I'm unable to reproduce this - cURL docs say exit code 43 is an internal error. Is it repeatable? If so, what happens if you run the command directly? (from the shell you have there, that'd be /cygdrive/c/Windows/system32/curl.exe --write-out '%{http_code}\n' --retry 3 --retry-delay 2 --user-agent opam/2.2.0 -L -o index.tar.gz.part -- https://opam.ocaml.org/index.tar.gz)

If it is a persistent problem, a workaround is almost certainly to install Cygwin's curl package, but it ought to be working with the Windows copy.

The first tip didn't work
The workaround worked: replacing the C:\Windows\System32\curl.exe with the Cygwin version
Thanks!

Glad that's worked around it (although I hope that was by shadowing C:\Windows\System32\curl.exe, not actually replacing files in the Windows system32 directory!).

I'm re-opening this as although I've not seen it do it at this point, I have seen Windows curl do this when downloading a series of packages and there are various reports in Discord seeing the same thing. In my particular instance some downloads succeeded while most of them ended up with this cURL error. As it happens, I was also using Windows 11 22H2 (rather than Windows 10, as I use both).

I'm tempted to:

• Add curl to the required programs for opam init for Windows (internal installation would naturally then get it and opam will recommend it for external Cygwin/MSYS2 installs via depext). That would be for 2.2.1+ potentially
• Distribute opam with an internal download agent (something a la ocurl - the benefit of libcurl is that it can be compiled against Windows SSL). That's quite a chunk of maintenance work, though.

I've opened #6142 as an implementation of solution 1

For anyone reading this, if you encounter the same error during opam init, the fix is to instead call:

opam init --cygwin-internal-install --cygwin-extra-packages=curl

Looks like curl/curl#13845 which has been fixed in curl 8.9.0.

I see this has already been mentioned in the implementation of solution 1.