OptionParser#warn

(?::_ToS mesg) -> void

warn(mesg = $!)

# File tmp/rubies/ruby-3.4.1/lib/optparse.rb, line 1337
def warn(mesg = $!)
  super("#{program_name}: #{mesg}")
end

Shows warning message with the program name

See also #version #ver also in 4.1 · 4.0 · 3.3 · 3.2 · 3.1 · 3.0