Scala“；“简单表达式的非法开始”；定义函数时

我在Scala上遇到以下错误：

scala> :pas
// Entering paste mode (ctrl-D to finish)

// sum takes a function that takes an integer and returns an integer then
// returns a function that takes two integers and returns an integer
def sum(f: Int => Int): (Int, Int) => Int =
  def sumf(a: Int, b: Int): Int = f(a) + f(b)
  sumf

// Exiting paste mode, now interpreting.

:4: error: illegal start of simple expression
  def sumf(a: Int, b: Int): Int = f(a) + f(b)
  ^

我不明白为什么def是一个illegal start of simple expression.我只是想声明一个函数。我是否违反了声明中的语法要求？非常感谢。

更新：这是我的Scala版本：

sbt:jaime> console
[info] Starting scala interpreter...
Welcome to Scala 2.12.10 (OpenJDK 64-Bit Server VM, Java 11.0.14).
Type in expressions for evaluation. Or try :help.

scala>