diff --git a/tests/unit/mod_private_type.jam b/tests/unit/mod_private_type.jam
new file mode 100644
index 0000000..ffcd1db
--- /dev/null
+++ b/tests/unit/mod_private_type.jam
@@ -0,0 +1,16 @@
+// Helper for test_private_type_signatures: a PRIVATE struct used in
+// this module's own pub-fn signatures. Private types register under
+// their qualified identity so these signatures resolve; importers can
+// only touch the values opaquely (through the pub fns), never name the
+// type.
+const Inner = struct {
+    value: u32,
+};
+
+pub fn makeInner(v: u32) Inner {
+    return Inner { value: v };
+}
+
+pub fn readInner(i: Inner) u32 {
+    return i.value;
+}
diff --git a/tests/unit/test_private_type_signatures.jam b/tests/unit/test_private_type_signatures.jam
new file mode 100644
index 0000000..19678a2
--- /dev/null
+++ b/tests/unit/test_private_type_signatures.jam
@@ -0,0 +1,15 @@
+// A module whose pub fns traffic in a PRIVATE struct. Private types
+// were once never registered at all — the module's own signatures
+// failed to resolve (`unknown module handle` on the compiler's own
+// qualified identity) unless some other module happened to define a
+// same-named type that collided into the same registry slot. They now
+// register under their qualified identity; the importer passes the
+// values through opaquely. (Privacy still holds: naming the type via a
+// handle is rejected — pinned by the C++ namespace-access tests.)
+
+const { assert } = import("test");
+const { makeInner, readInner } = import("mod_private_type");
+
+tfn privateTypeFlowsThroughPubFns() {
+    assert(readInner(makeInner(41)), 41);
+}