Implement LWG 2591 - Patch from K-Ballo

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@302724 91177308-0d34-0410-b5e6-96231b3b80d8

test/std/utilities/function.objects/func.wrap/func.wrap.func/func.wrap.func.targ/target.pass.cpp

@@ -63,6 +63,7 @@ int main()
     assert(A::count == 1);
     assert(f.target<A>());
     assert(f.target<int(*)(int)>() == 0);
+    assert(f.target<int>() == nullptr);
     }
     assert(A::count == 0);
     {
@@ -70,6 +71,7 @@ int main()
     assert(A::count == 0);
     assert(f.target<int(*)(int)>());
     assert(f.target<A>() == 0);
+    assert(f.target<int>() == nullptr);
     }
     assert(A::count == 0);
     {
@@ -77,6 +79,7 @@ int main()
     assert(A::count == 1);
     assert(f.target<A>());
     assert(f.target<int(*)(int)>() == 0);
+    assert(f.target<int>() == nullptr);
     }
     assert(A::count == 0);
     {
@@ -84,6 +87,7 @@ int main()
     assert(A::count == 0);
     assert(f.target<int(*)(int)>());
     assert(f.target<A>() == 0);
+    assert(f.target<int>() == nullptr);
     }
     assert(A::count == 0);
 }

www/cxx1z_status.html

@@ -387,7 +387,7 @@
 	<tr><td><a href="http://wg21.link/LWG2584">2584</a></td><td><regex> ECMAScript IdentityEscape is ambiguous</td><td>Issaquah</td><td></td></tr>
 	<tr><td><a href="http://wg21.link/LWG2588">2588</a></td><td>[fund.ts.v2] "Convertible to bool" requirement in conjunction and disjunction</td><td>Issaquah</td><td></td></tr>
 	<tr><td><a href="http://wg21.link/LWG2589">2589</a></td><td>match_results can't satisfy the requirements of a container</td><td>Issaquah</td><td>Complete</td></tr>
-	<tr><td><a href="http://wg21.link/LWG2591">2591</a></td><td>std::function's member template target() should not lead to undefined behaviour</td><td>Issaquah</td><td></td></tr>
+	<tr><td><a href="http://wg21.link/LWG2591">2591</a></td><td>std::function's member template target() should not lead to undefined behaviour</td><td>Issaquah</td><td>Complete</td></tr>
 	<tr><td><a href="http://wg21.link/LWG2598">2598</a></td><td>addressof works on temporaries</td><td>Issaquah</td><td>Complete</td></tr>
 	<tr><td><a href="http://wg21.link/LWG2664">2664</a></td><td>operator/ (and other append) semantics not useful if argument has root</td><td>Issaquah</td><td>Complete</td></tr>
 	<tr><td><a href="http://wg21.link/LWG2665">2665</a></td><td>remove_filename() post condition is incorrect</td><td>Issaquah</td><td>Complete</td></tr>