From c06165d3e0fa60269c952c0f351bc9c14b606804 Mon Sep 17 00:00:00 2001
From: Richard Smith <richard@metafoo.co.uk>
Date: Wed, 19 Aug 2026 17:45:34 +0000
Subject: [PATCH] Require exported field types to be complete in the clang AST.
 (#7653)

It's not enough for field types of Carbon classes to be complete in
SemIR. If the field is exported to Clang, we also need the type to be
complete in Clang's AST, since Clang assumes it has a definition
available for the types of all fields of a complete class.
---
 toolchain/check/cpp/export.cpp                 | 16 +++++++++++++++-
 .../interop/cpp/class/export/field.carbon      | 18 ++++++++++++++++++
 2 files changed, 33 insertions(+), 1 deletion(-)

diff --git a/toolchain/check/cpp/export.cpp b/toolchain/check/cpp/export.cpp
index bf8d85c013da..cffce8ade0ad 100644
--- a/toolchain/check/cpp/export.cpp
+++ b/toolchain/check/cpp/export.cpp
@@ -487,18 +487,32 @@ static auto CreateCppFieldDecl(Context& context,
     return nullptr;
   }
 
+  auto clang_loc = GetCppLocation(context, SemIR::LocId(field_inst_id));
+  bool invalid = false;
+
+  // The field is required to have a complete type in the Clang AST, not just in
+  // SemIR.
+  if (context.clang_sema().RequireCompleteSizedType(
+          clang_loc, cpp_type, clang::diag::err_field_incomplete_or_sizeless)) {
+    // Follow Clang in marking the class as invalid if it contains a field with
+    // an incomplete type. In Clang this prevents record layout; it's unclear if
+    // it's necessary for us.
+    record_decl->setInvalidDecl();
+    invalid = true;
+  }
+
   // Get the field's C++ identifier.
   auto* identifier_info = GetClangIdentifierInfo(context, field_decl.name_id);
   CARBON_CHECK(identifier_info, "field with non-identifier name {0}",
                field_decl.name_id);
 
   // Create the `clang::FieldDecl`.
-  auto clang_loc = GetCppLocation(context, SemIR::LocId(field_inst_id));
   auto* cpp_field_decl = clang::FieldDecl::Create(
       context.ast_context(), record_decl, /*StartLoc=*/clang_loc,
       /*IdLoc=*/clang_loc, identifier_info, cpp_type, /*TInfo=*/nullptr,
       /*BW=*/nullptr,
       /*Mutable=*/true, clang::ICIS_NoInit);
+  cpp_field_decl->setInvalidDecl(invalid);
 
   SetCppClassMemberAccess(class_scope, field_decl.name_id, cpp_field_decl);
 
diff --git a/toolchain/check/testdata/interop/cpp/class/export/field.carbon b/toolchain/check/testdata/interop/cpp/class/export/field.carbon
index 449bd7f92696..c584567e1746 100644
--- a/toolchain/check/testdata/interop/cpp/class/export/field.carbon
+++ b/toolchain/check/testdata/interop/cpp/class/export/field.carbon
@@ -48,6 +48,24 @@ void F() {
 }
 ''';
 
+// --- class_field.carbon
+library "[[@TEST_NAME]]";
+import Cpp;
+
+class ClassField {}
+
+class ClassWithClassField {
+  var c: ClassField;
+}
+
+inline Cpp '''
+void g() {
+  // In order for this to work, we need both class definitions to be exported to
+  // Clang.
+  Carbon::ClassWithClassField c;
+}
+''';
+
 // --- fail_nonexistent_field.carbon
 library "[[@TEST_NAME]]";
 import Cpp;