Proposed fix for PR22806

[mclow.lists]

Summary

In the single-argument case, tuple_cat of an rvalue tuple appears to move from, destroy, and then move from the argument again.

Provide a degenerate implementation for the single-argument case.

[mclow.lists]

We have a better fix.

include/tuple

                                             _VSTD::forward<_Tuples>(__tpls)...);
 }
 
+template <class _Tuple>
+inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX11
+typename __tuple_cat_return<_Tuple>::type
+tuple_cat(_Tuple&& __t0)
+{
+    return __t0;
+}
+
 template <class ..._Tp, class _Alloc>
 struct _LIBCPP_TYPE_VIS_ONLY uses_allocator<tuple<_Tp...>, _Alloc>
     : true_type {};