Re: [PATCH v2] c++: wrong error with constexpr array and value-init [PR108158]

[Marek Polacek <polacek@redhat.com>]

On Fri, Feb 03, 2023 at 01:53:48PM -0500, Jason Merrill wrote:
> On 2/3/23 13:08, Marek Polacek wrote:
> > On Thu, Feb 02, 2023 at 05:29:44PM -0500, Jason Merrill wrote:
> > > On 1/30/23 21:35, Marek Polacek wrote:
> > > > In this test case, we find ourselves evaluating 't' which is
> > > > ((const struct carray *) this)->data_[VIEW_CONVERT_EXPR<long int>(index)]
> > > > in cxx_eval_array_reference.  ctx->object is non-null, a RESULT_DECL, so
> > > > we replace it with 't':
> > > > 
> > > >     new_ctx.object = t; // result_decl replaced
> > > > 
> > > > and then we go to cxx_eval_constant_expression to evaluate an
> > > > AGGR_INIT_EXPR, where we end up evaluating an INIT_EXPR (which is in the
> > > > body of the constructor for seed_or_index):
> > > > 
> > > >     ((struct seed_or_index *) this)->value_ = NON_LVALUE_EXPR <0>
> > > > 
> > > > whereupon in cxx_eval_store_expression we go to the probe loop
> > > > where the 'this' is evaluated to
> > > > 
> > > >     ze_set.tables_.first_table_.data_[0]
> > > > 
> > > > so the 'object' is ze_set, but that isn't in ctx->global->get_value_ptr
> > > > so we fail with a bogus error.  ze_set is not there because it comes
> > > > from a different constexpr context (it's not in cv_cache either).
> > > > 
> > > > The problem started with r12-2304 where I added the new_ctx.object
> > > > replacement.  That was to prevent a type mismatch: the type of 't'
> > > > and ctx.object were different.
> > > > 
> > > > It seems clear that we shouldn't have replaced ctx.object here.
> > > > The cxx_eval_array_reference I mentioned earlier is called from
> > > > cxx_eval_store_expression:
> > > >    6257       init = cxx_eval_constant_expression (&new_ctx, init, vc_prvalue,
> > > >    6258                                            non_constant_p, overflow_p);
> > > > which already created a new context, whose .object we should be
> > > > using unless, for instance, INIT contained a.b and we're evaluating
> > > > the 'a' part, which I think was the case for r12-2304; in that case
> > > > ctx.object has to be something different.
> > > > 
> > > > A relatively safe fix should be to check the types before replacing
> > > > ctx.object, as in the below.
> > > 
> > > Agreed.  I'm trying to understand when the replacement could ever make
> > > sense, since 't' is not the target, it's the initializer.  The replacement
> > > comes from Patrick's fix for 98295, but that testcase no longer hits that
> > > code (likely due to changes in empty class handling).
> > > 
> > > If you add a gcc_checking_assert (false) to the replacement, does anything
> > > trip it?
> > 
> > It would trip in constexpr-101371.C, added in r12-2304.  BUT, and I would
> > have sworn that it ICEd when I tried, it's not necessary anymore.  So it
> > looks like we can simply remove the new_ctx.object line.  At least for
> > trunk, maybe 12 too.
> > 
> > Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?
> 
> OK, thanks.  Let's go with your original patch for 11/12.

Will do, thanks.  I think I'll wait for a few days before backporting.

Marek