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?
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
