TEN MONTHS ago the spectre of recession was haunting corporate America. Inflation was rampant, earnings were depressed and the Federal Reserve was rapidly tightening the screws. Instead, inflation has moderated, the jobs market remains tight and recession is no longer a certainty. The prospect of an elusive “soft landing” has combined with hype over the productivity-boosting promise of artificial intelligence (AI) to give investors a fillip. This year the S&P 500 index of big American firms is up by nearly a fifth. Markets are especially bullish about a handful of tech firms and carmakers. These are among the S&P 500’s...