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?
Marek
