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

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

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.

It no longer seems necessary to replace new_ctx.object (likely due to
changes in empty class handling).

	PR c++/108158

gcc/cp/ChangeLog:

	* constexpr.cc (cxx_eval_array_reference): Don't replace
	new_ctx.object.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp1y/constexpr-108158.C: New test.