[gcc r16-3832] c++: pack indexing is a non-deduced context [PR121795]

[Patrick Palka]

https://gcc.gnu.org/g:8e3230f4fed0d41e790d68c2534c057911bab35c

commit r16-3832-g8e3230f4fed0d41e790d68c2534c057911bab35c
Author: Patrick Palka <ppalka@redhat.com>
Date:   Fri Sep 12 14:21:25 2025 -0400

    c++: pack indexing is a non-deduced context [PR121795]
    
    We weren't explicitly treating a pack index specifier as a non-deduced
    context (as per [temp.deduct.type]/5), leading to an ICE for the first
    testcase below.
    
            PR c++/121795
    
    gcc/cp/ChangeLog:
    
            * pt.cc (unify) <case PACK_INDEX_TYPE>: New non-deduced context
            case.
    
    gcc/testsuite/ChangeLog:
    
            * g++.dg/cpp26/pack-indexing17.C: New test.
            * g++.dg/cpp26/pack-indexing17a.C: New test.
    
    Reviewed-by: Marek Polacek <polacek@redhat.com>
    Reviewed-by: Jason Merrill <jason@redhat.com>

Diff:
---
 gcc/cp/pt.cc                                  |  4 ++--
 gcc/testsuite/g++.dg/cpp26/pack-indexing17.C  | 11 +++++++++++
 gcc/testsuite/g++.dg/cpp26/pack-indexing17a.C | 12 ++++++++++++
 3 files changed, 25 insertions(+), 2 deletions(-)

diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
index 7f0d16f217e7..b7cb807d8daa 100644
--- a/gcc/cp/pt.cc
+++ b/gcc/cp/pt.cc
@@ -26301,8 +26301,8 @@ unify (tree tparms, tree targs, tree parm, tree arg, int strict,
     case TYPEOF_TYPE:
     case DECLTYPE_TYPE:
     case TRAIT_TYPE:
-      /* Cannot deduce anything from TYPEOF_TYPE, DECLTYPE_TYPE,
-	 or TRAIT_TYPE nodes.  */
+    case PACK_INDEX_TYPE:
+      /* These are non-deduced contexts.  */
       return unify_success (explain_p);
 
     case ERROR_MARK:
diff --git a/gcc/testsuite/g++.dg/cpp26/pack-indexing17.C b/gcc/testsuite/g++.dg/cpp26/pack-indexing17.C
new file mode 100644
index 000000000000..8450ed64bd5f
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp26/pack-indexing17.C
@@ -0,0 +1,11 @@
+// PR c++/121795
+// { dg-do compile { target c++26 } }
+
+template<class T, class... Ts>
+struct A;
+
+template<class... Ts>
+struct A<Ts...[sizeof...(Ts)-1], Ts...> { };
+
+A<int, int> x;
+A<char, int> y; // { dg-error "incomplete" }
diff --git a/gcc/testsuite/g++.dg/cpp26/pack-indexing17a.C b/gcc/testsuite/g++.dg/cpp26/pack-indexing17a.C
new file mode 100644
index 000000000000..445f0d94ad20
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp26/pack-indexing17a.C
@@ -0,0 +1,12 @@
+// PR c++/121795
+// A version of pack-indexing17.C using a nontype pack.
+// { dg-do compile { target c++26 } }
+
+template<int N, int... Ns>
+struct A;
+
+template<int... Ns>
+struct A<Ns...[sizeof...(Ns)-1], Ns...> { };
+
+A<0, 0> x;
+A<0, 1> y; // { dg-error "incomplete" }