libstdc++: Implement operator<< for std::unique_ptr (LWG 2948)

libstdc++-v3/ChangeLog:

	* include/bits/unique_ptr.h (operator<<): Define for C++20.
	* testsuite/20_util/unique_ptr/io/lwg2948.cc: New test.

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

@@ -39,6 +39,7 @@
 #include <bits/functional_hash.h>
 #if __cplusplus > 201703L
 # include <compare>
+# include <ostream>
 #endif
 
 namespace std _GLIBCXX_VISIBILITY(default)
@@ -934,7 +935,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       public __uniq_ptr_hash<unique_ptr<_Tp, _Dp>>
     { };
 
-#if __cplusplus > 201103L
+#if __cplusplus >= 201402L
   /// @relates unique_ptr @{
 #define __cpp_lib_make_unique 201304
 
@@ -971,7 +972,22 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     inline typename _MakeUniq<_Tp>::__invalid_type
     make_unique(_Args&&...) = delete;
   // @} relates unique_ptr
-#endif
+#endif // C++14
+
+#if __cplusplus > 201703L && __cpp_concepts
+  // _GLIBCXX_RESOLVE_LIB_DEFECTS
+  // 2948. unique_ptr does not define operator<< for stream output
+  /// Stream output operator for unique_ptr
+  template<typename _CharT, typename _Traits, typename _Tp, typename _Dp>
+    inline basic_ostream<_CharT, _Traits>&
+    operator<<(basic_ostream<_CharT, _Traits>& __os,
+	       const unique_ptr<_Tp, _Dp>& __p)
+    requires requires { __os << __p.get(); }
+    {
+      __os << __p.get();
+      return __os;
+    }
+#endif // C++20
 
   // @} group pointer_abstractions
 

libstdc++-v3/testsuite/20_util/unique_ptr/io/lwg2948.cc

@@ -0,0 +1,91 @@
+// 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 run { target c++2a } }
+
+#include <memory>
+#include <sstream>
+#include <testsuite_hooks.h>
+
+template<typename S, typename T>
+  concept streamable = requires (S& o, const T& p) { o << p; };
+
+template<typename T, typename D>
+  bool
+  check(const std::unique_ptr<T, D>& p)
+  {
+    std::ostringstream ss1, ss2;
+    ss1 << p;
+    ss2 << p.get();
+    return ss1.str() == ss2.str();
+  }
+
+void
+test01()
+{
+  static_assert( streamable<std::ostream, std::unique_ptr<int>> );
+
+  std::unique_ptr<int> p;
+  VERIFY( check(p) );
+  p = std::make_unique<int>();
+  VERIFY( check(p) );
+}
+
+template<typename>
+  struct deleter
+  {
+    struct pointer
+    {
+      pointer() { }
+      pointer(std::nullptr_t) { }
+      explicit operator bool() const { return false; }
+      bool operator==(pointer) const { return true; }
+    };
+
+    void operator()(pointer) const { }
+  };
+
+template<typename C, typename Traits>
+  int
+  operator<<(std::basic_ostream<C, Traits>& o, typename deleter<C>::pointer&&)
+  {
+    o << C{'P'};
+    return 1; // no requirement that this operator returns the stream
+  }
+
+template<typename D>
+  using Unique_ptr = std::unique_ptr<typename D::pointer, D>;
+
+static_assert( streamable<std::ostream, Unique_ptr<deleter<char>>> );
+static_assert( ! streamable<std::ostream, Unique_ptr<deleter<wchar_t>>> );
+static_assert( ! streamable<std::wostream, Unique_ptr<deleter<char>>> );
+static_assert( streamable<std::wostream, Unique_ptr<deleter<wchar_t>>> );
+
+void
+test02()
+{
+  Unique_ptr<deleter<char>> p;
+  VERIFY( check(p) );
+}
+
+int
+main()
+{
+  test01();
+  test02();
+}