On Wed, 26 Nov 2025, Marek Polacek wrote:
> On Mon, Nov 24, 2025 at 08:20:10PM -0500, Patrick Palka wrote:
> > On Mon, 24 Nov 2025, Marek Polacek wrote:
> >
> > > Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk/15?
> > >
> > > -- >8 --
> > > In my r15-6792 patch I added a call to tsubst in tsubst_pack_index
> > > to fully instantiate args#N in the pack.
> > >
> > > Here we are in an unevaluated context, but since the pack is
> > > a TREE_VEC, we call tsubst_template_args which has cp_evaluated
> > > at the beginning. That causes a crash because we trip on the
> > > assert in tsubst_expr/PARM_DECL:
> > >
> > > gcc_assert (cp_unevaluated_operand);
> > >
> > > because retrieve_local_specialization didn't find anything (becase
> > > there are no local_specializations yet).
> > >
> > > ISTM that we don't need a full instantiation in an unevaluated context
> > > so we can avoid the crash like this.
> >
> > Hmm, it doesn't seem right to avoid doing a substitution solely because
> > we're in an unevaluated context..
> >
> > Here the TREE_VEC is just a subexpression of the templated
> > PACK_INDEX_EXPR, not a template argument list, so we should still
> > substitute it normally, just without setting cp_evaluated, I think.
>
> I think you're right. How about I just walk the TREE_VEC and subst
> each element, like this?
In-place modification of a templated tree is unusual for tsubst, we
probably should just return a new TREE_VEC. I think we could use a
version of tsubst_template_args that doesn't do cp_evaluated
(tsubst_tree_vec?), define tsubst_template_args in terms of that, and
also use it here?
>
> Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?
>
> -- >8 --
> In my r15-6792 patch I added a call to tsubst in tsubst_pack_index
> to fully instantiate args#N in the pack.
>
> Here we are in an unevaluated context, but since the pack is
> a TREE_VEC, we call tsubst_template_args which has cp_evaluated
> at the beginning. That causes a crash because we trip on the
> assert in tsubst_expr/PARM_DECL:
>
> gcc_assert (cp_unevaluated_operand);
>
> because retrieve_local_specialization didn't find anything (becase
> there are no local_specializations yet).
>
> We can avoid the cp_evaluated by calling tsubst_template_arg on
> each element of the TREE_VEC. I'm still using tsubst_template_arg
> for convenience (tsubst for types, tsubst_expr otherwise).
>
> PR c++/121325
>
> gcc/cp/ChangeLog:
>
> * pt.cc (tsubst_pack_index): Instead of tsubst, call
> tsubst_template_arg on each element of the TREE_VEC.
>
> gcc/testsuite/ChangeLog:
>
> * g++.dg/cpp26/pack-indexing18.C: New test.
> ---
> gcc/cp/pt.cc | 3 +-
> gcc/testsuite/g++.dg/cpp26/pack-indexing18.C | 32 ++++++++++++++++++++
> 2 files changed, 34 insertions(+), 1 deletion(-)
> create mode 100644 gcc/testsuite/g++.dg/cpp26/pack-indexing18.C
>
> diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
> index e74e34d8149..6d8eca044b9 100644
> --- a/gcc/cp/pt.cc
> +++ b/gcc/cp/pt.cc
> @@ -14220,7 +14220,8 @@ tsubst_pack_index (tree t, tree args, tsubst_flags_t
> complain, tree in_decl)
> a partially instantiated closure. Let tsubst find the
> fully-instantiated one. */
> gcc_assert (TREE_CODE (pack) == TREE_VEC);
> - pack = tsubst (pack, args, complain, in_decl);
> + for (tree &arg : tree_vec_range (pack))
> + arg = tsubst_template_arg (arg, args, complain, in_decl);
> }
> if (TREE_CODE (pack) == TREE_VEC && TREE_VEC_LENGTH (pack) == 0)
> {
> diff --git a/gcc/testsuite/g++.dg/cpp26/pack-indexing18.C
> b/gcc/testsuite/g++.dg/cpp26/pack-indexing18.C
> new file mode 100644
> index 00000000000..d3e3730408c
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp26/pack-indexing18.C
> @@ -0,0 +1,32 @@
> +// PR c++/121325
> +// { dg-do compile { target c++26 } }
> +
> +void f(auto... a) requires requires { []<int i = 0>
> noexcept(noexcept(a...[i])) { }(); } {}
> +void g(auto... a) requires requires { []<int i = 0> {
> static_assert(noexcept(a...[i])); }(); } {}
> +
> +void
> +h ()
> +{
> + f (0);
> + g (0);
> +}
> +
> +void foo () {}
> +void bar () noexcept {}
> +template<bool B>
> +void baz () noexcept(B) {}
> +
> +template<typename... Ts>
> +void
> +x (Ts... ts) noexcept (noexcept (ts...[0]()))
> +{
> +}
> +
> +void
> +y ()
> +{
> + static_assert (!noexcept (x (foo)));
> + static_assert (noexcept (x (bar)));
> + static_assert (noexcept (x (baz<true>)));
> + static_assert (!noexcept (x (baz<false>)));
> +}
>
> base-commit: e97550a7d0e1a8b31a76b0877c0e90a0163da7ee
> --
> 2.51.1
>
>
