(Bloomberg) -- Expensive US equities are flashing a major warning sign that could see the S&P 500 sliding as much as 26% in the first half of this year, according to Morgan Stanley strategists.Most Read from BloombergYou Want at Least $3 Million in Savings to Retire ComfortablyThe Biden White House Operated Under Cloak-and-Dagger Secrecy to Plan His Ukraine TripUS-China Meeting Only Worsens Tensions Over Balloon, RussiaStocks Muted, US Futures Fall as China Bets Fade: Markets WrapRecent economic