Type checking bug when masking polymorphic effects

[anfelor]

Consider the following program using effect polymorphism (due to @thwfhk):

effect reader<s>
  fun ask() : s

fun test( p : () -> <reader<int>, reader<int>|e> a ) : e a
  with handler
    fun ask() 100
  with handler
    fun ask() 42
  p()

pub fun main(): console ()
  with test
  println(mask<reader<int>>(ask) : int)

As expected, it completes with output 100 and hovering over the call to ask we get the correct instantiation fun ask : () -> <reader<int>,console/console> int.

However, Koka surprisingly also accepts:

pub fun main(): console ()
  with test
  println(mask<reader<int>>(ask()) : int) // notice the call to ask

Hovering over ask again, we get the wrong instantiation fun ask : () -> <reader<() -> <console/console,reader<int>> int>|_e> (() -> <console/console,reader<int>> int). This program is accepted by the type checker but segfaults at runtime.