How do I get the return type of a function in a class?

for example

class A
{
    int foo() {return 1;}
    void goo( decltype(((A*)nullptr)->foo()) i) {}
};

      

        
        
        
      

    

decltype(((A*)nullptr)->foo())

doesn't work here because A is an incomplete class. How to do it?

On coliru: http://coliru.stacked-crooked.com/a/830fdd982167a4e8