[TypeChecker] NFC: Add a test-case for SR-12277

Author: xedin

test/Constraints/sr12277.swift

@@ -0,0 +1,45 @@
+// RUN: %target-typecheck-verify-swift
+
+enum Time {
+  case milliseconds(Int)
+}
+
+struct Scheduled<T> {
+  let futureResult: EventLoopFuture<T>
+}
+
+struct EventLoopFuture<T> {
+  func flatMap<V>(_ body: @escaping (T) -> EventLoopFuture<V>) -> EventLoopFuture<V> {
+    fatalError()
+  }
+}
+
+struct EventLoop {
+  func makeSucceededFuture<T>(_ v: T) -> EventLoopFuture<T> {
+    fatalError()
+  }
+
+  func scheduleTask<T>(in t: Time, _ body: @escaping () -> T) -> Scheduled<T> {
+    fatalError()
+  }
+}
+
+struct Thing {}
+
+func bar(eventLoop: EventLoop, process: @escaping (Thing) -> EventLoopFuture<Void>) -> EventLoopFuture<Void> {
+  func doIt(thingsLeft: ArraySlice<Thing>) -> EventLoopFuture<Void> {
+    var thingsLeft = thingsLeft
+    guard let first = thingsLeft.popFirst() else {
+      return eventLoop.makeSucceededFuture(())
+    }
+
+    return process(first).flatMap { [thingsLeft] in
+      return eventLoop.scheduleTask(in: .milliseconds(100)) {
+        return doIt(thingsLeft: thingsLeft)
+        // expected-error@-1 {{cannot convert value of type 'EventLoopFuture<Void>' to closure result type 'Void'}}
+      }.futureResult
+    }
+  }
+
+  return doIt(thingsLeft: [][...])
+}