[Bug c++/106395] New: [10/11 regression] [mingw] "redeclared without dllimport attribute: previous dllimport ignored" on C++ friend

[thiago at kde dot org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106395

            Bug ID: 106395
           Summary: [10/11 regression] [mingw] "redeclared without
                    dllimport attribute: previous dllimport ignored" on
                    C++ friend
           Product: gcc
           Version: 12.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: thiago at kde dot org
  Target Milestone: ---

__attribute__((dllimport)) void f();
class S
{
private:
    int i;
    friend __attribute__((dllimport)) void f();
};

With GCC 10.3.0 (OpenSUSE Tumbleweed):
$ x86_64-w64-mingw32-gcc -c -Werror /tmp/test.cpp        

With GCC 11.2.1 (Fedora 35 & 36):
$ x86_64-w64-mingw32-gcc -Wall -Wextra -Werror -c /tmp/test.cpp

With GCC 12.1.0 (Arch Linux, self built):
$ x86_64-w64-mingw32-g++ -Werror -c /tmp/test.cpp
/tmp/test.cpp:6:44: error: ‘void f()’ redeclared without dllimport attribute:
previous dllimport ignored [-Werror=attributes]
    6 |     friend __attribute__((dllimport)) void f();
      |                                            ^
cc1plus: all warnings being treated as errors

GCC 12 is obviously wrong because it is complaining that the attribute is
missing and then shows that the attribute is right there.