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

[Jason Merrill]

On 3/9/20 9:40 AM, Marek Polacek wrote:
> On Mon, Mar 09, 2020 at 09:19:30AM -0400, Jason Merrill wrote:
>> On 3/9/20 8:58 AM, Jakub Jelinek wrote:
>>> 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)];
>>
>> Ah, sure.  We're pretty loose with cv-quals in the constexpr code in
>> general, so it's probably not worth trying to change that here.  Getting
>> back to the patch:
> 
> Yes, here the additional const was caused by a const_cast adding a const.
> 
> But this could also happen with wrapper functions like this one from
> __array_traits in std::array:
> 
>        static constexpr _Tp&
>        _S_ref(const _Type& __t, std::size_t __n) noexcept
>        { return const_cast<_Tp&>(__t[__n]); }
> 
> where the ref-to-const parameter added the const.
> 
>>> +      if (TREE_CODE (obj) == COMPONENT_REF)
>>> +	{
>>> +	  tree op1 = TREE_OPERAND (obj, 1);
>>> +	  if (CP_TYPE_CONST_P (TREE_TYPE (op1)))
>>> +	    return true;
>>> +	  else
>>> +	    {
>>> +	      tree op0 = TREE_OPERAND (obj, 0);
>>> +	      /* The LHS of . or -> might itself be a COMPONENT_REF.  */
>>> +	      if (TREE_CODE (op0) == COMPONENT_REF)
>>> +		op0 = TREE_OPERAND (op0, 1);
>>> +	      return CP_TYPE_CONST_P (TREE_TYPE (op0));
>>> +	    }
>>> +	}
>>
>> Shouldn't this be a loop?
> 
> I don't think so, though my earlier patch had a call to
> 
> +static bool
> +cref_has_const_field (tree ref)
> +{
> +  while (TREE_CODE (ref) == COMPONENT_REF)
> +    {
> +      if (CP_TYPE_CONST_P (TREE_TYPE (TREE_OPERAND (ref, 1))))
> +       return true;
> +      ref = TREE_OPERAND (ref, 0);
> +    }
> +  return false;
> +}

> here.  A problem arised when I checked even the outermost expression (which is not a
> field_decl), then I saw another problematical error.
> 
> The more outer fields are expected to be checked in subsequent calls to
> modifying_const_object_p in next iterations of the
> 
> 4459   for (tree probe = target; object == NULL_TREE; )
> 
> loop in cxx_eval_store_expression.

OK, but then why do you want to check two levels here rather than just one?

Jason