On 1/27/26 4:17 AM, Patrick Palka wrote:
Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look
OK for trunk/backports?
-- >8 --
Here since the expression within the decltype C<Ts...> is not instantiation
dependent (we know its type is bool, and don't care about its value)
finish_decltype_type instantiates it immediately via the usual tsubst_expr
with NULL_TREE args. During which however tsubst_pack_expansion isn't
prepared to handle such a substitution due to an overly strict assert.
This patch relaxes the assert accordingly.
PR c++/123676
gcc/cp/ChangeLog:
* pt.cc (tsubst_pack_expansion): Relax unsubsituted_packs
assert to allow !processing_template_decl when args is
NULL_TREE.
gcc/testsuite/ChangeLog:
* g++.dg/cpp2a/concepts-decltype5.C: New test.
---
gcc/cp/pt.cc | 7 +++----
gcc/testsuite/g++.dg/cpp2a/concepts-decltype5.C | 10 ++++++++++
2 files changed, 13 insertions(+), 4 deletions(-)
create mode 100644 gcc/testsuite/g++.dg/cpp2a/concepts-decltype5.C
diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
index 4906de000e66..140ffbc2c458 100644
--- a/gcc/cp/pt.cc
+++ b/gcc/cp/pt.cc
@@ -14143,10 +14143,9 @@ tsubst_pack_expansion (tree t, tree args,
tsubst_flags_t complain,
}
else
{
- /* We can't substitute for this parameter pack. We use a flag as
- well as the missing_level counter because function parameter
- packs don't have a level. */
I don't see a reason to remove the comment explaining why we have the
unsubstituted_packs flag. OK leaving the comment alone.
- gcc_assert (processing_template_decl || is_auto (parm_pack));
+ /* We can't substitute for this parameter pack. */
+ gcc_assert (processing_template_decl || is_auto (parm_pack)
+ || args == NULL_TREE);
unsubstituted_packs = true;
}
}
diff --git a/gcc/testsuite/g++.dg/cpp2a/concepts-decltype5.C
b/gcc/testsuite/g++.dg/cpp2a/concepts-decltype5.C
new file mode 100644
index 000000000000..1b6f7f78f2c3
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/concepts-decltype5.C
@@ -0,0 +1,10 @@
+// PR c++/123676
+// { dg-do compile { target c++20 } }
+
+template<class...>
+concept C = true;
+
+template<class... Ts>
+auto f() -> decltype(C<Ts...>) {
+ return true;
+}
