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

[Jason Merrill <jason@redhat.com>]

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.

> -- >8 --
> 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.
> ---
>   gcc/cp/constexpr.cc                           |  4 ---
>   gcc/testsuite/g++.dg/cpp1y/constexpr-108158.C | 32 +++++++++++++++++++
>   2 files changed, 32 insertions(+), 4 deletions(-)
>   create mode 100644 gcc/testsuite/g++.dg/cpp1y/constexpr-108158.C
> 
> diff --git a/gcc/cp/constexpr.cc b/gcc/cp/constexpr.cc
> index 5b31f9c27d1..564766c8a00 100644
> --- a/gcc/cp/constexpr.cc
> +++ b/gcc/cp/constexpr.cc
> @@ -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;
>       }
> diff --git a/gcc/testsuite/g++.dg/cpp1y/constexpr-108158.C b/gcc/testsuite/g++.dg/cpp1y/constexpr-108158.C
> new file mode 100644
> index 00000000000..e5f5e9954e5
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp1y/constexpr-108158.C
> @@ -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());
> 
> base-commit: c9aef107ce697f58a34734d82f8d2514405c9be0