id	summary	reporter	owner	description	type	status	priority	component	version	resolution	keywords	cc
198	Forward Declaration Leads to Bad Function Pointer Types	ajbeach		"So I have encountered a rather convoluted case where a combination of a circular dependency and a polymorphic function pointer leads to an error in the generated code. It appears that a concrete implementation is not getting forward declared.

{{{
bug.cfa:12:72: warning: 'struct _conc_object0' declared inside parameter list will not be visible outside of this definition or declaration
   12 |  void (*object_function)(object(T) *);
      |
}}}

Here is the code to produce the error (the `_vtable` names are from how I discovered this but are not required, the virtual system seems to have nothing to do with this):
{{{
forall(otype T)
struct object_vtable;

forall(otype U)
struct object {
    object_vtable(U) * virtual_table;
};

forall(otype T)
struct object_vtable {
    void (*object_function)(object(T) *);
};

void func(object(int) * test) {}

object_vtable(int) _object_vtable_instance = { func };
}}}

The polymorphic variables currently have to be otypes because of #196. Once that bug is fixed they might be able to be changed to dtypes.

In `object` removing the virtual_table pointer or replacing it with a field that does not refer to `object_vtable` will cause the error to disappear. Whether the field is polymorphic or not doesn't matter, `object_vtable(char) *` produces the error but `U *` does not.

In `object_vtable` the function pointer field must refer to `object` and must be a function pointer. So the error remains with `void (*f)(object(int) *)` as long as the structure is polymorphic. But `object(U) * x` will compile just fine as will `void (*f)(U *)`.

Also if the star is removed from the function pointer argument (and the same star on `func`) there is a burst of possibly related errors. If the initialization at the end of the examples is also changed this becomes a single warning on assignment of incompatible pointer types. Not sure if that is the same problem or not.

The initialization of `_object_vtable_instance` is required for the error to occur. It also happens if you use `@=` to initial the structure, it leads to an extra error:
{{{
bug.cfa:19:82: warning: initialization of 'void (*)(struct _conc_object0 *)' from incompatible pointer type 'void (*)(struct _conc_object0 *)' [-Wincompatible-pointer-types]
   19 | object_vtable(int) _object_vtable_instance @= { func };
      |
             
bug.cfa:19:82: note: (near initialization for '_X23_object_vtable_instanceS13object_vtable_i__1._X15object_functionFv_PS6object_Y12__T_generic____1')
}}}"	defect	closed	major	cfa-cc	1.0	fixed