I briefly tested this one empirically and it looks good so far; I haven't done a proper code-review yet though

Hmm this doesn't seem to build on Windows :(

Distribution\Client\InstallSymlink.hs:36:10: error:
    Not in scope: type constructor or class `Binary'
   |
36 | instance Binary OverwritePolicy
   |          ^^^^^^

AppVeyor is showing the error too.

Thanks @fgaz, @hvr , it compiles now. A couple of observations:

1. Doing a cabal new-install pony seems to build a binary and then end with

Installing executable can-i-have-a-pony in R:\CabalStore\ghc-8.4.3\incoming\new-20796\CabalStore\ghc-8.4.3\pony-1.0-2eda844295defe2b7defaac8f0f9a0f26ee16330\bin
Warning: The directory
R:\CabalStore\ghc-8.4.3\incoming\new-20796\CabalStore\ghc-8.4.3\pony-1.0-2eda844295defe2b7defaac8f0f9a0f26ee16330\bin
is not in the system search path.
cabal.exe: installdir is not defined. Set it in your cabal config file or use
--installdir=<path>

Which led me to believe the binary was in R:\CabalStore\ghc-8.4.3\incoming\new-20796\CabalStore\ghc-8.4.3\pony-1.0-2eda844295defe2b7defaac8f0f9a0f26ee16330\bin, but it turns out the folder no longer exists after install. Sooo... where is it? :)

2. I then tried using --installdir as suggested

$ ./cabal.exe new-install --installdir=/r/bin pony
Resolving dependencies...
Up to date
Symlinking 'can-i-have-a-pony'
cabal.exe: Symlinking feature not available on Windows

So we'll need to pick a working default, maybe recommend people try a different --bindir-method=.

3. I then tried using --bindir-method=copy which failed with

$ ./cabal.exe new-install --installdir=/r/bin --bindir-method=copy pony
Resolving dependencies...
Up to date
Copying 'can-i-have-a-pony'
R:\CabalStore\ghc-8.4.3\pony-1.0-2eda844295defe2b7defaac8f0f9a0f26ee16330\bin\can-i-have-a-pony: copyFile:atomicCopyFileContents:withReplacementFile:copyFileToHandle:openBinaryFile: does not exist (No such file or directory)

Which failed because can-i-have-a-pony doesn't have an extension, it should be trying to copy can-i-have-a-pony.exe.

@Mistuke could you try now?

edit: nevermind, I can use appveyor

Tested on windows, ready for review. Docs update coming shortly.

editing this comment so I don't spam everyone: travis is red because I forgot [ci skip] on the last 2 commits and stopped it manually

Rebased against master.

@23Skidoo I assume we can consider the 2.4-backport #5869 accepted as well?

Yeah I guess, assuming it's the same patches but just backported -- I haven't looked at it.

The error is a timeout. Merging.