type Foo is a volatile type; cannot override a type with non-volatile upper bound

[Atry]

Welcome to Scala 2.12.4 (Java HotSpot(TM) 64-Bit Server VM, Java 1.8.0_152).
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scala> trait Base {
     |     type Foo
     |     type Bar
     | }
defined trait Base

scala> trait OK extends Base {
     |     trait InnerImpl
     |     type Foo <: Bar with InnerImpl
     | }
defined trait OK

scala> trait Error extends Base {
     |     trait InnerImpl
     |     type Foo <: InnerImpl with Bar 
     | }
<console>:14: error: overriding type Foo in trait Base with bounds;
 type Foo is a volatile type; cannot override a type with non-volatile upper bound
           type Foo <: InnerImpl with Bar
                ^

scala> trait Error2 extends Base {
     |     trait InnerImpl { def abstractMethod: Unit }
     |     type Foo <: Bar with InnerImpl
     | }
<console>:14: error: overriding type Foo in trait Base with bounds;
 type Foo is a volatile type; cannot override a type with non-volatile upper bound
           type Foo <: Bar with InnerImpl
                ^

The only difference between trait OK and trait Error is the order of super types of Foo. I expect all the three examples compile.

[Atry]

By the way, the same code compiles on Dotty:

$ dotr
Starting dotty REPL...
scala> trait Base {
         type Foo
         type Bar
       } 
// defined trait Base
scala> trait OK extends Base {
         trait InnerImpl
         type Foo <: Bar with InnerImpl
       } 
// defined trait OK
scala> trait Error extends Base {
         trait InnerImpl
         type Foo <: InnerImpl with Bar 
       } 
// defined trait Error

[joroKr21]

This is according to the spec of volatile types. I suspect the unsoundness potential comes from type projection on abstract types as described in this SO answer which is forbidden in Dotty. However I do not understand why this check applies to abstract methods as well.

[Atry]

Volatile type seems like an ill-defined concept to me. Scala 2 is unsound due to type selector #, not ill-defined concept volatile.

Since type projection has been disabled in scala/scala3#1050 , I hope the volatile check can be removed in the future.