编译错误

当我尝试编译此代码时

using namespace std;
namespace asf{
inline int operator|(int);
}

asf::operator|(int x){
return (x>1)?x*operator|(x-1):1;
}

int main(){
    cout<<5|;
}

我收到以下错误

[Error] 'int asf::operator|(int)' must have an argument of class or enumerated type
[Error] ISO C++ forbids declaration of 'operator|' with no type [-fpermissive]
[Error] 'int asf::operator|(int)' should have been declared inside 'asf'
[Error] 'int asf::operator|(int)' must have an argument of class or enumerated type
In function 'int main()':
[Error] expected primary-expression before ';' token

怎么了？请帮忙.