fix async generator typing in flows

src/api/flow.ts

@@ -5,7 +5,7 @@ let generatorId = 0
 export type CancellablePromise<T> = Promise<T> & { cancel(): void }
 
 export function flow<R, Args extends any[]>(
-    generator: (...args: Args) => Generator<any, R, any>
+    generator: (...args: Args) => Generator<any, R, any> | AsyncGenerator<any, R, any>
 ): (...args: Args) => CancellablePromise<R> {
     if (arguments.length !== 1)
         fail(
@@ -18,7 +18,9 @@ export function flow<R, Args extends any[]>(
         const ctx = this
         const args = arguments
         const runId = ++generatorId
-        const gen = action(`${name} - runid: ${runId} - init`, generator).apply(ctx, args)
+        const gen = action(`${name} - runid: ${runId} - init`, generator as (
+            ...args: Args
+        ) => Generator<any, R, any>).apply(ctx, args)
         let rejector: (error: any) => void
         let pendingPromise: CancellablePromise<any> | undefined = undefined
 

test/base/typescript-tests.ts

@@ -1557,9 +1557,9 @@ test("it should support asyncAction as decorator (ts)", async () => {
     class X {
         @observable a = 1
 
-        f = mobx.flow(function* f(initial: number): any {
+        f = mobx.flow(function* f(this: X, initial: number) {
             this.a = initial // this runs in action
-            this.a += yield Promise.resolve(5)
+            this.a += yield Promise.resolve(5) as any
             this.a = this.a * 2
             return this.a
         })
@@ -1590,7 +1590,7 @@ test("flow support async generators", async () => {
             total += number
         }
         return total
-    } as any) // TODO: fix typings
+    })
 
     const p = start()
     const res = await p
@@ -1615,7 +1615,7 @@ test("flow support throwing async generators", async () => {
             total += number
         }
         return total
-    } as any) // TODO: fix typings
+    })
 
     const p = start()
     try {
@@ -1647,3 +1647,14 @@ test("verify #1528", () => {
 
     expect(appState.timer).toBe(0)
 })
+
+test("type of flows that return promises", async () => {
+    mobx.configure({ enforceActions: "observed" })
+
+    const f = mobx.flow(function* f() {
+        return Promise.resolve(5)
+    })
+
+    const n: number = await f()
+    expect(n).toBe(5)
+})