Kotlin 如何用表达式体推断 function 的返回类型

Question

If function consists of only one expression, than its return type may be infered from that expression. And it sounds pretty simple:

fun max(a: Int, b: Int) = if (a > b) a else b

In this case the return type of function will be Int.

But what type will be in the next example?

fun max(a: Int, b: Int) = if (a > b) a else true

Answer 1

The inference just gives the closest ancestor of the types of the possible values of the expression.

Here your expression can yield a Boolean or an Int . The closest ancestor in this case is Comparable<*> , so that's the return type of your function.

If the types were completely unrelated, the common ancestor would be Any , which is in Kotlin the common ancestor of all non-nullable types. If one of the values can also be null, you may end up with a return type that is Any? , which is the ultimate parent of all types in Kotlin (similar to Java's Object ).