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 =
       decltype(foo());
};

And I get the following error (gcc 7.4):

<source>:6:25: error: use of 'static auto Thing::foo()' before deduction of 'auto'
            decltype(foo());
                         ^
<source>:6:25: error: use of 'static auto Thing::foo()' before deduction of 'auto'

Why has the compiler not yet deduced the return type?

Answer 1

Because for class definition, compiler will first determine all member names and types. Function body is analyzed after this is done.

That's why a class member function can call another member function declared after its own definition.

At the point compile is determining

using type_t = decltype(foo());

function foo()'s body has not yet been analyzed.

As a remedy, you can use

static auto foo() -> decltype(12) {
  return 12;
}

NOTE:

This phenomenon is only for class. The following code outside a class will compile:

auto bar() { return 12; }

using t = decltype(bar());