Fix possible ambiguous use of when(fulfilled:) (#1171)

Author: RomanPodymov

Tests/CorePromise/WhenTests.swift

@@ -218,7 +218,8 @@ class WhenTests: XCTestCase {
         let p2 = after(.milliseconds(100)).done { throw Error.straggler }
         let p3 = after(.milliseconds(200)).done { throw Error.straggler }
 
-        when(fulfilled: p1, p2, p3).catch { error -> Void in
+        let whenFulfilledP1P2P3: Promise<(Void, Void, Void)> = when(fulfilled: p1, p2, p3)
+        whenFulfilledP1P2P3.catch { error -> Void in
             XCTAssertTrue(Error.test == error as? Error)
             ex1.fulfill()
         }
@@ -241,7 +242,8 @@ class WhenTests: XCTestCase {
         let p2 = Promise<Void>(error: Error.test2)
         let p3 = Promise<Void>(error: Error.test3)
 
-        when(fulfilled: p1, p2, p3).catch { error in
+        let whenFulfilledP1P2P3: Promise<Void> = when(fulfilled: p1, p2, p3)
+        whenFulfilledP1P2P3.catch { error in
             XCTAssertTrue(error as? Error == Error.test1)
             ex.fulfill()
         }