[Bug fortran/41227] COMMON block, BIND(C) and LTO interoperability issues

["fxcoudert at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=41227

--- Comment #21 from Francois-Xavier Coudert <fxcoudert at gcc dot gnu.org> ---
(In reply to Tobias Burnus from comment #18)
> By your argument,
>   int i;
> and
>   struct { int i; } a;
> are interoperable.

No. The standard only defines interoperability as a one-to-one mapping between
one Fortran entity and one C entity.

An "extern int i" and "integer(int_c) :: i" are interoperable. By the standard,
a common block with i as single variable is interoperable with "extern struct {
int i; } a;" or "int i;" (where the question is whether "or" is exclusive or
not).

But I don't see how you can expand that to mean that the common block is
interoperable with "struct { struct { int i; } a; } a;" by a simple reading of
the standard. There are, in my reading, 2 or 3 (with the same "or" as before)
entities interoperable with this nested struct:

  - a derived type containing a derived type containing "integer(int_c) :: i"
  - a common containing the dt containing the dt
  - a common containing a derived type containing "integer(int_c) :: i"


> Otherwise, I stand to what I wrote before: I think the standard does not
> demand the interoperability.

Let's raise a formal interp, then.