Compiler panic when unifying inner skolem with an outer unification variable about compiler HOT 4 OPEN

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from compiler.

If you need a quick fix, try adding a type annotation to the module-level declaration. Issue seems similar to #1839

from compiler.

Re

The reason I even found this is that the error messages reported by both GHC and OCaml are quite poor.

What's the OCaml error message? This OCaml translation compiles and runs on at least OCaml 4.13.1 and 5.0.0:

let f x =
  let _g : (('a -> unit) -> unit) = fun h -> h x
  in x

https://try.ocamlpro.com/#code/let'f'x'=!let'_g':'(($,a'-$.'unit)'-$.'unit)'='fun'h'-$.'h'x!in'x

from compiler.

This is quite subtle, but the OCaml type you wrote is not actually equivalent to the one in Elm.

'a variables are not implicitly quantified type variables like in Elm or Haskell. They are actually just stand-ins for unification variables, which are closer to _a with PartialTypeSignatures in Haskell.
In other words, the following will compile and define a function f : Int -> int

let f : 'a -> 'a = fun x -> x + 1

To get the fully polymorphic behavior like in Elm or Haskell, one needs to use an explicit quantifier

let f x =
     let g : 'a. ('a -> unit) -> unit =
         fun h -> h x
     in x

This generates the "fantastic" error message

Error: This definition has type ('a -> unit) -> unit which is less general than
         'a0. ('a0 -> unit) -> unit

from compiler.