Re: [PATCH] c++: suppress all warnings on memper pointers to work around dICE [PR101219]

[Jason Merrill <jason@redhat.com>]

On 8/11/21 6:36 PM, Sergei Trofimovich wrote:
> On Wed, 11 Aug 2021 15:19:58 -0400
> Jason Merrill <jason@redhat.com> wrote:
> 
>> On 8/6/21 11:34 AM, Sergei Trofimovich wrote:
>>> On Thu, 29 Jul 2021 11:41:39 -0400
>>> Jason Merrill <jason@redhat.com> wrote:
>>>    
>>>> On 7/22/21 7:15 PM, Sergei Trofimovich wrote:
>>>>> From: Sergei Trofimovich <siarheit@google.com>
>>>>>
>>>>> r12-1804 ("cp: add support for per-location warning groups.") among other
>>>>> things removed warning suppression from a few places including ptrmemfuncs.
>>>>>
>>>>> Currently ptrmemfuncs don't have valid BINFO attached which causes ICEs
>>>>> in access checks:
>>>>>
>>>>>        crash_signal
>>>>>            gcc/toplev.c:328
>>>>>        perform_or_defer_access_check(tree_node*, tree_node*, tree_node*, int, access_failure_info*)
>>>>>            gcc/cp/semantics.c:490
>>>>>        finish_non_static_data_member(tree_node*, tree_node*, tree_node*)
>>>>>            gcc/cp/semantics.c:2208
>>>>>        ...
>>>>>
>>>>> The change suppresses warnings again until we provide BINFOs for ptrmemfuncs.
>>>>
>>>> We don't need BINFOs for PMFs, we need to avoid paths that expect them.
>>>>
>>>> It looks like the problem is with tsubst_copy_and_build calling
>>>> finish_non_static_data_member instead of build_ptrmemfunc_access_expr.
>>>
>>> Sounds good. I'm not sure what would be the best way to match it. Here is
>>> my attempt seems to survive all regtests:
>>>
>>> --- a/gcc/cp/pt.c
>>> +++ b/gcc/cp/pt.c
>>> @@ -20530,7 +20530,13 @@ tsubst_copy_and_build (tree t,
>>>           if (member == error_mark_node)
>>>             RETURN (error_mark_node);
>>>
>>> -       if (TREE_CODE (member) == FIELD_DECL)
>>> +       if (object_type && TYPE_PTRMEMFUNC_P(object_type)
>>> +           && TREE_CODE (member) == FIELD_DECL)
>>> +         {
>>> +           r = build_ptrmemfunc_access_expr (object, DECL_NAME(member));
>>> +           RETURN (r);
>>> +         }
>>> +       else if (TREE_CODE (member) == FIELD_DECL)
>>>             {
>>>               r = finish_non_static_data_member (member, object, NULL_TREE);
>>>               if (TREE_CODE (r) == COMPONENT_REF)
>>>    
>>>>> 	PR c++/101219
>>>>>
>>>>> gcc/cp/ChangeLog:
>>>>>
>>>>> 	* typeck.c (build_ptrmemfunc_access_expr): Suppress all warnings
>>>>> 	to avoid ICE.
>>>>>
>>>>> gcc/testsuite/ChangeLog:
>>>>>
>>>>> 	* g++.dg/torture/pr101219.C: New test.
>>>>
>>>> This doesn't need to be in torture; it has nothing to do with optimization.
>>>
>>> Aha, moved to gcc/testsuite/g++.dg/warn/pr101219.C.
>>>
>>> --- /dev/null
>>> +++ b/gcc/testsuite/g++.dg/warn/pr101219.C
>>> @@ -0,0 +1,11 @@
>>> +/* PR c++/101219 - ICE on use of uninitialized memfun pointer
>>> +   { dg-do compile }
>>> +   { dg-options "-Wall" } */
>>> +
>>> +struct S { void m(); };
>>> +
>>> +template <int> bool f() {
>>> +  void (S::*mp)();
>>> +
>>> +  return &S::m == mp; // no warning emitted here (no instantiation)
>>> +}
>>>
>>> Another question: Is it expected that gcc generates no warnings here?
>>> It's an uninstantiated function (-1 for warn), but from what I
>>> understand it's guaranteed to generate comparison with uninitialized
>>> data if it ever gets instantiated. Given that we used to ICE in
>>> warning code gcc could possibly flag it? (+1 for warn)
>>
>> Generally it's desirable to diagnose templates for which no valid
>> instantiation is possible.  It seems reasonable in most cases to also
>> warn about templates for which all instantiations would warn.
>>
>> But uninitialized warnings rely on flow analysis that we only do on
>> instantiated functions, and in any case the ICE doesn't depend on mp
>> being uninitialized; I get the same crash if I add = 0 to the declaration.
> 
> Aha. That makes sense. Let's just fix ICE then.
> 
>>> +	if (object_type && TYPE_PTRMEMFUNC_P(object_type)
>>
>> Missing space before (.
>>
>>> +	    && TREE_CODE (member) == FIELD_DECL)
>>> +	  {
>>> +	    r = build_ptrmemfunc_access_expr (object, DECL_NAME(member));
>>
>> And here.
> 
> Added both. Attached as v3.

OK, thanks.

Jason