[PATCH] c++: Fix wrong modifying const object error for COMPONENT_REF [PR94074]

[Jakub Jelinek]

On Fri, Mar 06, 2020 at 07:43:43PM -0500, Jason Merrill wrote:
> On 3/6/20 6:54 PM, Marek Polacek wrote:
> > 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).
> 
> What is folding the const into the COMPONENT_REF?

cxx_eval_component_reference when it is called on
((const struct array *) this)->elems
with /*lval=*/true and lval is true because we are evaluating
<retval> = (const int &) &((const struct array *) this)->elems[VIEW_CONVERT_EXPR<size_t>(n)];

	Jakub