Confusing error message with iso parameters

[andrenth]

Hi

As discussed with @SeanTAllen on IRC, we have found an instance of a confusing error message related to iso parameters:

use "random"
use "time"

class C
  let _v: U8
  
  new val create(rand: (Random iso | None) = None) =>
    let rng =
      match rand
      | let r: Random => r
      | None =>
        let now = Time.now()
        Rand(now._1.u64(), now._2.u64())
      end
    _v = rng.u8()

This leads to the following error:

Error:
main.pony:10:9: this capture violates capabilities, because the match would need to differentiate by capability at runtime instead of matching on type alone
      | let r: Random => r
        ^
    Info:
    main.pony:7:36: the match type allows for more than one possibility with the same type as pattern type, but different capabilities. match type: (Random iso! | None val)
      new val create(rand: (Random iso | None) = None) =>
                                       ^
    main.pony:10:9: pattern type: Random ref
          | let r: Random => r
            ^
    main.pony:10:16: matching Random iso! with Random ref could violate capabilities: iso! isn't a subcap of ref
          | let r: Random => r
                   ^
    main.pony:10:16: matching (Random iso! | None val) with Random ref could violate capabilities
          | let r: Random => r

The issue is fixed by adding the missing consume:

...
let rng =
    match consume rand
...

If the rand parameter's type is specified as Random iso^ (i.e. making it ephemeral), the code compiles, either with or without consume.

[jemc]

Probably the most consistent way to improve this would be to amend the third info message to say:

    main.pony:10:16: matching Random iso! with Random ref could violate capabilities: iso! isn't a subcap of ref (did you forget to consume?)
          | let r: Random => r
                   ^

That is, just adding a "(did you forget to consume?)" to any error message where we say "[iso|trn]! is not a subtype of [some other cap]".