[RFC] libstdc++: Fix pretty-printing old implementations of std::unique_ptr

[Andres Rodriguez]

On Mon, Aug 10, 2020 at 1:49 PM Jonathan Wakely <jwakely@redhat.com> wrote:
>
> On 10/08/20 09:45 -0400, Andres Rodriguez wrote:
> >*ping*
>
> As it says at https://gcc.gnu.org/lists.html all patches for libstdc++
> need to be sent to the libstdc++ mailing as well as the gcc-patches
> list. Otherwise I won't see them, and everybody else will ignore them.
>

Thanks for the heads up on the libstdc++ mailing list and for the
unique_ptr fix.

-Andres


> >On Tue, Aug 4, 2020 at 10:51 AM Andres Rodriguez <andresx7@gmail.com> wrote:
> >>
> >> On binaries compiled against gcc5 the impl_type parameter is None,
> >> which results in an exception being raised by is_specialization_of()
> >>
> >> These versions of std::unique_ptr have the tuple as a root element.
>
> The unique_ptr implementation in GCC 5 should already be handled by
> the second branch which handles a tuple member:
>
>          if is_specialization_of(impl_type, '__uniq_ptr_data') \
>              or is_specialization_of(impl_type, '__uniq_ptr_impl'):
>              tuple_member = val['_M_t']['_M_t']
>          elif is_specialization_of(impl_type, 'tuple'):
>              tuple_member = val['_M_t']
>          else:
>              raise ValueError("Unsupported implementation for unique_ptr: %s" % impl_type)
>
> We have a test that's supposed to verify that printing the old
> unique_ptr implementation still works:
> https://gcc.gnu.org/git/?p=gcc.git;a=blob;f=libstdc%2B%2B-v3/testsuite/libstdc%2B%2B-prettyprinters/compat.cc;h=f1c3b599634af2eb0a5a0889c528d9de2d21aacd;hb=HEAD
>
> So the right fix is to work out why that branch isn't taken, and fix
> it, not add a kluge.
>
> If impl_type is None that means the .tag field is missing, which seems
> to be because the _M_t member is declared using the __tuple_type
> typedef, rather than std::tuple itself.
>
> That's fixed by the attached patch.
>
> Tested x86_64-linux, pushed to master. I'll backport it to the
> branches too.
>
>