On Thu, 8 Jul 2021, Jason Merrill wrote:
> On 7/8/21 11:28 AM, Patrick Palka wrote:
> > Here we're crashing ultimately because the mechanism for delaying
> > substitution into a requires-expression (or constexpr if) doesn't
> > expect to see dependent args. But we end up capturing dependent
> > args here when substituting into the default template argument during
> > coerce_template_parms for the dependent specialization p<T>.
> >
> > This patch enables the commented out code in add_extra_args for
> > handling this situation. It turns out we also need to make a copy of
> > the captured arguments so that coerce_template_parms doesn't later
> > add to the argument, which would form an unexpected cycle. And we
> > need to make tsubst_template_args more forgiving about missing template
> > arguments, since the arguments we capture from coerce_template_parms are
> > incomplete.
> >
> > Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look OK for
> > trunk/11?
> >
> > PR c++/101181
> >
> > gcc/cp/ChangeLog:
> >
> > * constraint.cc (tsubst_requires_expr): Pass complain/in_decl to
> > add_extra_args.
> > * cp-tree.h (add_extra_args): Add complain/in_decl parameters.
> > * pt.c (build_extra_args): Make a copy of args.
> > (add_extra_args): Add complain/in_decl parameters. Handle the
> > case where the extra arguments are dependent.
> > (tsubst_pack_expansion): Pass complain/in_decl to
> > add_extra_args.
> > (tsubst_template_args): Handle missing template arguments.
> > (tsubst_expr) <case IF_STMT>: Pass complain/in_decl to
> > add_extra_args.
> >
> > gcc/testsuite/ChangeLog:
> >
> > * g++.dg/cpp2a/concepts-requires26.C: New test.
> > * g++.dg/cpp2a/lambda-uneval16.C: New test.
> > ---
> > gcc/cp/constraint.cc | 3 +-
> > gcc/cp/cp-tree.h | 2 +-
> > gcc/cp/pt.c | 31 +++++++++----------
> > .../g++.dg/cpp2a/concepts-requires26.C | 18 +++++++++++
> > gcc/testsuite/g++.dg/cpp2a/lambda-uneval16.C | 22 +++++++++++++
> > 5 files changed, 58 insertions(+), 18 deletions(-)
> > create mode 100644 gcc/testsuite/g++.dg/cpp2a/concepts-requires26.C
> > create mode 100644 gcc/testsuite/g++.dg/cpp2a/lambda-uneval16.C
> >
> > diff --git a/gcc/cp/constraint.cc b/gcc/cp/constraint.cc
> > index 99d3ccc6998..4ee5215df50 100644
> > --- a/gcc/cp/constraint.cc
> > +++ b/gcc/cp/constraint.cc
> > @@ -2266,7 +2266,8 @@ tsubst_requires_expr (tree t, tree args, sat_info
> > info)
> > /* A requires-expression is an unevaluated context. */
> > cp_unevaluated u;
> > - args = add_extra_args (REQUIRES_EXPR_EXTRA_ARGS (t), args);
> > + args = add_extra_args (REQUIRES_EXPR_EXTRA_ARGS (t), args,
> > + info.complain, info.in_decl);
> > if (processing_template_decl)
> > {
> > /* We're partially instantiating a generic lambda. Substituting
> > into
> > diff --git a/gcc/cp/cp-tree.h b/gcc/cp/cp-tree.h
> > index 58da7460001..0a5f13489cc 100644
> > --- a/gcc/cp/cp-tree.h
> > +++ b/gcc/cp/cp-tree.h
> > @@ -7289,7 +7289,7 @@ extern void add_mergeable_specialization (bool
> > is_decl, bool is_alias,
> > tree outer, unsigned);
> > extern tree add_to_template_args (tree, tree);
> > extern tree add_outermost_template_args (tree, tree);
> > -extern tree add_extra_args (tree, tree);
> > +extern tree add_extra_args (tree, tree, tsubst_flags_t,
> > tree);
> > extern tree build_extra_args (tree, tree,
> > tsubst_flags_t);
> > /* in rtti.c */
> > diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
> > index 06116d16887..e4bdac087ad 100644
> > --- a/gcc/cp/pt.c
> > +++ b/gcc/cp/pt.c
> > @@ -12928,7 +12928,9 @@ extract_local_specs (tree pattern, tsubst_flags_t
> > complain)
> > tree
> > build_extra_args (tree pattern, tree args, tsubst_flags_t complain)
> > {
> > - tree extra = args;
> > + /* Make a copy of the extra arguments so that they won't get changed
> > + from under us. */
> > + tree extra = copy_template_args (args);
> > if (local_specializations)
> > if (tree locals = extract_local_specs (pattern, complain))
> > extra = tree_cons (NULL_TREE, extra, locals);
> > @@ -12939,7 +12941,7 @@ build_extra_args (tree pattern, tree args,
> > tsubst_flags_t complain)
> > normal template args to ARGS. */
> > tree
> > -add_extra_args (tree extra, tree args)
> > +add_extra_args (tree extra, tree args, tsubst_flags_t complain, tree
> > in_decl)
> > {
> > if (extra && TREE_CODE (extra) == TREE_LIST)
> > {
> > @@ -12959,20 +12961,14 @@ add_extra_args (tree extra, tree args)
> > gcc_assert (!TREE_PURPOSE (extra));
> > extra = TREE_VALUE (extra);
> > }
> > -#if 1
> > - /* I think we should always be able to substitute dependent args into the
> > - pattern. If that turns out to be incorrect in some cases, enable the
> > - alternate code (and add complain/in_decl parms to this function). */
>
> Ah, because these cases aren't pack expansions, so we aren't trying to do the
> substitution; I wonder if it would be feasible to do so. But this approach is
> probably simpler. OK.
For constexpr if, it seems feasible/safe to do the substitution by
setting tf_partial when the arguments are dependent.
But for requires-exprs, if one of the arguments is non-dependent then
doing the substitution could cause us to evaluate the requirements out
of order. So this current approach might be better for requires-exprs
since it avoids out-of-order evaluation.
