v3/include/bits/ranges_uninitialized.h
+++ b/libstdc++-v3/include/bits/ranges_uninitialized.h
@@ -493,12 +493,16 @@ namespace ranges
   struct __construct_at_fn
   {
     template<typename _Tp, typename... _Args>
-      requires requires { ::new (declval<void*>()) _Tp(declval<_Args>()...); }
+      requires requires {
+	::new (std::declval<void*>()) _Tp(std::declval<_Args>()...);
+      }
       constexpr _Tp*
       operator()(_Tp* __location, _Args&&... __args) const
+      noexcept(noexcept(std::construct_at(__location,
+					  std::forward<_Args>(__args)...)))
       {
-	return ::new (__detail::__voidify(*__location))
-		     _Tp(std::forward<_Args>(__args)...);
+	return std::construct_at(__location,
+				 std::forward<_Args>(__args)...);
       }
   };
 
diff --git a/libstdc++-v3/testsuite/20_util/specialized_algorithms/construct_at/95788.cc b/libstdc++-v3/testsuite/20_util/specialized_algorithms/construct_at/95788.cc
new file mode 100644
index 0000000..895332b
--- /dev/null
+++ b/libstdc++-v3/testsuite/20_util/specialized_algorithms/construct_at/95788.cc
@@ -0,0 +1,41 @@
+// 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 <memory>
+
+constexpr bool
+test01()
+{
+  const int sz{1};
+  int* data{std::allocator<int>{}.allocate(sz)};
+  static_assert(noexcept(std::ranges::construct_at(data, 42)));
+  std::ranges::construct_at(data, 42);
+  if (*data != 42)
+    return false;
+  std::ranges::destroy_at(data);
+  std::allocator<int>{}.deallocate(data, sz);
+  return true;
+}
+
+static_assert(test01());
+
+struct S { S(); };
+S* p;
+static_assert(!noexcept(std::ranges::construct_at(p)));