Pfthb

object solution extends App 
 'n = 5

It gives the compile time Error: value update is not a member of object Symbol
println('n = 'n) which is understandable. Because literals are the fixed values in the source code. But what is the reason the above syntax is valid?

The reason the syntax is valid is … well … because it is:

implicit class UpdateableSymbol(val s: Symbol.type) extends AnyVal 
 def update[A](s: String, v: A) = println(s"`Symbol.update` called with s = $s and v = $v")


'n = 5
// `Symbol.update` called with s = n and v = 5

As you can see, there is absolutely nothing wrong with the syntax, so why should it be invalid? The error message tells you what the problem is: you are calling Symbol.update but that doesn't exist. A missing method is not a syntactic error, it is a semantic error.

Welcome to Scala 2.12.7 (Java HotSpot(TM) 64-Bit Server VM, Java 1.8.0_92).
Type in expressions for evaluation. Or try :help.

scala> import reflect.runtime.universe._
import reflect.runtime.universe._

scala> reify('n)
res0: reflect.runtime.universe.Expr[Symbol] = Expr[Symbol](Symbol.apply("n"))

scala> val a = 'n
a: Symbol = 'n

scala> a = 5
<console>:15: error: reassignment to val
 a = 5
 ^

scala> a.update(5)
<console>:16: error: value update is not a member of Symbol
 a.update(5)
 ^

Desugar it, and you will find the answer.

In Scala, operators are methods.

For Symbol, see https://github.com/scala/scala/blob/2.13.x/src/library/scala/Symbol.scala