diff --git a/changelog.d/10682-codegen-testbuild-missing-field.md b/changelog.d/10682-codegen-testbuild-missing-field.md
new file mode 100644
index 0000000000..7572d77d62
--- /dev/null
+++ b/changelog.d/10682-codegen-testbuild-missing-field.md
@@ -0,0 +1,11 @@
+**Fix the `perry-codegen` lib-test build, which `-D warnings --all-targets` rejects on `main`.**
+
+#10484's `constructor_has_synthetic_arguments` field was added to `ImportedClass`, but two test
+fixtures construct that struct literally and were never updated:
+`expr/instanceof_imported_rhs_tests.rs` and `lower_call/new_builtin_shadow_tests.rs`. Both now set
+it to `false`, which is the pre-#10484 behaviour they were written against.
+
+`cargo check -p perry-codegen --lib` does not compile `cfg(test)` code, so this is invisible to the
+per-crate preflight and only the workspace-wide `--all-targets` step sees it — which is why it
+reached `main`. Verified: `RUSTFLAGS="-D warnings" cargo check --workspace --all-targets` (with the
+usual cross-host UI exclusions) now finishes clean.
diff --git a/crates/perry-codegen/src/expr/instanceof_imported_rhs_tests.rs b/crates/perry-codegen/src/expr/instanceof_imported_rhs_tests.rs
index a58793068b..9471ce5a66 100644
--- a/crates/perry-codegen/src/expr/instanceof_imported_rhs_tests.rs
+++ b/crates/perry-codegen/src/expr/instanceof_imported_rhs_tests.rs
@@ -49,6 +49,7 @@ fn imported_class(name: &str, class_id: u32) -> ImportedClass {
         namespace: None,
         source_prefix: "lib_ts".to_string(),
         constructor_param_count: 0,
+        constructor_has_synthetic_arguments: false,
         has_own_constructor: true,
         constructor_has_rest: false,
         has_instance_fields: false,
diff --git a/crates/perry-codegen/src/lower_call/new_builtin_shadow_tests.rs b/crates/perry-codegen/src/lower_call/new_builtin_shadow_tests.rs
index 98cbad3866..8583a4b53e 100644
--- a/crates/perry-codegen/src/lower_call/new_builtin_shadow_tests.rs
+++ b/crates/perry-codegen/src/lower_call/new_builtin_shadow_tests.rs
@@ -121,6 +121,7 @@ fn imported_class_of_the_same_name_already_shadowed_the_builtin() {
         namespace: None,
         source_prefix: "lib_ts".to_string(),
         constructor_param_count: 0,
+        constructor_has_synthetic_arguments: false,
         has_own_constructor: false,
         constructor_has_rest: false,
         has_instance_fields: false,