diff --git a/libstdc++-v3/ChangeLog b/libstdc++-v3/ChangeLog
index 84fc03b048b..7bfd9be15c8 100644
--- a/libstdc++-v3/ChangeLog
+++ b/libstdc++-v3/ChangeLog
@@ -1,3 +1,13 @@
+2019-07-31  Jonathan Wakely  <jwakely@redhat.com>
+
+	PR libstdc++/91308
+	* include/bits/unique_ptr.h (unique_ptr::__safe_conversion_up): Remove
+	constraints on deleter that should only apply to the constructor.
+	(unique_ptr<T[], D>::__safe_conversion_up): Likewise.
+	(unique_ptr<T[], D>::unique_ptr(unique_ptr<U, D>&&)): Restore
+	constraints on deleter here.
+	* testsuite/20_util/unique_ptr/assign/91308.cc: New test.
+
 2019-07-29  Jonathan Wakely  <jwakely@redhat.com>
 
 	PR libstdc++/51333
diff --git a/libstdc++-v3/include/bits/unique_ptr.h b/libstdc++-v3/include/bits/unique_ptr.h
index 484c8b328e4..0c4eb256810 100644
--- a/libstdc++-v3/include/bits/unique_ptr.h
+++ b/libstdc++-v3/include/bits/unique_ptr.h
@@ -245,14 +245,9 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       // unique_ptr
       template<typename _Up, typename _Ep>
 	using __safe_conversion_up = __and_<
-	        is_convertible<typename unique_ptr<_Up, _Ep>::pointer, pointer>,
-                __not_<is_array<_Up>>,
-                __or_<__and_<is_reference<deleter_type>,
-                             is_same<deleter_type, _Ep>>,
-                      __and_<__not_<is_reference<deleter_type>>,
-                             is_convertible<_Ep, deleter_type>>
-                >
-              >;
+	  is_convertible<typename unique_ptr<_Up, _Ep>::pointer, pointer>,
+	  __not_<is_array<_Up>>
+        >;
 
     public:
       // Constructors.
@@ -492,16 +487,14 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       // helper template for detecting a safe conversion from another
       // unique_ptr
       template<typename _Up, typename _Ep,
-               typename _Up_up = unique_ptr<_Up, _Ep>,
-	       typename _Up_element_type = typename _Up_up::element_type>
+               typename _UPtr = unique_ptr<_Up, _Ep>,
+	       typename _UP_pointer = typename _UPtr::pointer,
+	       typename _UP_element_type = typename _UPtr::element_type>
 	using __safe_conversion_up = __and_<
           is_array<_Up>,
           is_same<pointer, element_type*>,
-          is_same<typename _Up_up::pointer, _Up_element_type*>,
-          is_convertible<_Up_element_type(*)[], element_type(*)[]>,
-          __or_<__and_<is_reference<deleter_type>, is_same<deleter_type, _Ep>>,
-                __and_<__not_<is_reference<deleter_type>>,
-                       is_convertible<_Ep, deleter_type>>>
+          is_same<_UP_pointer, _UP_element_type*>,
+          is_convertible<_UP_element_type(*)[], element_type(*)[]>
         >;
 
       // helper template for detecting a safe conversion from a raw pointer
@@ -590,8 +583,11 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	: _M_t()
         { }
 
-      template<typename _Up, typename _Ep,
-	       typename = _Require<__safe_conversion_up<_Up, _Ep>>>
+      template<typename _Up, typename _Ep, typename = _Require<
+	       __safe_conversion_up<_Up, _Ep>,
+	       typename conditional<is_reference<_Dp>::value,
+				    is_same<_Ep, _Dp>,
+				    is_convertible<_Ep, _Dp>>::type>>
 	unique_ptr(unique_ptr<_Up, _Ep>&& __u) noexcept
 	: _M_t(__u.release(), std::forward<_Ep>(__u.get_deleter()))
 	{ }
diff --git a/libstdc++-v3/testsuite/20_util/unique_ptr/assign/91308.cc b/libstdc++-v3/testsuite/20_util/unique_ptr/assign/91308.cc
new file mode 100644
index 00000000000..6500ea43f02
--- /dev/null
+++ b/libstdc++-v3/testsuite/20_util/unique_ptr/assign/91308.cc
@@ -0,0 +1,46 @@
+// Copyright (C) 2019 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-do compile { target c++11 } }
+
+#include <memory>
+
+struct D1
+{
+  void operator()(int* p) const noexcept { }
+};
+
+struct D2 : D1
+{
+  D2& operator=(D1&&) noexcept { return *this; }
+};
+
+void
+test01()
+{
+  std::unique_ptr<int, D1> d1;
+  std::unique_ptr<int, D2> d2;
+  d2 = std::move(d1);
+}
+
+void
+test02()
+{
+  std::unique_ptr<int[], D1> d1;
+  std::unique_ptr<int[], D2> d2;
+  d2 = std::move(d1);
+}