type Input<Output> = {
  [K in keyof Output]: { foo: () => Output[K] };
};

function convert<Output>(input: Input<Output>): Output {
  const keys = Object.keys(input) as (keyof Input<Output>)[];

  return keys.reduce(
    (output, key) => ({ ...output, [key]: input[key].foo() }),
    {} as Output
  );
}

const output = convert<{
  a: number;
  b: string;
  c: boolean;
}>({
  a: { foo: () => 1 },
  b: { foo: () => '!' },
  c: { foo: () => true },
});

console.log(output); // = { a: 1, b: '!', c: true }