Array Literal Types in Pony

arrays.pony

type MyUnionType is (U32|I32|U8)

actor Main
  new create(env: Env) =>
    let union_array: Array[MyUnionType] = [
      as MyUnionType: U32(1)
      I32(-1)
    ]

compiler_output.txt

$ CC=gcc ponyc --debug .
Building builtin -> /usr/lib/pony/0.12.3-3147.5a68e92/packages/builtin
Building . -> /home/mwahl/dev/pony/arrays
Error:
/home/mwahl/dev/pony/arrays/arrays.pony:5:41: right side must be a subtype of left side
    let union_array: Array[MyUnionType] = [
                                        ^
    Info:
    /home/mwahl/dev/pony/arrays/arrays.pony:1:30: U8 val is not a subtype of U32 val
    type MyUnionType is (U32|I32|U8)
                                 ^
    /home/mwahl/dev/pony/arrays/arrays.pony:1:30: U8 val is not a subtype of I32 val
    type MyUnionType is (U32|I32|U8)
                                 ^
    /home/mwahl/dev/pony/arrays/arrays.pony:1:30: U8 val is not a subtype of any element of (U32 val | I32 val)
    type MyUnionType is (U32|I32|U8)
                                 ^
    /home/mwahl/dev/pony/arrays/arrays.pony:1:30: not every element of (U32 val | I32 val | U8 val) is a subtype of (U32 val | I32 val)
    type MyUnionType is (U32|I32|U8)
                                 ^
    /usr/lib/pony/0.12.3-3147.5a68e92/packages/builtin/array.pony:9:3: Array[(U32 val | I32 val)] ref has different type arguments than Array[(U32 val | I32 val | U8 val)] ref
      new create(len: USize = 0) =>
      ^

To me it seems that Array[(I32|U32)], the type of the array literal, actually is a subtype of the variable type Array[MyUnionType] -> Array[(I32IU32IU8)]. In other words, the array literal seems to fulfil the constraints given by the variable type it is assigned to.