From 9bcfc343cb73c1d7d4e9380b615224ca128832f4 Mon Sep 17 00:00:00 2001
From: Dan Katz <katzdm@gmail.com>
Date: Wed, 25 Jun 2025 10:30:22 -0400
Subject: [PATCH] Handle StructuralValue-type template arguments in
 define_aggregate.

Fixes #159.
---
 clang/lib/Sema/SemaReflect.cpp                | 11 ++++++--
 .../reflection/define-aggregate.pass.cpp      | 27 +++++++++++++++++++
 2 files changed, 36 insertions(+), 2 deletions(-)

diff --git a/clang/lib/Sema/SemaReflect.cpp b/clang/lib/Sema/SemaReflect.cpp
index 07324d30240f..a224219e216f 100644
--- a/clang/lib/Sema/SemaReflect.cpp
+++ b/clang/lib/Sema/SemaReflect.cpp
@@ -551,8 +551,15 @@ public:
                                      SourceLocation());
             break;
           }
-          // TODO(P2996): Handle other kinds of TemplateArgument
-          // (e.g., structural).
+          case TemplateArgument::StructuralValue: {
+            Expr *E = new (S.Context) OpaqueValueExpr(
+                DefinitionLoc, TArg.getStructuralValueType(), VK_PRValue);
+            E = ConstantExpr::Create(S.Context, E, TArg.getAsStructuralValue());
+            ParsedTArgs.emplace_back(ParsedTemplateArgument::NonType, E,
+                                     SourceLocation());
+            break;
+          }
+          // TODO(P2996): Handle other kinds of TemplateArgument.
           default:
             llvm_unreachable("unimplemented");
           }
diff --git a/libcxx/test/std/experimental/reflection/define-aggregate.pass.cpp b/libcxx/test/std/experimental/reflection/define-aggregate.pass.cpp
index 8c7e7ab2e460..1a9e8e1f4a30 100644
--- a/libcxx/test/std/experimental/reflection/define-aggregate.pass.cpp
+++ b/libcxx/test/std/experimental/reflection/define-aggregate.pass.cpp
@@ -396,4 +396,31 @@ int_holder_template<int_holder{0}> o2;
 
 }  // namespace bb_clang_p2996_issue_145_regression_test
 
+                  // ========================================
+                  // bb_clang_p2996_issue_159_regression_test
+                  // ========================================
+
+namespace bb_clang_p2996_issue_159_regression_test {
+template <auto V>
+struct VS;
+
+template <>
+struct VS<^^float>{};
+
+consteval {
+  std::meta::define_aggregate(^^VS<0>, {});
+  std::meta::define_aggregate(
+      std::meta::substitute(^^VS, {std::meta::reflect_constant(1)}), {});
+
+  std::meta::define_aggregate(
+      std::meta::substitute(^^VS, {std::meta::reflect_constant(^^int)}),
+      {});
+}
+
+VS<1> v1;
+VS<^^int> v2;
+
+}  // namespace bb_clang_p2996_issue_159_regression_test
+
+
 int main() { }