Update release notes for PR #5006

Author: ponylang-main

.release-notes/fix-reify-typeparamref-nested-lambdas.md

@@ -63,3 +63,19 @@ partial call involved
 
 The workaround was to use a `match` expression instead. This has been fixed. The `as` form now compiles correctly, including when chaining method calls before the field assignment (e.g., `(a as Wumpus).some_method().hunger = 1`).
 
+## Fix segfault when using Generator.map with PonyCheck shrinking
+
+Using `Generator.map` to transform values from one type to another would segfault during shrinking when a property test failed. For example, this program would crash:
+
+```pony
+let gen = recover val
+  Generators.u32().map[String]({(n: U32): String^ => n.string()})
+end
+PonyCheck.for_all[String](gen, h)(
+  {(sample: String, ph: PropertyHelper) =>
+    ph.assert_true(sample.size() > 0)
+  })?
+```
+
+The underlying compiler bug affected any code where a lambda appeared inside an object literal inside a generic method and was then passed to another generic method. The lambda's `apply` method was silently omitted from the vtable, causing a segfault when called at runtime.
+