OCaml 4.06.0 build failure

[ilovezfs]

CC @gabelevi @mroch

Testing with flow 0.58.0 with Homebrew and OCaml 4.06.0 (See Homebrew/homebrew-core#20239)

The build fails while install gen with the following error:

# Error: This expression has type bytes but an expression was expected of type
#          string

See https://jenkins.brew.sh/job/Homebrew%20Testing/333/version=sierra/testReport/junit/brew-test-bot/sierra/install_flow/
(duplicate since that log will eventually expire https://gist.github.com/ilovezfs/dbf31ea424ed3392da3aff497aeda207)

I can workaround that by setting -unsafe-string via the `OCAMLPARAM environment variable:

export OCAMLPARAM="safe-string=0,_"

This is a typical issue with 4.06.0 since the default has switched from unsafe-string to safe-string. (See Homebrew/homebrew-core#20277)

Once that's set the OPAM dependencies all install fine. However, the build of flow itself then fails:

File "src/typing/flow_js.ml", line 10205, characters 2-334:
Error: This kind of expression is not allowed as right-hand side of `let rec'
Command exited with code 2.
make[1]: *** [build-flow] Error 10
make: *** [all-homebrew] Error 2

Full log here: https://gist.github.com/ilovezfs/05d070b5c347a585a378a20107733fc3

It would be great if we could get a patch for that build failure so that we don't have to resort to opam switch 4.05.0 via Homebrew/homebrew-core#20298.

[mroch]

I started looking into fixing the safe-string warnings, which look like they'll be pretty extensive, so those fixes will take a while, across multiple commits. the override looks like the way for homebrew to go for now.

will try to get some time to dig into the let rec thing too.

[ilovezfs]

Thanks @mroch!

[mroch]

@ilovezfs ff9c103 and 12dd84a should be backportable and avoid needing to set OCAMLPARAM (or just the latter, plus OCAMLPARAM)

[ilovezfs]

@mroch you're awesome. Backporting done: https://github.com/Homebrew/homebrew-core/pull/20427/files