[Bug c++/91222] [10 Regression] 507.cactuBSSN_r build fails in warn_types_mismatch at ipa-devirt.c:1006 since r273571

[jason at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91222

Jason Merrill <jason at gcc dot gnu.org> changed:

           What    |Removed                     |Added
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                 CC|                            |jason at gcc dot gnu.org

--- Comment #8 from Jason Merrill <jason at gcc dot gnu.org> ---
(In reply to Jan Hubicka from comment #2)
> 2.ii:62:3: warning: ‘ml_bssnrest_’ violates the C++ One Definition Rule
> [-Wodr]
>    62 | } ml_bssnrest_;
>       |   ^
> 1.ii:2:8: note: type ‘struct <anon>’ defined in anonymous namespace cannot
> match across the translation unit boundary
>     2 | struct {
>       |        ^
> 2.ii:2:15: note: the incompatible type defined in another translation unit
>     2 | extern struct {
>       |               ^
> 1.ii:62:3: note: ‘ml_bssnrest_’ was previously declared here
>    62 | } ml_bssnrest_;
>       |   ^
> 
> Now I wonder why C++ FE makes the struct anonymous namespace when it is
> declared with extern "C".
> I have checked that type_in_anonymous_namespace_p(prevailing_type) returns
> true because mangled name is <anon> which comes from C++ FE.
> 
> I will check in the patch to avoid ICE. Jason, can you please look if it is
> correct to consider these types anonymous?

They aren't in the anonymous namespace, but they are themselves anonymous, so
they have no linkage.  The standard says,

A type without linkage shall not be used as the type of a variable or function
with external linkage unless
— the entity has C language linkage (7.5), or
— the entity is declared within an unnamed namespace (7.3.1), or
— the entity is not odr-used (3.2) or is defined in the same translation unit.

Here 1.ii is OK under the third bullet, and 2.ii under the first bullet. 
Within extern "C" I guess we need to do structural comparison for anonymous
types rather than rely on the ODR.