Re: [PATCH] c++: fix ICE with is_really_empty_class [PR110106]

[Jason Merrill <jason@redhat.com>]

On 7/20/23 15:51, Marek Polacek wrote:
> On Thu, Jul 20, 2023 at 02:37:07PM -0400, Jason Merrill wrote:
>> On 7/20/23 14:13, Marek Polacek wrote:
>>> On Wed, Jul 19, 2023 at 10:11:27AM -0400, Patrick Palka wrote:
>>>> On Tue, 18 Jul 2023, Marek Polacek via Gcc-patches wrote:
>>>>
>>>>> Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk and branches?
>>>>
>>>> Looks reasonable to me.
>>>
>>> Thanks.
>>>> Though I wonder if we could also fix this by not checking potentiality
>>>> at all in this case?  The problematic call to is_rvalue_constant_expression
>>>> happens from cp_parser_constant_expression with 'allow_non_constant' != 0
>>>> and with 'non_constant_p' being a dummy out argument that comes from
>>>> cp_parser_functional_cast, so the result of is_rvalue_constant_expression
>>>> is effectively unused in this case, and we should be able to safely elide
>>>> it when 'allow_non_constant && non_constant_p == nullptr'.
>>>
>>> Sounds plausible.  I think my patch could be applied first since it
>>> removes a tiny bit of code, then I can hopefully remove the flag below,
>>> then maybe go back and optimize the call to is_rvalue_constant_expression.
>>> Does that sound sensible?
>>>
>>>> Relatedly, ISTM the member cp_parser::non_integral_constant_expression_p
>>>> is also effectively unused and could be removed?
>>>
>>> It looks that way.  Seems it's only used in cp_parser_constant_expression:
>>> 10806   if (allow_non_constant_p)
>>> 10807     *non_constant_p = parser->non_integral_constant_expression_p;
>>> but that could be easily replaced by a local var.  I'd be happy to see if
>>> we can actually do away with it.  (I wonder why it was introduced and when
>>> it actually stopped being useful.)
>>
>> It was for the C++98 notion of constant-expression, which was more of a
>> parser-level notion, and has been supplanted by the C++11 version.  I'm
>> happy to remove it, and therefore remove the is_rvalue_constant_expression
>> call.
> 
> Wonderful.  I'll do that next.
>   
>>>>> -- >8 --
>>>>>
>>>>> is_really_empty_class is liable to crash when it gets an incomplete
>>>>> or dependent type.  Since r11-557, we pass the yet-uninstantiated
>>>>> class type S<0> of the PARM_DECL s to is_really_empty_class -- because
>>>>> of the potential_rvalue_constant_expression -> is_rvalue_constant_expression
>>>>> change in cp_parser_constant_expression.  Here we're not parsing
>>>>> a template so we did not check COMPLETE_TYPE_P as we should.
>>>>>
>>>>> 	PR c++/110106
>>>>>
>>>>> gcc/cp/ChangeLog:
>>>>>
>>>>> 	* constexpr.cc (potential_constant_expression_1): Check COMPLETE_TYPE_P
>>>>> 	even when !processing_template_decl.
>>>>>
>>>>> gcc/testsuite/ChangeLog:
>>>>>
>>>>> 	* g++.dg/cpp0x/noexcept80.C: New test.
>>>>> ---
>>>>>    gcc/cp/constexpr.cc                     |  2 +-
>>>>>    gcc/testsuite/g++.dg/cpp0x/noexcept80.C | 12 ++++++++++++
>>>>>    2 files changed, 13 insertions(+), 1 deletion(-)
>>>>>    create mode 100644 gcc/testsuite/g++.dg/cpp0x/noexcept80.C
>>>>>
>>>>> diff --git a/gcc/cp/constexpr.cc b/gcc/cp/constexpr.cc
>>>>> index 6e8f1c2b61e..1f59c5472fb 100644
>>>>> --- a/gcc/cp/constexpr.cc
>>>>> +++ b/gcc/cp/constexpr.cc
>>>>> @@ -9116,7 +9116,7 @@ potential_constant_expression_1 (tree t, bool want_rval, bool strict, bool now,
>>>>>          if (now && want_rval)
>>>>>    	{
>>>>>    	  tree type = TREE_TYPE (t);
>>>>> -	  if ((processing_template_decl && !COMPLETE_TYPE_P (type))
>>>>> +	  if (!COMPLETE_TYPE_P (type)
>>>>>    	      || dependent_type_p (type)
>>
>> There shouldn't be a problem completing the type here, so it seems to me
>> that we're missing a call to complete_type_p, at least when
>> !processing_template_decl.  Probably need to move the dependent_type_p check
>> up as a result.
> 
> Like so?
> 
> Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?

OK.

> -- >8 --
> is_really_empty_class is liable to crash when it gets an incomplete
> or dependent type.  Since r11-557, we pass the yet-uninstantiated
> class type S<0> of the PARM_DECL s to is_really_empty_class -- because
> of the potential_rvalue_constant_expression -> is_rvalue_constant_expression
> change in cp_parser_constant_expression.  Here we're not parsing
> a template so we did not check COMPLETE_TYPE_P as we should.
> 
> It should work to complete the type before checking COMPLETE_TYPE_P.
> 
> 	PR c++/110106
> 
> gcc/cp/ChangeLog:
> 
> 	* constexpr.cc (potential_constant_expression_1): Try to complete the
> 	type when !processing_template_decl.
> 
> gcc/testsuite/ChangeLog:
> 
> 	* g++.dg/cpp0x/noexcept80.C: New test.
> ---
>   gcc/cp/constexpr.cc                     |  5 +++--
>   gcc/testsuite/g++.dg/cpp0x/noexcept80.C | 12 ++++++++++++
>   2 files changed, 15 insertions(+), 2 deletions(-)
>   create mode 100644 gcc/testsuite/g++.dg/cpp0x/noexcept80.C
> 
> diff --git a/gcc/cp/constexpr.cc b/gcc/cp/constexpr.cc
> index 6e8f1c2b61e..fb94f3cefcb 100644
> --- a/gcc/cp/constexpr.cc
> +++ b/gcc/cp/constexpr.cc
> @@ -9116,8 +9116,9 @@ potential_constant_expression_1 (tree t, bool want_rval, bool strict, bool now,
>         if (now && want_rval)
>   	{
>   	  tree type = TREE_TYPE (t);
> -	  if ((processing_template_decl && !COMPLETE_TYPE_P (type))
> -	      || dependent_type_p (type)
> +	  if (dependent_type_p (type)
> +	      || !COMPLETE_TYPE_P (processing_template_decl
> +				   ? type : complete_type (type))
>   	      || is_really_empty_class (type, /*ignore_vptr*/false))
>   	    /* An empty class has no data to read.  */
>   	    return true;
> diff --git a/gcc/testsuite/g++.dg/cpp0x/noexcept80.C b/gcc/testsuite/g++.dg/cpp0x/noexcept80.C
> new file mode 100644
> index 00000000000..3e90af747e2
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp0x/noexcept80.C
> @@ -0,0 +1,12 @@
> +// PR c++/110106
> +// { dg-do compile { target c++11 } }
> +
> +template<int> struct S
> +{
> +};
> +
> +struct G {
> +  G(S<0>);
> +};
> +
> +void y(S<0> s) noexcept(noexcept(G{s}));
> 
> base-commit: 4b8878fbf7b74ea5c3405c9f558df0517036f131