Effect bound for branch is not computed conservatively

[anlunx]

Assume that the function fun : Unit -> Unit has an effect {system.FFI}, the following code compiles

def foo() : {} Unit
  val x : Unit = if (1>0)
      fun()
    else
      unit
  unit

However, the function foo should have the effect annotation {system.FFI}

[JonathanAldrich]

The solution I'd like to see for this is as follows:

• make the underlying if() function effect-parametric
• implement inference of effect parameters by using the effect bound of the actual function parameters. Scala does something similar (there's a talk and I think a paper by Odersky and colleagues)