foo() + 1

ESInfer complains about this because you are adding a string with a number, which sometimes will result in an unexpected value in runtime, and you hardly find it out. So ESInfer, on purpose, disallows this kind of type casting automatically.

As for the error message:

> expecting possible candidates all failed: candidate number failed: expecting number but got string. candidate string failed: expecting string but got number. but got number.

It's saying two possible situations: #1 number + number and #2 string + string are all failed, which is precisely the error. However, this may be a lack of readability if someone is the first time using ESInfer. I'll write some wikis on how to read error messages efficiently as a starting point and, finally and hopefully, evolve a better error reporting system.

So if you ask me how to deal with this kind of code with ESInfer, I'd suggest making the compiler happy by converting the string to a number or the number to a string.

(BTW, try to use the format as suggested, hope it works :D)