[Bug c++/81764] [6/7/8 Regression] Visibility attributes for explicitly instantiated template class get warned if it has been implicitly instantiated

2018-04-06 Thread msebor at gcc dot gnu.org 
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81764

--- Comment #3 from Martin Sebor  ---
This is another one of those cases where implementations diverge:

Clang and ICC silently accept the program in comment #0 and give an error for
the one in comment #1.
GCC issues -Wattributes for both.
MSVC silently accepts the program in comment #1 but gives a warning when the
alignment specified on the specialization is different than on the primary.

MSVC doesn't understand visibility attributes but it does accept __declspec
dllexport and dllimport.  It allows the example in comment #0 with __declspec
(dllimport) and also with the primary declared dllexport and the instantiation
declared dllimport.  That makes sense because both declarations would typically
appear this way in a library header and an explicit instantiation directive
declared dllexport in some source file of the library.  The C++ constraint that
"no attribute-specifier-seq shall appertain to an explicit instantiation"
disallows this use case (one of the main uses cases in Windows class template
DLLs).

GCC for i686-pc-cygwin also issues a warning for this modified test case and so
is incompatible with MSVC:

$ cat pr81764.C && i686-pc-cygwin-g++ -S -Wall pr81764.C
template class g;

template
class __declspec (dllexport) g
{
g() = default;

static int f(int x)
{
return g<>::yyy::u(x); // g<> instantiated here
}


struct yyy
{
static int u(int x){
return x+ sizeof...(T);
}
};
};

extern template class __declspec (dllimport) g<>; 
template class  g<>;
pr81764.C:22:46: warning: type attributes ignored after type is already defined
[-Wattributes]
 extern template class __declspec (dllimport) g<>;
  ^~~

[Bug c++/81764] [6/7/8 Regression] Visibility attributes for explicitly instantiated template class get warned if it has been implicitly instantiated

2018-04-05 Thread jason at gcc dot gnu.org 
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81764

Jason Merrill  changed:

   What|Removed |Added

 Status|NEW |RESOLVED
 CC||jason at gcc dot gnu.org
 Resolution|--- |INVALID

--- Comment #2 from Jason Merrill  ---
Yes, this behavior is intended; just like you can't explicitly specialize a
class after it's been instantiated, you can't specify attributes either.

The C++ standard is even stricter, saying "No attribute-specifier-seq shall
appertain to an explicit instantiation."

[Bug c++/81764] [6/7/8 Regression] Visibility attributes for explicitly instantiated template class get warned if it has been implicitly instantiated

2018-03-06 Thread rguenth at gcc dot gnu.org 
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81764

Richard Biener  changed:

   What|Removed |Added

   Priority|P3  |P2

[Bug c++/81764] [6/7/8 Regression] Visibility attributes for explicitly instantiated template class get warned if it has been implicitly instantiated

2018-03-06 Thread rguenth at gcc dot gnu.org 
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81764

Richard Biener  changed:

   What|Removed |Added

   Target Milestone|--- |6.5

[Bug c++/81764] [6/7/8 Regression] Visibility attributes for explicitly instantiated template class get warned if it has been implicitly instantiated

2018-02-15 Thread msebor at gcc dot gnu.org 
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81764

Martin Sebor  changed:

   What|Removed |Added

 Status|UNCONFIRMED |NEW
  Known to work||4.1.0
   Keywords||diagnostic
   Last reconfirmed||2018-02-15
 CC||msebor at gcc dot gnu.org
   See Also||https://gcc.gnu.org/bugzill
   ||a/show_bug.cgi?id=84318
 Ever confirmed|0   |1
Summary|Visibility attributes for   |[6/7/8 Regression]
   |explicitly instantiated |Visibility attributes for
   |template class get warned   |explicitly instantiated
   |if it has been implicitly   |template class get warned
   |instantiated|if it has been implicitly
   ||instantiated
  Known to fail||4.2.2, 4.3.2, 4.6.0, 5.4.0,
   ||6.3.0, 7.2.0, 8.0

--- Comment #1 from Martin Sebor  ---
The warning might be helpful if the explicit specialization declaration
attempted to specify different visibility that the primary template (assuming
the declaration of the specialization inherits the attribute from the template)
but it's pointless when it simply reiterates the same visibility, and it would
be wrong if the specialization didn't "inherit" its attributes from the
primary.  With that, I would say that it's not desirable (even though it is
intended in other cases). With that I confirm it as a bug.

The warning wasn't issued prior to r115086 (committed into GCC 4.2) so it's
also aa regression, albeit a very old one.

As the test case below shows, the warning isn't unique to the visibility
attribute.  It affects other type attributes as well.  Here too the warning is
pointless regardless of how attributes are treated.

Unfortunately, specifying an attribute on the declaration of an explicit
instantiation isn't specified either in C++ or in G++ and it not clear that we
know or have consensus what we want it to mean (see for example bug 84318), so
this bug isn't likely to get a lot attention until the confusion around
attributes and templates has been cleared up.

$ cat t.C && gcc -S -Wall -Wextra -Wpedantic t.C
template  class A;

template 
struct alignas (32) A
{
  static int f () { return A<>::f (); }
};

extern template struct alignas (32) A<>;

static_assert (alignof (A<>) == 32, "");
t.C:9:37: warning: type attributes ignored after type is already defined
[-Wattributes]
 extern template struct alignas (32) A<>;
 ^~~