Unexpected corecursive call

Question

This (trimmed out) corecursive function definition in Isabelle

primcorec tree :: "'form fset ⇒ 'vertex ⇒ 'preform ⇒ (('form fset × 'form), ('rule × 'preform) NatRule) dtree" where
 "tree Γ v p =
    (case undefined of Hyp h c ⇒ undefined | Reg c ⇒
    Node undefined (fimage (tree Γ v) undefined)
    )"

yields

Unexpected corecursive call in "case undefined of Reg c ⇒ Node undefined (tree Γ v |`| undefined)" at
  "case case undefined of Reg c ⇒ Node undefined (tree Γ v |`| undefined) of Node uu uua ⇒ uua"

but if I simplify it further to

primcorec tree :: "'form fset ⇒ 'vertex ⇒ 'preform ⇒ (('form fset × 'form), ('rule × 'preform) NatRule) dtree" where
   "tree Γ v p =
      (Node undefined (fimage (tree Γ v) undefined))"

it works.

I also tried to use the deconstructor view, i.e.

primcorec tree :: "'form fset ⇒ 'vertex ⇒ 'preform ⇒ (('form fset × 'form), ('rule × 'preform) NatRule) dtree" where
 "cont (tree Γ v p) = (case undefined of Hyp h c ⇒ undefined | Reg c ⇒ (fimage (tree Γ v) undefined))"

And now I get a different error message: Invalid map function at "case undefined of Reg c ⇒ tree Γ v |`| undefined".

What might be the cause?

With other case expressions it works, and I don’t find any mention of a restriction in the documentation (§ 5.1.1 in the datatype documentation.)

Answer 1

Via private communication I was told that the solution is to add the (discs_sel) option to the datatype wher the Hyp constructor comes from.