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I got a report that building Chromium fails with the "modifying a const object" error. After some poking I realized it's a bug in GCC, not in their codebase. Much like with ARRAY_REFs, which can be const even though the array itself isn't, COMPONENT_REFs can be const although neither the object nor the field were declared const. So let's dial down the checking. Here the COMPONENT_REF was const because of the "const_cast<const U &>(m)" thing -- cxx_eval_component_reference then builds a COMPONENT_REF with TREE_TYPE (t). While looking into this I noticed that we don't detect modifying a const object in certain cases like in <https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94074#c2>. That's because we never evaluate an X::X() CALL_EXPR -- there's none. Fixed as per Jason's suggestion by setting TREE_READONLY on a CONSTRUCTOR after initialization in cxx_eval_store_expression. 2020-03-11 Marek Polacek <polacek@redhat.com> Jason Merrill <jason@redhat.com> PR c++/94074 - wrong modifying const object error for COMPONENT_REF. * constexpr.c (cref_has_const_field): New function. (modifying_const_object_p): Consider a COMPONENT_REF const only if any of its fields are const. (cxx_eval_store_expression): Mark a CONSTRUCTOR of a const type as readonly after its initialization has been done. * g++.dg/cpp1y/constexpr-tracking-const17.C: New test. * g++.dg/cpp1y/constexpr-tracking-const18.C: New test. * g++.dg/cpp1y/constexpr-tracking-const19.C: New test. * g++.dg/cpp1y/constexpr-tracking-const20.C: New test. * g++.dg/cpp1y/constexpr-tracking-const21.C: New test. * g++.dg/cpp1y/constexpr-tracking-const22.C: New test.
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