Re: [PATCH 1/2] c++: requires-exprs and partial constraint subst [PR112769]

[Patrick Palka <ppalka@redhat.com>]

On Fri, 2 Feb 2024, Jason Merrill wrote:

> On 2/2/24 14:41, Patrick Palka wrote:
> > Bootstrapped and regtested on x86_64-pc-linux-gnu, does this
> > look OK for trunk?
> > 
> > -- >8 --
> > 
> > In r11-3261-gb28b621ac67bee we made tsubst_requires_expr never partially
> > substitute into a requires-expression so as to avoid checking its
> > requirements out of order during e.g. generic lambda regeneration.
> > 
> > Unfortunately we still do need to partially substitute into a
> > requires-expression in rare cases, in particular when it's used in
> > associated constraints that we are directly substituting for sake of
> > declaration matching or dguide constraint rewriting.  We can identify
> > this situation by checking processing_constraint_expression_p, so this
> > patch uses this predicate to control whether we defer substitution or
> > partially substitute.  The entering_scope=true change in tsubst_baselink
> > is needed to avoid ICEing from tsubst_baselink during name lookup when
> > rewriting std::ranges::ref_view's dguide constraints.
> 
> Actually, I don't think we want to enter the scope when rewriting constraints.
> Would tsubst_scope work instead?

Oops yes, because of the maybe_dependent_member_ref stuff, the handling
for which doesn't trigger here for some reason.  Ah, I think it's
because the tf_dguide flag gets dropped during tsubst_requires_expr
since it uses tf_none rather than complain & ~tf_warning_or_error
which would preserve special tsubst flags such as tf_dguide.  I'll fix...

> 
> Jason
> 
>