Is there someone above your boss that you can talk to about this? Because this could get you slapped with sexual harassment complaints.

Get this in writing to cover your a** when you go to HR and she throws you under the bus

You can go with "Hello. My boss (Boss's name) would like me to inform you that due to the new dress code, the turtleneck and bell bottoms combo is now mandatory for every employee."

Go to HR FIRST!!! Let them be the bad guy. also, let them know what you were expected to do and you don't need HR reports against you.

In most companies there’s a policy that anything having to do with female specific dress code, a senior female employee is to take the lead on informing other female staff members (or member). Everyone in the comments are right, go to HR, this is way too tricky of a tightrope to walk. To finish the metaphor, get a safety net in place first!

The good news is you don't have to "discuss" anything. When you see visible cleavage, just playfully motorboat them and they'll cover up right quick. Rarely is it so fun to be a corporate policy enforcer. It will certainly work with no possible downside.

Just tell them if you can motorboat, they need to cover up.

They should have a woman do that. In some places, workers can't tell the opposite gender how to dress because it's like saying they were staring.