unison @2.51.2_3+aqua installed on macOS Mojave (10.14) built for macOS Catalina (10.15) and won't run

[ShadSterling (Shad Sterling)]

Invoking the executable inside the app bundle yields an OS version error:

$ /Applications/MacPorts/Unison.app/Contents/MacOS/Unison
application requires at least OS X version 10.15.0 (10.15), but is being run on 10.14.6 ( 10.14.6/18G6020), and so is exiting.Abort trap: 6

(Invoking the executable directly for reasons in #61060)

How can I tell MacPorts to build Unison for the OS it's running on?

[ryandesign (Ryan Carsten Schmidt)]

Replying to ShadSterling:

How can I tell MacPorts to build Unison for the OS it's running on?

It should already be doing that, and on my 10.13 system, it appears to do so.

Could you rebuild the port, keeping the log (sudo port -kns upgrade --force unison) and then attach the main.log file (port logfile unison) to this ticket so that we can see how it built on your system?

[ShadSterling (Shad Sterling)]

main.log after sudo port -kns upgrade --force unison

[ryandesign (Ryan Carsten Schmidt)]

Ok, I do see

export MACOSX_DEPLOYMENT_TARGET=10.15

in that log, which explains the problem, but I don't yet know where that comes from (it's not in the source code). The source code (in the Xcode project) sets MACOSX_DEPLOYMENT_TARGET=10.6, and we add a patch which removes that. We do similar things in other ports with the intention that it would use the deployment target that MacPorts sets globally. Maybe that no longer works with the "new build system" introduced in Xcode 10.