Fix return type of std::tuple_cat.

When the arguments to tuple cat were const, the const was incorrectly
propagated into the type of the resulting tuple. For example:

const std::tuple<int> t(42);
auto r = std::tuple_cat(t, t);
// Incorrect! should be std::tuple<int, int>.
static_assert(is_same_v<decltype(r), std::tuple<const int, const int>>);

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@359255 91177308-0d34-0410-b5e6-96231b3b80d8

include/tuple

@@ -1211,7 +1211,7 @@ template <class ..._Types, class _Tuple0>
 struct __tuple_cat_return_1<tuple<_Types...>, true, _Tuple0>
 {
     typedef typename __tuple_cat_type<tuple<_Types...>,
-            typename __make_tuple_types<typename remove_reference<_Tuple0>::type>::type>::type
+            typename __make_tuple_types<typename __uncvref<_Tuple0>::type>::type>::type
                                                                            type;
 };
 
@@ -1220,7 +1220,7 @@ struct __tuple_cat_return_1<tuple<_Types...>, true, _Tuple0, _Tuple1, _Tuples...
     : public __tuple_cat_return_1<
                  typename __tuple_cat_type<
                      tuple<_Types...>,
-                     typename __make_tuple_types<typename remove_reference<_Tuple0>::type>::type
+                     typename __make_tuple_types<typename __uncvref<_Tuple0>::type>::type
                  >::type,
                  __tuple_like<typename remove_reference<_Tuple1>::type>::value,
                  _Tuple1, _Tuples...>

test/std/utilities/tuple/tuple.tuple/tuple.creation/tuple_cat.pass.cpp

@@ -238,6 +238,21 @@ int main(int, char**)
         );
         assert(t2 == std::make_tuple(std::make_tuple(1), std::make_tuple(2)));
     }
+    {
+        int x = 101;
+        std::tuple<int, int&, const int&, int&&> t(42, x, x, std::move(x));
+        const auto& ct = t;
+        std::tuple<int, int&, const int&> t2(42, x, x);
+        const auto& ct2 = t2;
+
+        auto r = std::tuple_cat(std::move(t), std::move(ct), t2, ct2);
 
+        ASSERT_SAME_TYPE(decltype(r), std::tuple<
+            int, int&, const int&, int&&,
+            int, int&, const int&, int&&,
+            int, int&, const int&,
+            int, int&, const int&>);
+        ((void)r);
+    }
   return 0;
 }