On Wed, Nov 26, 2025 at 01:29:35PM -0500, Patrick Palka wrote:
On Wed, 26 Nov 2025, Patrick Palka wrote:
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?
Actually never mind about the idea of defining tsubst_template_args
in terms of tsubst_tree_vec... tsubst_template_args does a lot of
stuff that's specific to template arguments which wouldn't be suitable
for TREE_VEC expression substitution. I suppose we could just use a
standalone tsubst_tree_vec routine that tsubsts each element and returns
a new TREE_VEC.
Fair enough.
(I believe TRAIT_EXPR is another tree that has a TREE_VEC subexpression
and probably has a similiar bug wrt cp_evaluated being set, so we could
use tsubst_tree_vec there as well.)
I've not done that yet.
How does this look? Thanks,
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 the new tsubst_tree_vec,
which creates a new TREE_VEC and substitutes each element.
PR c++/121325
gcc/cp/ChangeLog:
* pt.cc (tsubst_tree_vec): New.
(tsubst_pack_index): Call it.
gcc/testsuite/ChangeLog:
* g++.dg/cpp26/pack-indexing18.C: New test.
Reviewed-by: Patrick Palka <[email protected]>
---
gcc/cp/pt.cc | 21 ++++++++++++-
gcc/testsuite/g++.dg/cpp26/pack-indexing18.C | 32 ++++++++++++++++++++
2 files changed, 52 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..4dc8f980d0d 100644
--- a/gcc/cp/pt.cc
+++ b/gcc/cp/pt.cc
@@ -14203,6 +14203,25 @@ tsubst_pack_expansion (tree t, tree args,
tsubst_flags_t complain,
return result;
}
+/* Substitute ARGS into T, which is a TREE_VEC. This function creates a new
+ TREE_VEC rather than substituting the elements in-place. */
+
+static tree
+tsubst_tree_vec (tree t, tree args, tsubst_flags_t complain, tree in_decl)
+{
+ const int len = TREE_VEC_LENGTH (t);
+ tree r = make_tree_vec (len);
+ for (int i = 0; i < len; ++i)
+ {
+ tree arg = TREE_VEC_ELT (t, i);
+ if (TYPE_P (arg))
+ TREE_VEC_ELT (r, i) = tsubst (arg, args, complain, in_decl);
+ else
+ TREE_VEC_ELT (r, i) = tsubst_expr (arg, args, complain, in_decl);
+ }
+ return r;
+}
+
/* Substitute ARGS into T, which is a pack index (i.e., PACK_INDEX_TYPE or
PACK_INDEX_EXPR). Returns a single type or expression, a PACK_INDEX_*
node if only a partial substitution could be performed, or ERROR_MARK_NODE
@@ -14220,7 +14239,7 @@ 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);
+ pack = tsubst_tree_vec (pack, args, complain, in_decl);
