Can't detect binary incompatibility when trait extends abstract class

• scala version 2.12.6
• mima version 0.3.0
• complete example repository https://github.com/xuwei-k/mima-trait-extends-class

library x v1

package com.example

trait A

library x v2

package com.example

abstract class B {
  def foo: Int = 42
}

trait A extends B

mima said library x v1 => v2 binary compatible.

another library y

• depends on library x v1

package com.example

object C {
  val a = new com.example.A {}
}

main

• depends on library y
• also depends on library x v2 (override x v1 transitive dependency from y)

package com.example

object Main {
  def main(args: Array[String]): Unit = println(C.a.foo)
}

run main

[error] (run-main-0) java.lang.ClassCastException: com.example.C$$anon$1 cannot be cast to com.example.B
[error] java.lang.ClassCastException: com.example.C$$anon$1 cannot be cast to com.example.B
[error] 	at com.example.Main$.main(Main.scala:6)
[error] 	at com.example.Main.main(Main.scala)
[error] 	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
[error] 	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)

[adriaanm]

That’s why a @PureInterface annotation would be useful: the compiler could check that your trait doesn’t have any of this baggage.

[dwijnand]

Thanks, it’s slightly different, but fundamentally the same problem.

Note to self’s brain-MiMa:

• beware what your open traits extend (prefer extending Object)
• beware what your open traits self subtype

In addition to the other binary compatibility hazardous problems, like super calls, fields, and etc…

Maybe I’ll just avoid traits altogether…