WASHINGTON—President-elect Donald Trump hasn’t taken office yet, but he is already reshaping the global growth outlook.

The International Monetary Fund on Monday bumped up its economic growth forecasts for the U.S., saying output could grow nearly a half-percentage point faster than previously thought over this year and next, thanks to Mr. Trump’s plans to cut taxes and boost infrastructure spending. That would put U.S. economic expansion at 2.3% this year and 2.5% next year.

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