Hard mismatches in single point types

Question

I have several questions about singleton types, but since they are both very closely related, I send them under the same thread.

Q1. Why does # 1 not compile, but # 2 does?

def id(x: Any): x.type = x // #1 def id(x: AnyRef): x.type = x // #2 

Q2. The type is correctly inferred in the case of String , but not in the case of other reference types that I tried. Why is this so?

 scala> id("hello") res3: String = hello scala> id(BigInt(9)) res4: AnyRef = 9 scala> class Foo defined class Foo scala> id(new Foo) res5: AnyRef = Foo@7c5c5601 

Answer 1

Singleton types can only refer to AnyRef descendants. See Why String Literals Match Scala Singleton for more details.

An argument to which the id(BigInt(9)) application id(BigInt(9)) cannot be referenced through a stable path, therefore, therefore, it does not have an interesting singleton type.

 scala> id(new {}) res4: AnyRef = $anon$1@7440d1b0 scala> var o = new {}; id(o) o: Object = $anon$1@68207d99 res5: AnyRef = $anon$1@68207d99 scala> def o = new {}; id(o) o: Object res6: AnyRef = $anon$1@2d76343e scala> val o = new {}; id(o) // Third time the charm! o: Object = $anon$1@3806846c res7: o.type = $anon$1@3806846c 

Answer 2

I also get the error at # 2 (using Scala 2.9.1.final):

 error: illegal dependent method type def id(x: AnyRef): x.type = x; ^ one error found 

I believe the correct solution is to use make id polymorphic using a type parameter:

 def id[T <: Any](x: T): T = x;