How to fix compilation error with gcc8 while specializing swap function/

I'm trying to compile code which specialize the function std::swap for my class. But I'm facing an issue that occurs because of rvalue constructor (it works when I comment the line)

I am compiling with g++ (GCC) 8.2.1 20180905 (Red Hat 8.2.1-3) and g++ -std=c++14 -Wall those options

#include <iostream>
#include <utility>

namespace ns1
{
class Foo 
{
public:
    Foo();
    Foo(const Foo& that) {};
    Foo(Foo&& that) {};  // <--- work when commented
    ~Foo();  
    void swap(Foo &that) {};
};

inline void swap(Foo &lhs, Foo &rhs)
{
   lhs.swap(rhs);
}
}  // namespace ns1

namespace std {

template <>
inline void swap(ns1::Foo &lhs, ns1::Foo &rhs)
{
   lhs.swap(rhs);
}

} // namespace std

I have the following error message :

 error: template-id 'swap<>' for 'void std::swap(ns1::Foo&, ns1::Foo&)' does not match any template declaration
 inline void swap(ns1::Foo &lhs, ns1::Foo &rhs)
             ^~~~
In file included from /opt/rh/devtoolset-8/root/usr/include/c++/8/string:52,
                 from /opt/rh/devtoolset-8/root/usr/include/c++/8/bits/locale_classes.h:40,
                 from /opt/rh/devtoolset-8/root/usr/include/c++/8/bits/ios_base.h:41,
                 from /opt/rh/devtoolset-8/root/usr/include/c++/8/ios:42,
                 from /opt/rh/devtoolset-8/root/usr/include/c++/8/ostream:38,
                 from /opt/rh/devtoolset-8/root/usr/include/c++/8/iostream:39,
                 from toto.cpp:1:
/opt/rh/devtoolset-8/root/usr/include/c++/8/bits/basic_string.h:6276:5: note: candidates are: 'template<class _CharT, class _Traits, class _Alloc> void std::swap(std::basic_string<_CharT, _Traits, _Alloc>&, std::basic_string<_CharT, _Traits, _Alloc>&)'
     swap(basic_string<_CharT, _Traits, _Alloc>& __lhs,
     ^~~~
In file included from /opt/rh/devtoolset-8/root/usr/include/c++/8/bits/stl_algobase.h:64,
                 from /opt/rh/devtoolset-8/root/usr/include/c++/8/bits/char_traits.h:39,
                 from /opt/rh/devtoolset-8/root/usr/include/c++/8/ios:40,
                 from /opt/rh/devtoolset-8/root/usr/include/c++/8/ostream:38,
                 from /opt/rh/devtoolset-8/root/usr/include/c++/8/iostream:39,
                 from toto.cpp:1:

The error you are seeing is because providing a user defined move-constructor prevents compiler from synthesizing move-assignment operator, therefore making your class non move-assignable. As per cppreferece, T in swap must be move-assignable.

To fix it, provide move-assignment operator as well, demo

Foo& operator=(Foo&& other) {return *this;}
Foo& operator= (const Foo& other) { return *this; }

Also take a look at rule of five