c++ - How to get the return type of a function in class? -

c++ - How to get the return type of a function in class? -

for example

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

the decltype(((a*)nullptr)->foo()) doesn't work here because incomplete class. how it?

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

c++ class c++11 decltype