Overrides do not work when changing the effect type

[TimWhiting]

Discussed in #600

^{Originally posted by chtenb October 24, 2024}

pub effect yield<a>
  ctl yield(elem : a) : ()
  
pub fun map(it : () -> <yield<a>|e> (), f : a -> b) : <yield<b>|e> ()
  with override ctl yield(elem)
    yield(f(elem))
    resume(())
  it()

This does not work

repro.kk(6, 3): type error: abstract types do not match
  context        :   with override ctl yield(elem)
                       yield(f(elem))
                       resume(())
  term           :   with override ctl yield(elem)
                       yield(f(elem))
                       resume(())
  inferred effect: <yield<$a>,yield<$a>|_e1>
  expected effect: <yield<$a>,yield<$b>|_e>
  hint           : an higher-rank type escapes its scope?

If we replace the type b with a, it does work.
I'm unsure why the inferred effect is <yield<$a>,yield<$a>|_e1>, because clearly the result of f is b.
Why does this fail, and how can we make it work?

[TimWhiting]

Using the desugaring directly doesn't work either.