[Bug libstdc++/87822] [6/7/8/9 Regression] Binary incompatibility in std::pair introduced by PR 86751

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=87822

--- Comment #12 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Author: redi
Date: Wed Oct 31 13:16:10 2018
New Revision: 265683

URL: https://gcc.gnu.org/viewcvs?rev=265683&root=gcc&view=rev
Log:
PR libstdc++/87822 fix layout change for nested std::pair

The introduction of the empty __pair_base base class for PR 86751
changed the layout of std::pair<std::pair<...>, ...>. The outer pair and
its first member both have a base class of the same type, which cannot
exist at the same address. This causes the first member to be at a
non-zero offset.

The solution is to make the base class depend on the template
parameters, so that each pair type has a different base class type,
which allows the base classes of the outer pair and its first member to
have the same address.

        PR libstdc++/87822
        * include/bits/stl_pair.h (__pair_base): Change to class template.
        (pair): Make base class type depend on template parameters.
        * testsuite/20_util/pair/87822.cc: New test.

Added:
    branches/gcc-7-branch/libstdc++-v3/testsuite/20_util/pair/87822.cc
Modified:
    branches/gcc-7-branch/libstdc++-v3/ChangeLog
    branches/gcc-7-branch/libstdc++-v3/include/bits/stl_pair.h