“For investors with $12 trillion of negative-rate bonds worldwide, U.S. junk securities and their 6.9 percent average yield look like a gold mine. But with so many investors streaming into the market, the debt is now yielding almost 3 percentage points less than the average of the past two decades, Bank of America Merrill Lynch index data show. And they’re buying it up at the same time that junk-rated borrowers default at the fastest pace in six years.”