./decompile.sh
Fix osx with:
ModCoderPack/MCPBot-Issues#501 (comment)
in MinecraftDiscovery.py
replace:

        if 'natives' in library:
            libFilename = "%s-%s-%s.jar"%(libSubdir, libVersion, substitueString(library['natives'][osKeyword]))

with:

        if 'natives' in library:
            if osKeyword not in library['natives']:
                continue
            libFilename = "%s-%s-%s.jar"%(libSubdir, libVersion, substitueString(library['natives'][osKeyword]))

Import eclipse/Client in intellij

Cmd+; (File -> project structure)
Modules
Fix module sdk
Fix paths

Create new application:

Main class: Start
VM options: -Djava.library.path=versions/1.12/1.12-natives
Working directory: ...mcp940/jars

Osx:
java.lang.NoClassDefFoundError: ca/weblite/objc/NSObject
Add: libraries/ca/weblite/java-objc-bridge/1.0.0/java-objc-bridge-1.0.0.jar to your modules

./recompile.sh
./reobfuscate.sh
cp ./jars/versions/1.12/1.12.jar ./reobf/minecraft/
cd reobf/minecraft/
jar uf 1.12.jar *
zip -d 1.12.jar "META-INF/*"