From 62f25af06c39636d5e09d85753d3e958f264eec1 Mon Sep 17 00:00:00 2001
From: Ryan Ofsky <ryan@ofsky.org>
Date: Wed, 1 Jul 2026 18:23:58 -0400
Subject: [PATCH] test: check ReadList return value

ReadList was originally declared void and did not return the result of
read_dest.update(), silently discarding the return value. When callers
pass a ReadDestTemp or other ReadDest whose update() returns a
constructed value, that value was lost.

Add a test to demonstrate the bug: extend FooCustom with a v3
:List(Int32) / std::vector<int> field and use the assignment pattern
  value.v3 = ReadField(..., ReadDestTemp<std::vector<int>>())
inside CustomReadField. This fails to compile if ReadList returns void
and succeeds with the fix.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
---
 test/mp/test/foo-types.h | 2 ++
 test/mp/test/foo.capnp   | 1 +
 test/mp/test/foo.h       | 1 +
 test/mp/test/test.cpp    | 2 ++
 4 files changed, 6 insertions(+)

diff --git a/test/mp/test/foo-types.h b/test/mp/test/foo-types.h
index b96eabfc..1bc6c523 100644
--- a/test/mp/test/foo-types.h
+++ b/test/mp/test/foo-types.h
@@ -46,6 +46,7 @@ void CustomBuildField(TypeList<FooCustom>, Priority<1>, InvokeContext& invoke_co
 {
     BuildField(TypeList<std::string>(), invoke_context, output, value.v1);
     output.setV2(value.v2);
+    BuildField(TypeList<std::vector<int>>(), invoke_context, output, value.v3);
 }
 
 template <typename Input, typename ReadDest>
@@ -55,6 +56,7 @@ decltype(auto) CustomReadField(TypeList<FooCustom>, Priority<1>, InvokeContext&
     return read_dest.update([&](FooCustom& value) {
         value.v1 = ReadField(TypeList<std::string>(), invoke_context, mp::Make<mp::ValueField>(custom.getV1()), ReadDestTemp<std::string>());
         value.v2 = custom.getV2();
+        value.v3 = ReadField(TypeList<std::vector<int>>(), invoke_context, mp::Make<mp::ValueField>(custom.getV3()), ReadDestTemp<std::vector<int>>());
     });
 }
 
diff --git a/test/mp/test/foo.capnp b/test/mp/test/foo.capnp
index 9e6213fd..26890ea5 100644
--- a/test/mp/test/foo.capnp
+++ b/test/mp/test/foo.capnp
@@ -66,6 +66,7 @@ struct FooStruct $Proxy.wrap("mp::test::FooStruct") {
 struct FooCustom $Proxy.wrap("mp::test::FooCustom") {
     v1 @0 :Text;
     v2 @1 :Int32;
+    v3 @2 :List(Int32);
 }
 
 struct FooEmpty $Proxy.wrap("mp::test::FooEmpty") {
diff --git a/test/mp/test/foo.h b/test/mp/test/foo.h
index 779c8db1..a8fd9805 100644
--- a/test/mp/test/foo.h
+++ b/test/mp/test/foo.h
@@ -34,6 +34,7 @@ struct FooCustom
 {
     std::string v1;
     int v2;
+    std::vector<int> v3;
 };
 
 struct FooEmpty
diff --git a/test/mp/test/test.cpp b/test/mp/test/test.cpp
index f5f35437..715b6d91 100644
--- a/test/mp/test/test.cpp
+++ b/test/mp/test/test.cpp
@@ -233,9 +233,11 @@ KJ_TEST("Call FooInterface methods")
     FooCustom custom_in;
     custom_in.v1 = "v1";
     custom_in.v2 = 5;
+    custom_in.v3 = {10, 20, 30};
     FooCustom custom_out = foo->passCustom(custom_in);
     KJ_EXPECT(custom_in.v1 == custom_out.v1);
     KJ_EXPECT(custom_in.v2 == custom_out.v2);
+    KJ_EXPECT(custom_in.v3 == custom_out.v3);
 
     foo->passEmpty(FooEmpty{});