Regression in inference for keys of mapped type with empty object

[DanielRosenwasser]

interface Style {
  flashy: any;
}

declare function mergeStyleSets<K extends string>(
  ...cssSets: {[P in K]?: Style }[]): {[P in K]: Style };

// Expected:
//   let x: {
//       a: Style;
//       b: Style;
//   }
let x = mergeStyleSets(
  {},
  {
    a: { flashy: true },
  },
  {
    b: { flashy: true },
  },
)

This code currently causes an error, but previously would lead x to have the type { a: Style, b: Style }. Removing the {}` argument seems to restore the original behavior.