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

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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.
This commit is contained in:
Marek Polacek 2023-01-30 09:02:00 -05:00
parent 60fca1802a
commit 27ac6a707e
2 changed files with 32 additions and 4 deletions
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4
gcc/cp/constexpr.cc
View file

@ -4301,10 +4301,6 @@ cxx_eval_array_reference (const constexpr_ctx *ctx, tree t,
if (!SCALAR_TYPE_P (elem_type))
{
new_ctx = *ctx;
if (ctx->object)
/* If there was no object, don't add one: it could confuse us
into thinking we're modifying a const object. */
new_ctx.object = t;
new_ctx.ctor = build_constructor (elem_type, NULL);
ctx = &new_ctx;
}

32
gcc/testsuite/g++.dg/cpp1y/constexpr-108158.C Normal file
View file

@ -0,0 +1,32 @@
// PR c++/108158
// { dg-do compile { target c++14 } }
template <class T, int N> struct carray {
T data_[N]{};
constexpr T operator[](long index) const { return data_[index]; }
};
struct seed_or_index {
private:
long value_ = 0;
};
template <int M> struct pmh_tables {
carray<seed_or_index, M> first_table_;
template <typename KeyType, typename HasherType>
constexpr void lookup(KeyType, HasherType) const {
first_table_[0];
}
};
template <int N> struct unordered_set {
int equal_;
carray<int, N> keys_;
pmh_tables<N> tables_;
constexpr unordered_set() : equal_{} {}
template <class KeyType, class Hasher>
constexpr auto lookup(KeyType key, Hasher hash) const {
tables_.lookup(key, hash);
return keys_;
}
};
constexpr unordered_set<3> ze_set;
constexpr auto nocount = ze_set.lookup(4, int());
constexpr auto nocount2 = unordered_set<3>{}.lookup(4, int());