# Display a first-level warning.
	#
	# First-level warnings are warnings that SHOULD be corrected,
	# and COULD usually be immediately corrected.
	#
	# * There is a simple correction
	# * There is no reason to let the code this way (no reasonable @supresswarning-like annotation)
	# * They always are real issues (no false positive)
	#
	# First-level warnings are displayed by default (except if option `-q` is given).
	#
	# Return the message (to add information) or null if the warning is disabled
	fun warning(l: nullable Location, tag: String, text: String): nullable Message
	do
		if is_warning_blacklisted(l, tag) then return null
		var m = new Message(l, tag, text, 1)
		if messages.has(m) then return null
		if l != null then l.add_message m
		if opt_warning.value.has("no-{tag}") then return null
		if not opt_warning.value.has(tag) and opt_warn.value <= 0 then return null
		messages.add m
		warning_count = warning_count + 1
		if opt_stop_on_first_error.value then check_errors
		return m
	end