Compiler can't deduce the return type?

Question

I am trying to use the decltype keyword on an auto function:

struct Thing {
   static auto foo() {
     return 12;
   }
   using type_t =
               


        

Answer 1

It's because a using inside a class or struct see the declaration but not the definition of members. So see auto but doesn't see return 12;.

If different, would be dangerous because the definition of members can use the defined (using or typedef) types.

Imagine something as follows

struct Thing {
   static auto foo() {
     return type_t{};
   }
   using type_t =
       decltype(foo());
};