Re: [PATCH v2] c++: ICE with VEC_INIT_EXPR and defarg [PR106925]

[Jason Merrill <jason@redhat.com>]

On 10/13/22 09:58, Marek Polacek wrote:
> On Wed, Oct 12, 2022 at 02:23:40PM -0400, Marek Polacek wrote:
>> On Wed, Oct 12, 2022 at 01:12:57PM -0400, Marek Polacek wrote:
>>> On Wed, Oct 12, 2022 at 12:47:21PM -0400, Jason Merrill wrote:
>>>> On 10/12/22 12:27, Marek Polacek wrote:
>>>>> On Tue, Oct 11, 2022 at 04:28:11PM -0400, Jason Merrill wrote:
>>>>>> On 10/11/22 16:00, Marek Polacek wrote:
>>>>>>> Since r12-8066, in cxx_eval_vec_init we perform expand_vec_init_expr
>>>>>>> while processing the default argument in this test.
>>>>>>
>>>>>> Hmm, why are we calling cxx_eval_vec_init during parsing of the default
>>>>>> argument?  In particular, any expansion that depends on the enclosing
>>>>>> function context should be deferred until the default arg is used by a call.
>>>>>
>>>>> I think this is part of the semantic constraints checking [dcl.fct.default]/5
>>>>> talks about, as in, this doesn't compile even though the default argument is
>>>>> not executed:
>>>>>
>>>>> struct S {
>>>>>     S() = delete;
>>>>> };
>>>>> void foo (S = S()) { }
>>>>> In the test below we parse '= MyVector<1>()' and end up calling mark_used
>>>>> on the implicit "constexpr MyVector<1>::MyVector() noexcept (<uninstantiated>)"
>>>>> ctor.  mark_used calls maybe_instantiate_noexcept.  Since the ctor has
>>>>> a DEFERRED_NOEXCEPT, we have to figure out if the ctor should be noexcept
>>>>> or not using get_defaulted_eh_spec.  That means walking the members of
>>>>> MyVector.  Thus we reach
>>>>>     /* Core 1351: If the field has an NSDMI that could throw, the
>>>>>        default constructor is noexcept(false).  */
>>>>
>>>> Maybe we need a cp_unevaluated here?  The operand of noexcept should be
>>>> unevaluated.
>>>
>>> That wouldn't help since get_nsdmi specifically does "cp_evaluated ev;",
>>> so...
>>>   
>>>>> and call get_nsdmi on 'data'.  There we digest its initializer which is {}.
>>>>> massage_init_elt calls digest_init_r on the {} and produces
>>>>>     TARGET_EXPR <D.2518, <<< Unknown tree: vec_init_expr
>>>>>       D.2518
>>>>>       {} >>>>
>>>>> and the subsequent fold_non_dependent_init leads to cxx_eval_vec_init
>>>>> -> expand_vec_init_expr.
>>>>>
>>>>> I think this is all correct except that the fold_non_dependent_init is
>>>>> somewhat questionable to me; do we really have to fold in order to say
>>>>> if the NSDMI init can throw?  Sure, we need to digest the {}, maybe
>>>>> the field's ctors can throw, but I don't know about the folding.
>>>>
>>>> And we can check cp_unevaluated_operand to avoid the
>>>> fold_non_dependent_init?
>>>
>>> ...we'd still fold.  I'm not sure if we want a LOOKUP_ flag that says
>>> "we're just checking if we can throw, don't fold".
>>
>> Eh, a new flag is overkill.  Maybe don't do cp_evaluated in get_nsdmi if
>> we're called from walk_field_subobs would be worth a try?
> 
> FWIW, my experiments with cp_unevaluated_operand failed because then we'd
> miss warnings as in g++.dg/ext/cond5.C which warns from the
> get_defaulted_eh_spec context -- so I'd have no way to distinguish that
> from the test in this PR.  Should we just go back to my patch?

Your patch is still OK.

Jason