[Bug c++/109654] New: unnecessary "cannot bind packed field to reference" error when referenced type has aligned(1) attribute

[Bug c++/109654] New: unnecessary "cannot bind packed field to reference" error when referenced type has aligned(1) attribute

[richard-gccbugzilla at metafoo dot co.uk]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109654

            Bug ID: 109654
           Summary: unnecessary "cannot bind packed field to reference"
                    error when referenced type has aligned(1) attribute
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: richard-gccbugzilla at metafoo dot co.uk
  Target Milestone: ---

As a workaround for people hitting #36566, I think GCC should accept cases like
this:


typedef __attribute__((aligned(1))) int packed_int;

struct __attribute__((packed)) Foo {
    int i;
    packed_int& get() { return i; }
};


Unfortunately GCC rejects:


<source>:5:32: error: cannot bind packed field '((Foo*)this)->Foo::i' to
'packed_int&' {aka 'int&'}
    5 |     packed_int& get() { return i; }
      |    


And conversely, GCC accepts this code, which has a genuine misalignment issue:


typedef __attribute__((aligned(1))) int packed_int;

struct __attribute__((packed)) Foo {
    packed_int i;
    int& get() { return i; }
};


I wonder if the check is mistakenly looking at the alignment of the source type
instead of the alignment of the referent of the reference type?

[Bug c++/109654] unnecessary "cannot bind packed field to reference" error when referenced type has aligned(1) attribute

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109654

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
           See Also|                            |https://gcc.gnu.org/bugzill
                   |                            |a/show_bug.cgi?id=36566
         Resolution|---                         |INVALID

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
This won't work for templates or function overloading since GCC does NOT mangle
the alignment.

This is why it was started to be rejected in the first place.

That is:
```
typedef __attribute__((aligned(1))) int packed_int;

template <class T> T load(T &a);

int load1(int *a)
{
  return load<packed_int>(*a);
}
int load2(int *a)
{
  return load<int>(*a);
}
```

Will both still call load<int> in both cases.

[Bug c++/109654] unnecessary "cannot bind packed field to reference" error when referenced type has aligned(1) attribute

[richard-gccbugzilla at metafoo dot co.uk]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109654

--- Comment #2 from Richard Smith <richard-gccbugzilla at metafoo dot co.uk> ---
Hm, that doesn't explain why the second example I gave is accepted. But I
suppose what's happening there is probably just that the `packed` attribute is
ignored entirely for fields with alignment 1, so this behaves the same as

```
packed_int i;
int &r = i;
```

... which indeed doesn't produce an error or even a warning, presumably for the
same reason (the alignment isn't part of the canonical type of i).

[Bug c++/109654] unnecessary "cannot bind packed field to reference" error when referenced type has aligned(1) attribute

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109654

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |stsp at users dot sourceforge.net

--- Comment #3 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
*** Bug 109824 has been marked as a duplicate of this bug. ***