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From: "alex.wabik at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug c++/104834] New: visibility=hidden ignored for template
 instantiations for certain dependent types
Date: Tue, 08 Mar 2022 08:07:45 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D104834

            Bug ID: 104834
           Summary: visibility=3Dhidden ignored for template instantiations
                    for certain dependent types
           Product: gcc
           Version: 11.2.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: alex.wabik at gmail dot com
  Target Milestone: ---

Created attachment 52579
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=3D52579&action=3Dedit
Testcase

Public visibility templates generate instances with public visiblity if a
template parameter is an enum or class enum declared in the translation unit
which uses -fvisibility=3Dhidden, or even explicitly marked with hidden
visibility attribute.

This behaviour does not reproduce on clang, which will only instantiate pub=
lic
template if the enum has attribute visibility=3Ddefault, otherwise the
instantiated template has hidden visibility.

The std::thread's constructor instantiates the std::thread::_State_impl typ=
e,
which generates functions with default visibility if std::thread is constru=
cted
with a lambda defined in the hidden-visiblity class. This behaviour also do=
es
not reproduce on clang.

I have also seen similar problems with std::function's template constructor,
and with future base instantiations when using std::async, but I was not ab=
le
to produce a minimal testcase; The testcase I attach demonstrates the probl=
em
for enums and std::thread only.

The workaround for the enum problem and the lambda problem is to define the=
se
types in the anonymous namespace.

Attaching the test code. Inspecting the compiled object reveals the followi=
ng
default visibility symbols, which I believe should be hidden:

std::thread::_State_impl<std::thread::_Invoker<std::tuple<ThreadTest::Threa=
dTest()::{lambda()#1}>
> >::~_State_impl() where the whole ThreadTest class has hidden visibility.

aaa::Wrapper<ThisIsEnum>::get() where the ThisIsEnum type is an enum with
hidden visibility.=