[llvm-bugs] [Bug 48360] New: Lack of visibility specification on forward declaration of class template may hide derived symbols

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=48360

            Bug ID: 48360
           Summary: Lack of visibility specification on forward
                    declaration of class template may hide derived symbols
           Product: clang
           Version: trunk
          Hardware: PC
                OS: Windows NT
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: C++
          Assignee: unassignedclangbugs at nondot.org
          Reporter: predelnik at gmail.com
                CC: blitzrakete at gmail.com, dgregor at apple.com,
                    erik.pilkington at gmail.com, llvm-bugs at lists.llvm.org,
                    richard-llvm at metafoo.co.uk

Take a look at the following program

#include <any>

template <typename T>
class PublicTemplateClass;

class [[gnu::visibility("default")]] D {};

class E
{
  PublicTemplateClass<D> f ();
};

template <typename T>
class [[gnu::visibility("default")]] PublicTemplateClass
{
};

PublicTemplateClass<D> E::f ()
{
    PublicTemplateClass<D> v;
    std::any a (v);
    return v;
}

Running the following command:

clang++ -fvisibility=hidden -std=c++17 -shared -fPIC a.cpp && readelf -s a.out
| grep Manager

I see the following output:

15: 0000000000001c70    39 FUNC    LOCAL  HIDDEN    12
_ZNSt3any17_Manager_inter
16: 0000000000001b90   178 FUNC    LOCAL  HIDDEN    12
_ZNSt3any17_Manager_inter

Manager...thingie is basically a function pointer which is compared in any_cast
in libstdc++. So the problem is that when it's hidden any_cast to correct type
will fail because user of the library will have different pointer to this
function.

If however I add [[gnu::visibility("default")]] to forward declaration of
PublicTemplateClass like this:
template <typename T>
class [[gnu::visibility("default")]] PublicTemplateClass;

I receive the following:

10: 0000000000001ef0   178 FUNC    WEAK   DEFAULT   12
_ZNSt3any17_Manager_inter
13: 0000000000001fd0    39 FUNC    WEAK   DEFAULT   12
_ZNSt3any17_Manager_inter
28: 0000000000001fd0    39 FUNC    WEAK   DEFAULT   12
_ZNSt3any17_Manager_inter
29: 0000000000001ef0   178 FUNC    WEAK   DEFAULT   12
_ZNSt3any17_Manager_inter

I'm not 100% sure that this is not intended behavior but this seems to be very
unfortunate because:
1. Error is very subtle, hard to find, runtime only &c.
2. gcc does not behave like this
3. Normal classes do not seem to trigger such behavior with requiring
visibility specification for forward declarations.

If you need me to built full fledged example where executable + so lib fail to
perform any cast in such case I would be happy to provide that but I hope the
problem is already clear from this short sample.

-- 
You are receiving this mail because:
You are on the CC list for the bug.
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-bugs/attachments/20201202/48271f99/attachment.html>