freq-with-sig-abstract: compilation succeeds

[dijkstracula]

Dear Real World OCaml authors,

When following the signatures and abstract types section of Chapter 4, the compilation that should fail for lack of concrete type information hiding actually succeeds.

I can confirm my code matches the version from the book repo:

$ for FILE in "jbuild" "counter.mli" "counter.ml" "freq.ml"; do test $(curl -s https://raw.githubusercontent.com/realworldocaml/examples/v2-sexp/code/files-modules-and-programs/freq-with-sig-abstract/$FILE | md5) = $(md5 -q $FILE) && echo "$FILE matches repo" || echo "$FILE differs"; done
jbuild matches repo
counter.mli matches repo
counter.ml matches repo
freq.ml matches repo

But instead of the expected error message, I get

$ jbuilder clean
$ jbuilder build freq.bc
    ocamldep freq.depends.ocamldep-output
    ocamldep freq.dependsi.ocamldep-output
      ocamlc counter.{cmi,cmti}
      ocamlc counter.{cmo,cmt}
      ocamlc freq.bc
$ echo $?
0

Mysteriously, however, the expected error appears if I manually import Counter in the toplevel:

─( 14:18:23 )─< command 1 >───────────────────────────────────────────────────────────────────────────{ counter: 0 }─
utop # open Counter;;
─( 14:18:24 )─< command 2 >───────────────────────────────────────────────────────────────────────────{ counter: 0 }─
utop # let build_counts () =
  In_channel.fold_lines In_channel.stdin ~init:[] ~f:Counter.touch;;
Error: This expression has type t -> string -> t                                                                            
but an expression was expected of type 'a list -> Base.string -> 'a list
Type t is not compatible with type 'a list

So perhaps jbuilder is being overly smart? I'm running $ jbuilder --version 1.0+beta10.

My opam configuration:

# Global variables from the environment

ocaml-version        4.04.1               # The version of the currently used OCaml compiler
opam-version         1.2.1                # The currently running OPAM version
compiler             4.04.1               # The name of the current OCaml compiler (may be more specific than the version, eg: "4.01.0+fp", or "system")
preinstalled         false                # Whether the compiler was preinstalled on the system, or installed by OPAM
switch               4.04.1               # The local name (alias) of the current switch
jobs                 1                    # The number of parallel jobs set up in OPAM configuration
ocaml-native         true                 # Whether the OCaml native compilers are available
ocaml-native-tools   true                 # Whether the native ".opt" version of the OCaml toolchain is available
ocaml-native-dynlink true                 # Whether native dynlink is available on this installation
arch                 x86_64               # The current arch, as returned by "uname -m"

Thanks!
Nathan