Why does 'const' make structure pointers incompatible?

[David Daney <ddaney@xxxxxxxxxxxxxxxxxx>]

Consider this code (simplified from Linux kernel):

----------- foo.c -----------
struct B {
  int f1;
  int f2;
  int f3;
};

struct A {
  int (* foo)(struct B *);
};

int bar(const struct B *a)
{
  return a->f1 + a->f2 + a->f3;
}

struct A baz = {bar};
-----------8<---------------

$ gcc --version
gcc (GCC) 4.3.0 20080428 (Red Hat 4.3.0-8)
[...]

$ gcc -Wall -c foo.c
foo.c:16: warning: initialization from incompatible pointer type

Why do I get the warning?

In most cases a pointer to an object can be passed to functions taking a 
pointer to a const object with no warning being issued.

Thanks,
David Daney