[PATCH] c++: pack indexing is a non-deduced context [PR121795]

[Patrick Palka]

Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look
OK for trunk/15?

-- >8 --

We weren't explicitly treating a pack index specifier as a non-deduced
context.

        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.
---
 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(-)
 create mode 100644 gcc/testsuite/g++.dg/cpp26/pack-indexing17.C
 create mode 100644 gcc/testsuite/g++.dg/cpp26/pack-indexing17a.C

diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
index 7f0d16f217e7..870c80a91533 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" }
-- 
2.51.0.193.g4975ec3473