[Bug c++/100209] multiple inheritance with crtp pattern fails on sequentioal member access

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100209

--- Comment #4 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Patrick Palka <ppalka@gcc.gnu.org>:

https://gcc.gnu.org/g:0120cd9382728fdc99d4cfdcb72cd0f55aca2ce3

commit r12-136-g0120cd9382728fdc99d4cfdcb72cd0f55aca2ce3
Author: Patrick Palka <ppalka@redhat.com>
Date:   Mon Apr 26 17:30:39 2021 -0400

    c++: constexpr pointer indirection with negative offset [PR100209]

    During constexpr evaluation, a base-to-derived conversion may yield an
    expression like (Derived*)(&D.2217.D.2106 p+ -4) where D.2217 is the
    derived object and D.2106 is the base.  But cxx_fold_indirect_ref
    doesn't know how to resolve an INDIRECT_REF thereof to just D.2217,
    because it doesn't handle POINTER_PLUS_EXPR of a COMPONENT_REF with
    negative offset well: when the offset N is positive, it knows that
    '&x p+ N' is equivalent to '&x.f p+ (N - bytepos(f))', but it doesn't
    know about the reverse transformation, that '&x.f p+ N' is equivalent
    to '&x p+ (N + bytepos(f))' when N is negative, which is important for
    resolving such base-to-derived conversions and for accessing subobjects
    backwards.  This patch teaches cxx_fold_indirect_ref this reverse
    transformation.

    gcc/cp/ChangeLog:

            PR c++/100209
            * constexpr.c (cxx_fold_indirect_ref): Try to canonicalize the
            object/offset pair for a POINTER_PLUS_EXPR of a COMPONENT_REF
            with a negative offset into one whose offset is nonnegative
            before calling cxx_fold_indirect_ref_1.

    gcc/testsuite/ChangeLog:

            PR c++/100209
            * g++.dg/cpp1y/constexpr-base1.C: New test.
            * g++.dg/cpp1y/constexpr-ptrsub1.C: New test.