Bad error message when matching on a struct in a union

[dipinhora]

Use case: I'm working with FFI calls where an argument can either be a pointer to a struct or NULL (where it will then use defaults). I set up a variable as either (struct | None) but I'm running into a compiler error when trying to craft the appropriate MaybePointer (I get a similar error when trying to cast it with an as also). It is quite possible that I'm going about this the wrong way (in which case a pointer in the right direction would be appreciated).

Pony Playpen link: http://pony-playpen.lietar.net/?gist=4d388470321b9585cbb33f22f9578b74

Code:

struct Z
  var a: U32 = 0

  new create() => None

actor Main
  new create(env:Env) =>
    let arg: (Z | None) = None

    let y = match arg
      | let x: Z => MaybePointer[Z](x)
      else
        MaybePointer[Z].none()
      end

Error:

0.14.0 [release]
compiled with: llvm 3.9.1 -- cc (Ubuntu 5.4.0-6ubuntu1~16.04.4) 5.4.0 20160609
Error:
main.pony:11:9: this capture violates capabilities
      | let x: Z => MaybePointer[Z](x)
        ^
    Info:
    main.pony:8:15: match type: (Z ref | None val)
        let arg: (Z | None) = None
                  ^
    main.pony:11:9: pattern type: Z ref
          | let x: Z => MaybePointer[Z](x)
            ^

[Praetonus]

structs can't be part of unions because they don't have a type descriptor. I'm not sure why the compiler isn't telling you that, though.

You want your object to be a MaybePointer from the beginning when working with optional structs.

I'll keep the issue open for the error message.

[dipinhora]

@Praetonus Thanks for the quick response. I'll refactor my code accordingly.

[SeanTAllen]

What should the error message for this be?

[Praetonus]

It should be something like a struct cannot be part of a union type.