Make the PromiseValue type handle nested promises (#112)

Co-authored-by: Sindre Sorhus <sindresorhus@gmail.com>

source/promise-value.d.ts

@@ -1,5 +1,6 @@
 /**
 Returns the type that is wrapped inside a `Promise` type.
+If the type is a nested Promise, it is unwrapped recursively until a non-Promise type is obtained.
 If the type is not a `Promise`, the type itself is returned.
 
 @example
@@ -15,6 +16,12 @@ let data: Data = await asyncData;
 // Here's an example that shows how this type reacts to non-Promise types.
 type SyncData = PromiseValue<string>;
 let syncData: SyncData = getSyncData();
+
+// Here's an example that shows how this type reacts to recursive Promise types.
+type RecursiveAsyncData = Promise<Promise<string> >;
+let recursiveAsyncData: PromiseValue<RecursiveAsyncData> = Promise.resolve(Promise.resolve('ABC'));
 ```
 */
-export type PromiseValue<PromiseType, Otherwise = PromiseType> = PromiseType extends Promise<infer Value> ? Value : Otherwise;
+export type PromiseValue<PromiseType, Otherwise = PromiseType> = PromiseType extends Promise<infer Value>
+	? { 0: PromiseValue<Value>; 1: Value }[PromiseType extends Promise<unknown> ? 0 : 1]
+	: Otherwise;

test-d/promise-value.ts

@@ -2,11 +2,15 @@ import {expectAssignable} from 'tsd';
 import {PromiseValue} from '..';
 
 type NumberPromise = Promise<number>;
+type NestedPromise = Promise<NumberPromise>;
 type Otherwise = object;
 
 // Test the normal behaviour.
 expectAssignable<PromiseValue<NumberPromise>>(2);
 
+// Test the nested behaviour.
+expectAssignable<PromiseValue<NestedPromise>>(2);
+
 // Test what happens when the `PromiseValue` type is not handed a `Promise` type.
 expectAssignable<PromiseValue<number>>(2);