On 10/12/22 14:23, 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?
It seems that we treat DMI instantiations as evaluated even if they're
triggered from unevaluated context so sharing lambdas between different
uses of the DMI works properly. I don't think this is worth messing
with at this point; thanks for satisfying my curiosity.
Jason
