libstdc++: Implement std::make_unique_for_overwrite

This is the std::unique_ptr part of P1020R1 (as amended by P1973R1) for
C++20. The std::shared_ptr part still needs to be done.

libstdc++-v3/ChangeLog:

	* include/bits/unique_ptr.h (make_unique_for_overwrite): Define
	for C++20.
	* testsuite/20_util/unique_ptr/creation/array_neg.cc: Remove
	unused header. Adjust standard reference.
	* testsuite/20_util/unique_ptr/creation/for_overwrite.cc: New test.
	* testsuite/20_util/unique_ptr/creation/for_overwrite__neg.cc: New test.

libstdc++-v3/include/bits/unique_ptr.h

@@ -969,8 +969,28 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 
   /// Disable std::make_unique for arrays of known bound
   template<typename _Tp, typename... _Args>
-    inline typename _MakeUniq<_Tp>::__invalid_type
+    typename _MakeUniq<_Tp>::__invalid_type
     make_unique(_Args&&...) = delete;
+
+#if __cplusplus > 201703L
+  /// std::make_unique_for_overwrite for single objects
+  template<typename _Tp>
+    inline typename _MakeUniq<_Tp>::__single_object
+    make_unique_for_overwrite()
+    { return unique_ptr<_Tp>(new _Tp); }
+
+  /// std::make_unique_for_overwrite for arrays of unknown bound
+  template<typename _Tp>
+    inline typename _MakeUniq<_Tp>::__array
+    make_unique_for_overwrite(size_t __n)
+    { return unique_ptr<_Tp>(new remove_extent_t<_Tp>[__n]); }
+
+  /// Disable std::make_unique_for_overwrite for arrays of known bound
+  template<typename _Tp, typename... _Args>
+    typename _MakeUniq<_Tp>::__invalid_type
+    make_unique_for_overwrite(_Args&&...) = delete;
+#endif // C++20
+
   // @} relates unique_ptr
 #endif // C++14
 

libstdc++-v3/testsuite/20_util/unique_ptr/creation/array_neg.cc

@@ -17,10 +17,9 @@
 // with this library; see the file COPYING3.  If not see
 // <http://www.gnu.org/licenses/>.
 
-// 20.9.1.4 unique_ptr creation [unique.ptr.create]
+// C++14 20.8.1.4 unique_ptr creation [unique.ptr.create]
 
 #include <memory>
-#include <testsuite_hooks.h>
 
 struct A { };
 

libstdc++-v3/testsuite/20_util/unique_ptr/creation/for_overwrite.cc

@@ -0,0 +1,65 @@
+// { dg-options "-std=gnu++20" }
+// { dg-do run { target c++2a } }
+
+// 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/>.
+
+// C++20 20.11.1.5 unique_ptr creation [unique.ptr.create]
+
+#include <memory>
+#include <cstdlib>
+#include <cstring>
+#include <testsuite_hooks.h>
+
+void* operator new(std::size_t n)
+{
+  void* p = std::malloc(n);
+  std::memset(p, 0xaa, n);
+  return p;
+}
+
+void operator delete(void* p) { std::free(p); }
+void operator delete(void* p, std::size_t) { std::free(p); }
+
+void
+test01()
+{
+  std::unique_ptr<int> a = std::make_unique_for_overwrite<int>();
+  VERIFY( a != nullptr );
+  unsigned char buf[sizeof(int)];
+  std::memcpy(buf, a.get(), sizeof(buf));
+  for (unsigned char c : buf)
+    VERIFY( c == 0xaa );
+}
+
+void
+test02()
+{
+  std::unique_ptr<int[]> a = std::make_unique_for_overwrite<int[]>(3);
+  VERIFY( a != nullptr );
+  unsigned char buf[3 * sizeof(int)];
+  std::memcpy(buf, a.get(), sizeof(buf));
+  for (unsigned char c : buf)
+    VERIFY( c == 0xaa );
+}
+
+int
+main()
+{
+  test01();
+  test02();
+}

libstdc++-v3/testsuite/20_util/unique_ptr/creation/for_overwrite__neg.cc

@@ -0,0 +1,34 @@
+// { dg-options "-std=gnu++20" }
+// { dg-do compile { target c++2a } }
+
+// 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/>.
+
+// C++20 20.11.1.5 unique_ptr creation [unique.ptr.create]
+
+#include <memory>
+
+struct A { };
+
+auto p1 = std::make_unique_for_overwrite<A>(1);   // { dg-error "no matching function" }
+auto p1 = std::make_unique_for_overwrite<A[]>();  // { dg-error "no matching function" }
+auto p2 = std::make_unique_for_overwrite<A[]>(1, 2); // { dg-error "no matching function" }
+auto p3 = std::make_unique_for_overwrite<A[1]>(); // { dg-error "deleted" }
+auto p4 = std::make_unique_for_overwrite<A[1]>(1);// { dg-error "deleted" }
+
+// { dg-prune-output "declared here" }
+// { dg-prune-output "no type named" }