Struct declaration NOT sufficient?

Having the following C code:

struct Point2_s;
struct Point1_s{
  int x;
  int y;
  Point2_s P2;
} Point1;

struct Point2_s{
  int x;
  int y;
} ;


int main() {
    ...
    return 0;
}

I'm getting an error:

unknown type name ‘Point2_s’

Can anyone can please explain me WHY it doesn't work? Why doesn't the struct Point2_s declaration is insufficient when defining the Point1_s member P2?

In this line

struct Point2_s;

there is declared an incomplete structure specifier struct Point2_s.

In this declaration

struct Point1_s{
  int x;
  int y;
  Point2_s P2;
} Point1;

there is used an unknown name Point2_s. It is not the same as struct Point2_s.

But even if you will write

struct Point1_s{
  int x;
  int y;
  struct Point2_s P2;
} Point1;

nevertheless you may not use an incomplete type in a data member declaration.

From the C Standard (6.7.2.1 Structure and union specifiers)

3 A structure or union shall not contain a member with incomplete or function type (hence, a structure shall not contain an instance of itself, but may contain a pointer to an instance of itself), except that the last member of a structure with more than one named member may have incomplete array type; such a structure (and any union containing, possibly recursively, a member that is such a structure) shall not be a member of a structure or an element of an array.

Instead you need to write

struct Point2_s{
  int x;
  int y;
} ;

struct Point1_s{
  int x;
  int y;
  struct Point2_s P2;
} Point1;

Or you could write

typedef struct Point2_s{
  int x;
  int y;
} Point2_s;

struct Point1_s{
  int x;
  int y;
  Point2_s P2;
} Point1;

In this case the name Point2_s is an alias for the type specifier struct Point2_s.

On the other hand, as it is pointed out in other answers you may use pointers to incomplete types because pointers themselves are always complete types. That is you may write

struct Point2_s;
struct Point1_s{
  int x;
  int y;
  struct Point2_s *P2;
} Point1;