Optional parameters and option types using F#

Question

Consider the following code:

type Test () =
  member o.fn1 (?bo) = 1
  member o.fn2 (?bo) = o.fn1 bo

  member o.fn3 (?bo) = 1 + bo.Value
  member o.fn4 (?bo) = o.fn3 bo

While fn1 and fn2 work just fine, fn4 produces the following error:

init.fsx(6,30): error FS0001: This expression was expected to have type int but here has type 'a option

MSDN states:

Optional parameters are interpreted as the F# option type, so you can query them in the regular way that option types are queried, by using a match expression with Some and None.

To me, optional parameters are not interpreted as the F# option type otherwise the code would compile. Moreover I do not understand why, when I hover over ?bo in fn3 the tooltip says val bo: int option but from outside expects only int. I would expect a behavior of accepting nothing, int, Some int and None. And as the last note, I do not understand why fn2 works but fn4 does not.

Thanks for clarification

Answer 1

I have to reconsider the correct answer. Based on this question (and answer):

Propagating optional arguments

it seams that the correct answer is the following:

type Test () =
  member o.fn1 (?bo) = 1
  member o.fn2 (?bo) = o.fn1 bo

  member o.fn3 (?bo) = 1 + bo.Value
  member o.fn4 (?bo) = o.fn3 (?bo = bo)

It is a neat feature and credits for the answer go to desco!