0.19 Overrestrictive Type Inference?

[rue]

The following code (from An Outsider's Guide to Statically Typed Functional Programming, @marick) produces a type mismatch error in 0.19:

module SumTypes.WholeStorySolution exposing (..)

type Silly a b
  = AsA a
  | AsB b
  | AlsoAsA a

  | AsInt Int

  | AsFloatString Float String
  | AsBoth a b
  | AsIntB Int b

  | AsMaybe (Maybe b)
  | AsMaybeInt (Maybe Int)
  | DeeplySilly (Silly a a) (Silly Int b)
  | NoArg

bVals : Silly a b -> Maybe b
bVals silly =
  case silly of 
    AsB b -> Just b
    AsBoth _ b -> Just b
    AsIntB _ b -> Just b
    AsMaybe (Just b) -> Just b  -- could just be AsMaybe maybe -> maybe
    DeeplySilly _ nested -> bVals nested            {- <--- Problem here -}
_ -> Nothing

Error:

Detected errors in 1 module.                                         
-- TYPE MISMATCH --------------------------------------------- src/SumTypes/WholeStorySolution.elm

The 1st argument to `bVals` is not what I expect:

171|             bVals nested
                   ^^^^^^^^^^
This `nested` value is a:

    Silly Int b

But `bVals` needs the 1st argument to be:

    Silly a b
[…]

Additional possibly interesting info:

> bVals
<function> : Silly Int a -> Maybe a      -- The inferred type (without annotation)

> AsFloatString 4.5 "hi" |> bVals
Nothing : Maybe b                                -- …Why is this not an error, it doesn't conform to the inferred type?

It seems like Silly a b should allow Silly Int b. Is this intended behavior?

[marick]

The book was written against 0.18.

I translated the code into PureScript. It works there as it did in 0.18.