Flowjs：谓词函数用作对象属性时无法按预期工作

Question

请检查以下代码，我在做错什么吗？

/* @flow */

function isObject (y: any): boolean %checks {
  return typeof y === "object" && y !== null;
}

const obj = { isObject };

function foo(x: mixed) {
  if(isObject(x)) { // Works as expected
    return x.abc;
  }
}

function foo1(x: mixed) {
  if(obj.isObject(x)) { // Not working as expected
    return x.abc;
  }
}

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