libstdc++: Implement proposed resolution to LWG 3548

This has been tentatively approved by LWG. The deleter from a unique_ptr
can be moved into the shared_ptr (at least, since LWG 2802). This uses
std::forward<_Del>(__r.get_deleter()) not std::move(__r.get_deleter())
because we don't want to convert the deleter to an rvalue when _Del is
an lvalue reference type.

This also adds a missing is_move_constructible_v<D> constraint to the
shared_ptr(unique_ptr<Y, D>&&) constructor, which is inherited from the
shared_ptr(Y*, D) constructor due to the use of "equivalent to" in the
specified effects.

libstdc++-v3/ChangeLog:

	* include/bits/shared_ptr_base.h (__shared_count(unique_ptr&&)):
	Initialize a non-reference deleter from an rvalue, as per LWG
	3548.
	(__shared_ptr::_UniqCompatible): Add missing constraint.
	* testsuite/20_util/shared_ptr/cons/lwg3548.cc: New test.
	* testsuite/20_util/shared_ptr/cons/unique_ptr_deleter.cc: Check
	constraints.

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

@@ -684,8 +684,11 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	  using _Alloc_traits = allocator_traits<_Alloc>;
 	  _Alloc __a;
 	  _Sp_cd_type* __mem = _Alloc_traits::allocate(__a, 1);
+	  // _GLIBCXX_RESOLVE_LIB_DEFECTS
+	  // 3548. shared_ptr construction from unique_ptr should move
+	  // (not copy) the deleter
 	  _Alloc_traits::construct(__a, __mem, __r.release(),
-				   __r.get_deleter());  // non-throwing
+				   std::forward<_Del>(__r.get_deleter()));
 	  _M_pi = __mem;
 	}
 
@@ -1070,9 +1073,11 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       // Constraint for construction from unique_ptr:
       template<typename _Yp, typename _Del, typename _Res = void,
 	       typename _Ptr = typename unique_ptr<_Yp, _Del>::pointer>
-	using _UniqCompatible = typename enable_if<__and_<
-	  __sp_compatible_with<_Yp*, _Tp*>, is_convertible<_Ptr, element_type*>
-	  >::value, _Res>::type;
+	using _UniqCompatible = __enable_if_t<__and_<
+	  __sp_compatible_with<_Yp*, _Tp*>,
+	  is_convertible<_Ptr, element_type*>,
+	  is_move_constructible<_Del>
+	  >::value, _Res>;
 
       // Constraint for assignment from unique_ptr:
       template<typename _Yp, typename _Del>

libstdc++-v3/testsuite/20_util/shared_ptr/cons/lwg3548.cc

@@ -0,0 +1,16 @@
+// { dg-do compile { target c++11 } }
+
+#include <memory>
+
+// LWG 3548
+// shared_ptr construction from unique_ptr should move (not copy) the deleter
+
+struct D
+{
+  D() { }
+  D(D&&) { }
+  void operator()(int* p) const { delete p; }
+};
+
+std::unique_ptr<int, D> u;
+std::shared_ptr<int> s1(std::move(u));

libstdc++-v3/testsuite/20_util/shared_ptr/cons/unique_ptr_deleter.cc

@@ -58,10 +58,25 @@ test02()
   VERIFY( D::count == 0 ); // LWG 2415
 }
 
+void
+test03()
+{
+  struct D
+  {
+    D() = default;
+    D(const D&) = delete; // not copyable or movable
+    void operator()(int* p) const { delete p; }
+  };
+
+  using namespace std;
+  static_assert( ! is_constructible<shared_ptr<int>, unique_ptr<int, D>>(),
+		 "Constraints: is_move_constructible_v<D> is true" );
+}
+
 int
 main()
 {
   test01();
   test02();
-  return 0;
+  test03();
 }