Fix Flow Types in RelayObservable

Summary:
: Leveraging latest Flow features fix the `isObservable` and `isPromise` functions to now work correctly.

This allows us to finally fix `ObservableFromValue` so that it can use `Subscribable<T>` instead of `RelayObservable<T>`.

Reviewed By: captbaritone

Differential Revision: D54539298

fbshipit-source-id: bff95e07e2ac75c460e38c6d62df3eb490c5d067

packages/relay-runtime/network/RelayObservable.js

@@ -71,9 +71,7 @@ export interface Subscribable<+T> {
   subscribe(observer: Observer<T> | Sink<T>): Subscription;
 }
 
-// Note: This should accept Subscribable<T> instead of RelayObservable<T>,
-// however Flow cannot yet distinguish it from T.
-export type ObservableFromValue<+T> = RelayObservable<T> | Promise<T> | T;
+export type ObservableFromValue<+T> = Subscribable<T> | Promise<T> | T;
 
 let hostReportError:
   | ((Error, isUncaughtThrownError: boolean) => mixed)
@@ -144,9 +142,9 @@ class RelayObservable<+T> implements Subscribable<T> {
    * useful for accepting the result of a user-provided FetchFunction.
    */
   static from<V>(obj: ObservableFromValue<V>): RelayObservable<V> {
-    return isObservable(obj)
+    return isObservable<V>(obj)
       ? fromObservable(obj)
-      : isPromise(obj)
+      : isPromise<V>(obj)
         ? fromPromise(obj)
         : fromValue(obj);
   }
@@ -447,9 +445,7 @@ class RelayObservable<+T> implements Subscribable<T> {
 }
 
 // Use declarations to teach Flow how to check isObservable.
-// $FlowFixMe[deprecated-type]
-declare function isObservable(p: mixed): boolean %checks(p instanceof
-  RelayObservable);
+declare function isObservable<T>(obj: mixed): obj is Subscribable<T>;
 
 function isObservable(obj: mixed) {
   return (

packages/relay-runtime/util/isPromise.js

@@ -11,11 +11,10 @@
 
 'use strict';
 
-// $FlowFixMe[deprecated-type]
-declare function isPromise(p: mixed): boolean %checks(p instanceof Promise);
+declare function isPromise<T>(p: mixed): p is Promise<T>;
 
-function isPromise(p: $FlowFixMe): boolean {
-  return !!p && typeof p.then === 'function';
+function isPromise(p: mixed) {
+  return p != null && typeof p === 'object' && typeof p.then === 'function';
 }
 
 module.exports = isPromise;