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

[Marek Polacek <polacek@redhat.com>]

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".

Marek