#12315 introduced a bug in the typing of let rec, causing recursive functions with optional parameters that have defaults to no longer typecheck.

The issue is that the typing of patterns on optional parameters is slightly different depending on whether a default is present:

let f ?(x : int option) () = ()
let g ?(x : int = 5) () = ()

In both cases, the type is ?x:int -> unit -> unit, represented internally as something like this:

(Optional "x", int option) -> ((Nolabel, unit) -> unit)

The type in the function arrow is internally always int option, but the type of the pattern should be int option only when there is no default, and int if there is a default.

#12315 always typechecked like f, causing type errors on code with defaults. (Since the change was to type_approx, this only affects such functions when they are part of a recursive group).