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 <ppa...@gcc.gnu.org>: https://gcc.gnu.org/g:0120cd9382728fdc99d4cfdcb72cd0f55aca2ce3 commit r12-136-g0120cd9382728fdc99d4cfdcb72cd0f55aca2ce3 Author: Patrick Palka <ppa...@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.
