Singleton type has wrong bounds.

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A singleton type is always inhabited by an object reference, and null. Hence the type Singleton should have bounds >: Null <: AnyRef. However, this is not currently the case:

scala> trait T { type X >: Null <: AnyRef }
defined trait T

scala> new T { type X = Singleton }
<console>:6: error: error overriding type X in trait T with bounds >: Null <: AnyRef;
 type X has incompatible type Singleton
       new T { type X = Singleton }

In particular it seems obviously erroneous that Null <: Singleton doesn't hold, since Null <: a.type and a.type <: Singleton for any reference a. So the transitivity of type conformance seems to be broken.

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Imported From: https://issues.scala-lang.org/browse/SI-1273?orig=1
Reporter: Lauri Alanko (lealanko)

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@odersky said:
The status of singleton types and null is most likely to be reviewed soon. Postponed until then.

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@odersky said:
Milestone postponed deleted

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@paulp said:
Can I rescue this from postponement? I think the ticket as reported unintentionally misrepresents the issue, possibly making it appear deeper than it is. It's actually broken worse than is shown, but at the same time, is easier to fix. The problem is that SingletonClass is assigned Any as a parent, not AnyRef. So forget the lower bound, even <: AnyRef is enough to exclude it.

The good news though is that that's the same reason the lower bound is wrong, and assigning AnyRef as the parent fixes that too. Making this change doesn't seem to bother the compiler in the least. All builds, all passes.

Specwise, from my reading of it all singleton types must descend from AnyRef, but the same is not explicitly said of scala.Singleton. Someone reading it would be rather likely to think Singleton <: AnyRef. Perhaps it should be worded more clearly regardless of outcome.

A singleton type is of the form p.type, where p is a path pointing to a value expected to conform (�6.1) to scala.AnyRef. The type denotes the set of values consisting of null and the value denoted by p.
A stable type is either a singleton type or a type which is declared to be a subtype of trait scala.Singleton.

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@milessabin said:
As of the work on SIP-23 we want Singleton <: Any so I think the current behaviour becomes correct once that branch is merged.

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@SethTisue said:
scala/scala#5310