[Bug libstdc++/97729] [11 Regression] Link failure due to basic_stringbuf and others when building DLL on MinGW

["cvs-commit at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97729

--- Comment #7 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Jonathan Wakely <redi@gcc.gnu.org>:

https://gcc.gnu.org/g:710508c7b1a2c8e1d75d4c4f1ac79473dbf2b2bb

commit r11-4749-g710508c7b1a2c8e1d75d4c4f1ac79473dbf2b2bb
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Thu Nov 5 16:19:15 2020 +0000

    libstdc++: Fix multiple definitions of std::exception_ptr functions [PR
97729]

    This fixes some multiple definition errors caused by the changes for
    PR libstdc++/90295. The previous solution for inlining the members of
    std::exception_ptr but still exporting them from the library was to
    suppress the 'inline' keyword on those functions when compiling
    libsupc++/eh_ptr.cc, so they get defined in that file. That produces ODR
    violations though, because there are now both inline and non-inline
    definitions in the library, due to the use of std::exception_ptr in
    other files sucg as src/c++11/future.cc.

    The new solution is to define all the relevant members as 'inline'
    unconditionally, but use __attribute__((used)) to cause definitions to
    be emitted in libsupc++/eh_ptr.cc as before. This doesn't quite work
    however, because PR c++/67453 means the attribute is ignored on
    constructors and destructors. As a workaround, the old solution
    (conditionally inline) is still used for those members, but they are
    given the always_inline attribute so that they aren't emitted in
    src/c++11/future.o as inline definitions.

    libstdc++-v3/ChangeLog:

            PR libstdc++/97729
            * include/std/future (__basic_future::_M_get_result): Use
            nullptr for null pointer constant.
            * libsupc++/eh_ptr.cc (operator==, operator!=): Remove
            definitions.
            * libsupc++/exception_ptr.h (_GLIBCXX_EH_PTR_USED): Define
            macro to conditionally add __attribute__((__used__)).
            (operator==, operator!=, exception_ptr::exception_ptr())
            (exception_ptr::exception_ptr(const exception_ptr&))
            (exception_ptr::~exception_ptr())
            (exception_ptr::operator=(const exception_ptr&))
            (exception_ptr::swap(exception_ptr&)): Always define as
            inline. Add macro to be conditionally "used".