Problems with polymorphic method, Nothing, Invariant return type

[scabug]

Happening in Scala 2.7.1 and 2.7.2 at least:

scala> trait Test[A]
defined trait Test

scala> trait Test2[+A]
defined trait Test2

scala> def f[A](a:Test[A]):Test[A] = null
f: [A](Test[A])Test[A]

scala> def f2[A](a:Test[A]):Test2[A] = null
f2: [A](Test[A])Test2[A]

scala> val x:Test[Nothing] = null
x: Test[Nothing] = null

scala> f(x)
<console>:8: error: type mismatch;
 found   : Test[Nothing]
 required: Test[A]
      f(x)
        ^

scala> f[Nothing](x)
res2: Test[Nothing] = null

scala> f2(x)
res1: Test2[Nothing] = null

This happens only for A=Nothing. And only if the return type is invariant in a type variable being Nothing. There is probably not much sense in operating on Nothing types, but what is the rationale behind this behaviour?

[scabug]

Imported From: https://issues.scala-lang.org/browse/SI-1570?orig=1
Reporter: Johannes Rudolph (jrudolph100)

[scabug]

Mohsen Lesani [X] (lesani) said:

This is still a problem in Trunk.

[scabug]

@soc said:
Slightly different error for f(x) in 2.9.0.r24145-b20110130020134

scala> f(x)
<console>:9: error: type mismatch;
 found   : Test[Nothing]
 required: Test[A]
Note: Nothing <: A, but trait Test is invariant in type A.
You may wish to define A as +A instead. (SLS 4.5)
       f(x)
         ^

[scabug]

@adriaanm said:
see also #4509

[scabug]

Daniel Darabos (darabos) said:
What an old and annoying issue. I wouldn't know where to start fixing it. But if someone can give a rough outline of how this could be fixed, I can offer to work on the implementation. Thanks.

[dima-starosud]

Guys, should we try to adopt that as a quick-and-dirty fix for this?