Can optimizing inlining and removing dead code prevent the creation of patterns?

Question

Given the following code example

struct S;

template<typename>
class C
{
public:
   void f(bool b)
   {
      if (b)
        g();
   }

   void g()
   {
     S{};
   }
};

int main()
{
   C<int>{}.f(false);
}

GCC correctly reports the following:

example.cpp: In instantiation of 'void C< <template-parameter-1-1> >::g() [with <template-parameter-1-1> = int]':
10 : required from 'void C< <template-parameter-1-1> >::f(bool) [with <template-parameter-1-1> = int]'
21 : required from here
15 : error: invalid use of incomplete type 'struct S'

Now my question is: Is this a guaranteed behavior that has some kind of rule in a standard or any other document?

To clarify my question:

C - , f() g() . f() main(). g() ( ), . g() if- f(). . , / , , g() ( ). ( ).

, , , , - (, ), -.

.

Answer 1

?

.

- , , . " ", .

. C<int> , main & hellip; . , , "". , "" .

, , , , . , , " " ^{& dagger;}. , , , -.

_{^{& dagger;} , , " ". , , , . ( .)}

Answer 2

- , . S undefined, , . .