Add libponyc-run regression test

packages/promises/promise.pony

@@ -203,7 +203,7 @@ actor Promise[A: Any #share]
       let p: Promise[B] = outer
 
       fun ref apply(value: A) =>
-        let fulfill' = f = _PendingFulFill[A, B]
+        let fulfill' = f = _PromiseFulFill[A, B]
 
         try
           let inner = (consume fulfill').apply(value)?
@@ -421,9 +421,9 @@ actor _Join[A: Any #share]
 primitive _None
 
 
-class iso _PendingFulFill[A: Any #share, B: Any #share] is Fulfill[A, Promise[B]]
+class iso _PromiseFulFill[A: Any #share, B: Any #share] is Fulfill[A, Promise[B]]
   """
-  Fulfill returning a promise that is never fulfilled.
+  Fulfill discarding its input value of `A` and returning a promise of type `B`.
   """
   new iso create() => None
-  fun ref apply(value: A): Promise[B] => Promise[B]
+  fun ref apply(value: A): Promise[B] => Promise[B]

test/libponyc-run/promise-flatten-next-regression-3856/main.pony

@@ -0,0 +1,24 @@
+use "promises"
+
+actor FlattenNextProbe
+  new create(handler: Handler iso) =>
+    let promise = Promise[String]
+    // We need a call to next to be present with a call to flatten_next to trigger the issue during reachability analysis
+    promise.next[Any tag](recover this~behaviour() end)
+    (consume handler)(recover String end, promise)
+
+  be behaviour(value: String) =>
+    None
+
+
+class Handler
+  fun ref apply(line: String, prompt: Promise[String]) =>
+    let p = Promise[String]
+    // BUG: Can't change the flatten_next
+    p.flatten_next[String]({ (x: String) => Promise[String] })
+
+
+actor Main
+  new create(env: Env) =>
+    // BUG: Can't change this
+    let term = FlattenNextProbe(Handler)