E05250: abstract type cannot be in 'allow only' list — only concrete classes are permitted

What went wrong

An abstract class has been listed in a trait's 'allow only' clause. The 'allow only' clause defines the exhaustive set of concrete runtime types that may implement the sealed trait. Abstract classes cannot be instantiated, so listing them in 'allow only' is meaningless — they can never be concrete runtime instances of the trait.

Abstract classes ARE allowed to implement sealed traits (they are exempt from the E05240 enforcement check). But they should not appear in the 'allow only' list itself, because the list should enumerate only the concrete types that the developer intends to exist at runtime.

Worked examples

Compilable examples demonstrating this error and the correct fix:

• ek9 -q 850 (Can an abstract class appear in a trait's allow only list?)

How to fix it

Remove the abstract class from the 'allow only' list. The list should contain only concrete classes that are intended as the complete set of runtime implementations. Abstract classes can still implement the trait freely — they just should not appear in the permitted list.

Common causes

• Listing a base class that was later made abstract during refactoring
• Confusing 'allow only' (concrete runtime types) with 'extends' hierarchy
• Including an abstract helper class that intermediates between the trait and concrete implementations

See also

• E05240: type is not permitted to implement this sealed trait
• E05100: incompatible genus (allow only requires classes)
• E05110: incompatible category (allow only rejects templates)

Examples

Every example is compiled by the EK9 compiler: the first fails exactly as captioned, the others fix it.

Fails with E05250: abstract type cannot be in 'allow only' list — only concrete classes are permitted

#!ek9
defines module error.e05250.bad

  defines trait

    Workable allow only AbstractHelper, ConcreteWorker
      process() as abstract

  defines class

    AbstractHelper as abstract
      doWork() as abstract

    ConcreteWorker
      doWork()
        content <- "Working"
        require content?

Fixed: List only concrete classes after 'allow only'

#!ek9
defines module error.e05250.good

  defines trait

    Workable allow only ConcreteWorker, OtherWorker
      process() as abstract

  defines class

    ConcreteWorker with trait of Workable
      override process()
        content <- "Working"
        require content?

    OtherWorker with trait of Workable
      override process()
        content <- "Also working"
        require content?

Questions and answers

These Q&A entries show E05250 and its fix:

• Q850: Can an abstract class appear in a trait's allow only list?