[Bug c++/106633] New: Hidden visibility on forward declarations impacts visibility of default-visibility class methods

[mh+gcc at glandium dot org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106633

            Bug ID: 106633
           Summary: Hidden visibility on forward declarations impacts
                    visibility of default-visibility class methods
           Product: gcc
           Version: 12.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: mh+gcc at glandium dot org
  Target Milestone: ---

This was not happening before GCC 12.

Reduced testcase from a Firefox build failure we got in Debian on armhf:

```
#pragma GCC visibility push(hidden)
namespace std { class type_info; }
#pragma GCC visibility pop
#include <typeinfo>
bool foo(std::type_info& a, std::type_info& b) { return a == b; }
```

Compile with `-O2 -D__GXX_TYPEINFO_EQUALITY_INLINE=0` (the define makes the
problem appear on all platforms, rather than armhf only)

What happens in this example is that the symbol reference to
`std::type_info::operator==` ends up with hidden visibility, instead of default
visibility. With GCC 11 and older versions, it would end up with default
visibility.

On godbolt:
https://godbolt.org/z/hYKjWGaeq

(Ironically(?), that forward declaration comes from ICU working around an old
bug in libstdc++ headers)