[libc++] Mark N4089 as complete

The paper has been implemented since at least LLVM 5, maybe earlier.

Author: philnik777

libcxx/docs/Status/Cxx17Papers.csv

@@ -1,6 +1,6 @@
 "Paper #","Paper Name","Meeting","Status","First released version","Notes"
 "`N3911 <https://wg21.link/N3911>`__","TransformationTrait Alias ``void_t``\ .","2014-11 (Urbana)","|Complete|","3.6",""
-"`N4089 <https://wg21.link/N4089>`__","Safe conversions in ``unique_ptr<T[]>``\ .","2014-11 (Urbana)","|In Progress|","3.9",""
+"`N4089 <https://wg21.link/N4089>`__","Safe conversions in ``unique_ptr<T[]>``\ .","2014-11 (Urbana)","|Complete|","5",""
 "`N4169 <https://wg21.link/N4169>`__","A proposal to add invoke function template","2014-11 (Urbana)","|Complete|","3.7",""
 "`N4190 <https://wg21.link/N4190>`__","Removing auto_ptr, random_shuffle(), And Old <functional> Stuff.","2014-11 (Urbana)","|Complete|","15",""
 "`N4258 <https://wg21.link/N4258>`__","Cleaning-up noexcept in the Library.","2014-11 (Urbana)","|In Progress|","3.7",""

libcxx/test/std/utilities/smartptr/unique.ptr/unique.ptr.class/unique.ptr.asgn/move_convert.runtime.pass.cpp

@@ -103,6 +103,12 @@ void test_sfinae() {
     using UA2 = std::unique_ptr<A[], const DAC&>;
     static_assert(!std::is_assignable<UA1, UA2&&>::value, "");
   }
+  { // cannot move-convert with reference deleters with different qualifiers
+    using UA1 = std::unique_ptr<A[], DA&>;
+    using UA2 = std::unique_ptr<A[], const DA&>;
+    static_assert(!std::is_assignable<UA1, UA2&&>::value, "");
+    static_assert(!std::is_assignable<UA2, UA1&&>::value, "");
+  }
   { // cannot move-convert from unique_ptr<Single>
     using UA1 = std::unique_ptr<A[]>;
     using UA2 = std::unique_ptr<A>;