how scala infer method's parameterss

I notice by chance that scala can infer the type of some method's parameter. But I don't understand the exact rule. Can someone explain me why the test1 method work and why the test2 method does not work

object Test {
  def test1[A]: A => A = a => a
  def test2[A]: A = a
}

I can't find a good title for my question since I don't understand what is happening in this two lines. Do you have any idea?

---

def test1[A]: A => A         =    a => a
              |____|              |____|

         the return type       an anonymous function
     (a function from A to A)  (`a` is a parameter of this function)


def test2[A]: A =                 a
              |                   |
        the return type      an unbound value
             (A)         (i.e. not in scope, a is not declared)

的疑难杂症的是，在第一个例子中a是匿名函数的参数，而在第二个例子中a是从来没有声明。

---

test1 is a method that takes no input and returns a function A => A . The name a is given as the imput parameter of the funtion and the function simple reutrns a , it's input.

test2 is a method that takes no input returns a value of type A . The method is defined to return the variable named a but that variable has never been declared so you get an error. you could redefine the method to be def test2[A](a: A): A = a and it would work, because now a has been declared as a variable of type A , it is the parameter of the method.

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