From cdcd3ab66cba468007c49a026f27b148760dbab6 Mon Sep 17 00:00:00 2001
From: Nicholas Bishop <nicholasbishop@google.com>
Date: Mon, 9 Mar 2026 10:49:31 -0400
Subject: [PATCH] Handle pack expansion for dependent non-type template params
 (#6850)

https://github.com/carbon-language/carbon-lang/issues/6717
---
 toolchain/check/cpp/call.cpp                  | 25 +++++++++++++------
 .../cpp/template/non_type_param.carbon        | 14 +++++++++++
 2 files changed, 31 insertions(+), 8 deletions(-)

diff --git a/toolchain/check/cpp/call.cpp b/toolchain/check/cpp/call.cpp
index ed835c2093a5..558583e91129 100644
--- a/toolchain/check/cpp/call.cpp
+++ b/toolchain/check/cpp/call.cpp
@@ -112,7 +112,8 @@ static auto MakePlaceholderTemplateArg(Context& context, SemIR::InstId arg_id)
 static auto ConvertArgToTemplateArg(
     Context& context, clang::TemplateDecl* template_decl,
     clang::NamedDecl* param_decl, SemIR::InstId arg_id,
-    clang::SmallVector<clang::TemplateArgument>* template_args, bool diagnose)
+    clang::SmallVector<clang::TemplateArgument>* template_args,
+    unsigned argument_pack_index, bool diagnose)
     -> std::optional<clang::TemplateArgumentLoc> {
   if (isa<clang::TemplateTypeParmDecl>(param_decl)) {
     auto type = ExprAsType(context, SemIR::LocId(arg_id), arg_id, diagnose);
@@ -153,6 +154,9 @@ static auto ConvertArgToTemplateArg(
 
   if (auto* non_type = dyn_cast<clang::NonTypeTemplateParmDecl>(param_decl)) {
     auto param_type = non_type->getType();
+    if (non_type->isParameterPack() && non_type->isExpandedParameterPack()) {
+      param_type = non_type->getExpansionType(argument_pack_index);
+    }
 
     // Handle non-type parameters with a dependent type. For example:
     //
@@ -179,9 +183,13 @@ static auto ConvertArgToTemplateArg(
                                                   /*Final=*/true);
 
       mltal.addOuterRetainedLevels(non_type->getDepth());
-      // TODO: handle pack expansion by passing in the pack index from
-      // `ConvertArgsToTemplateArgs`.
-      if (!param_type->getAs<clang::PackExpansionType>()) {
+      if (const auto* pet = param_type->getAs<clang::PackExpansionType>()) {
+        clang::Sema::ArgPackSubstIndexRAII subst_index(context.clang_sema(),
+                                                       argument_pack_index);
+        param_type = context.clang_sema().SubstType(pet->getPattern(), mltal,
+                                                    non_type->getLocation(),
+                                                    non_type->getDeclName());
+      } else {
         param_type = context.clang_sema().SubstType(param_type, mltal,
                                                     non_type->getLocation(),
                                                     non_type->getDeclName());
@@ -292,12 +300,13 @@ auto ConvertArgsToTemplateArgs(Context& context,
     llvm::ArrayRef<SemIR::InstId> args_for_param =
         param_decl->isTemplateParameterPack() ? std::exchange(arg_ids, {})
                                               : arg_ids.consume_front();
-    for (auto arg_id : args_for_param) {
-      if (auto arg =
-              ConvertArgToTemplateArg(context, template_decl, param_decl,
-                                      arg_id, &template_args, diagnose)) {
+    for (auto [argument_pack_index, arg_id] : llvm::enumerate(args_for_param)) {
+      if (auto arg = ConvertArgToTemplateArg(context, template_decl, param_decl,
+                                             arg_id, &template_args,
+                                             argument_pack_index, diagnose)) {
         arg_list.addArgument(*arg);
         template_args.push_back(arg->getArgument());
+        argument_pack_index++;
       } else {
         return false;
       }
diff --git a/toolchain/check/testdata/interop/cpp/template/non_type_param.carbon b/toolchain/check/testdata/interop/cpp/template/non_type_param.carbon
index 98eb68f31229..dd55afc88103 100644
--- a/toolchain/check/testdata/interop/cpp/template/non_type_param.carbon
+++ b/toolchain/check/testdata/interop/cpp/template/non_type_param.carbon
@@ -164,6 +164,20 @@ var n: i32;
 // CHECK:STDERR:
 var x: Cpp.TwoNonType(1, &n);
 
+// --- dependent_parameter_pack.carbon
+
+library "[[@TEST_NAME]]";
+
+import Cpp inline '''
+template<typename ...T> struct X {
+  template<T... N> static int n;
+};
+''';
+
+fn G() -> i32 {
+  return Cpp.X(i32, i32).n(1, 2);
+}
+
 // CHECK:STDOUT: --- valid.carbon
 // CHECK:STDOUT:
 // CHECK:STDOUT: constants {