diff --git a/map.js b/map.js
index 65cbc17..ad2c642 100644
--- a/map.js
+++ b/map.js
@@ -18,9 +18,9 @@ export const create = () => new Map()
  * Copy a Map object into a fresh Map object.
  *
  * @function
- * @template X,Y
- * @param {Map<X,Y>} m
- * @return {Map<X,Y>}
+ * @template K,V
+ * @param {Map<K,V>} m
+ * @return {Map<K,V>}
  */
 export const copy = m => {
   const r = create()
@@ -37,12 +37,12 @@ export const copy = m => {
  * ```
  *
  * @function
- * @template V,K
- * @template {Map<K,V>} MAP
+ * @template {Map<any, any>} MAP
+ * @template {MAP extends Map<any,infer V> ? function():V : unknown} CF
  * @param {MAP} map
- * @param {K} key
- * @param {function():V} createT
- * @return {V}
+ * @param {MAP extends Map<infer K,any> ? K : unknown} key
+ * @param {CF} createT
+ * @return {ReturnType<CF>}
  */
 export const setIfUndefined = (map, key, createT) => {
   let set = map.get(key)
diff --git a/map.test.js b/map.test.js
index 55f5432..2f47bf4 100644
--- a/map.test.js
+++ b/map.test.js
@@ -26,3 +26,30 @@ export const testMap = _tc => {
   t.assert(!map.all(m, (v, k) => k === 1 && v === 2))
   t.assert(map.all(m, (v) => v === 2 || v === 3 || v === numberOfWrites))
 }
+
+/**
+ * @param {t.TestCase} _tc
+ */
+export const testTypeDefinitions = _tc => {
+  // setIfUndefined supports inheritance properly: See https://github.com/dmonad/lib0/issues/82
+  class A {
+    constructor () {
+      this.a = 4
+    }
+  }
+  class B extends A {
+    constructor () {
+      super()
+      this.b = 4
+    }
+  }
+  /**
+   * @type {Map<number, A>}
+   */
+  const m = map.create()
+  /**
+   * @type {B}
+   */
+  const b = map.setIfUndefined(m, 0, () => new B())
+  console.log(b)
+}