[gcc r10-9933] libstdc++: Implement proposed resolution to LWG 3548

[gcc r10-9933] libstdc++: Implement proposed resolution to LWG 3548

[Jonathan Wakely]

https://gcc.gnu.org/g:15d6f574615b78c5e783ea51924f6e452a3ff45b

commit r10-9933-g15d6f574615b78c5e783ea51924f6e452a3ff45b
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Mon May 10 20:46:38 2021 +0100

    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.
    
    (cherry picked from commit 5edc0c15f1667cc2a5deb664b25c007b35d259f6)

Diff:
---
 libstdc++-v3/include/bits/shared_ptr_base.h             | 13 +++++++++----
 .../testsuite/20_util/shared_ptr/cons/lwg3548.cc        | 16 ++++++++++++++++
 .../20_util/shared_ptr/cons/unique_ptr_deleter.cc       | 17 ++++++++++++++++-
 3 files changed, 41 insertions(+), 5 deletions(-)

diff --git a/libstdc++-v3/include/bits/shared_ptr_base.h b/libstdc++-v3/include/bits/shared_ptr_base.h
index f0232c36be9..219007b2c07 100644
--- a/libstdc++-v3/include/bits/shared_ptr_base.h
+++ b/libstdc++-v3/include/bits/shared_ptr_base.h
@@ -716,8 +716,11 @@ _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;
 	}
 
@@ -1104,9 +1107,11 @@ _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>
diff --git a/libstdc++-v3/testsuite/20_util/shared_ptr/cons/lwg3548.cc b/libstdc++-v3/testsuite/20_util/shared_ptr/cons/lwg3548.cc
new file mode 100644
index 00000000000..d6ec7b1d057
--- /dev/null
+++ b/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));
diff --git a/libstdc++-v3/testsuite/20_util/shared_ptr/cons/unique_ptr_deleter.cc b/libstdc++-v3/testsuite/20_util/shared_ptr/cons/unique_ptr_deleter.cc
index 3205cc37f1f..506e1823c8f 100644
--- a/libstdc++-v3/testsuite/20_util/shared_ptr/cons/unique_ptr_deleter.cc
+++ b/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();
 }