Repro crystal-lang/crystal#3946

shard.yml

name: bug-3946
version: 0.1.0

crystal: 0.20.5

dependencies:
  callback:
    github: mosop/callback
    version: 0.6.2

  cli:
    github: mosop/cli
    version: 0.6.4

  optarg:
    github: mosop/optarg
    version: 0.5.4

  string_inflection:
    github: mosop/string_inflection
    version: 0.2.0

test.cr

require "cli"
require "json"

class Command
    JSON.mapping({
      content: String
    })
end



class Main < Cli::Supercommand
    class Server < Cli::Command
      def run
        Command.from_json("")
      end
    end
end

Main.run ARGV