Install Apple Clang 12 next to Apple Clang 13

Install Apple Clang 12 next to Apple Clang 13

# download 'Command_Line_Tools_for_Xcode_12.5.1.dmg' from Apple's developer portal:
# https://developer.apple.com/download/all/?q=xcode

# mount 'Command_Line_Tools_for_Xcode_12.5.1.dmg', then unpack it
❯ pkgutil --expand-full /Volumes/Command\ Line\ Developer\ Tools/Command\ Line\ Tools.pkg /tmp/CLTools

# rename existing CLTools v13
❯ sudo mv /Library/Developer/CommandLineTools /Library/Developer/CommandLineTools@13

# move CLTools v12 into place
❯ sudo mv /tmp/CLTools/CLTools_Executables.pkg/Payload/Library/Developer/CommandLineTools /Library/Developer/CommandLineTools

# symlink shared SDKs
❯ sudo ln -s /Library/Developer/CommandLineTools@13/SDKs /Library/Developer/CommandLineTools/SDKs

# clean up
❯ rm -rf /tmp/CLTools

❯ clang --version
Apple clang version 12.0.5 (clang-1205.0.22.11)

# if you want to restore Apple Clang 13 just do the reverse:
❯ sudo mv /Library/Developer/CommandLineTools /Library/Developer/CommandLineTools@12
❯ sudo mv /Library/Developer/CommandLineTools@13 /Library/Developer/CommandLineTools

❯ clang --version
Apple clang version 13.0.0 (clang-1300.0.29.30)