openarray/varargs: Nothing match Foo(bar: openarray[int] or PFoo)

[mratsim]

This issue gave me the idea of https://github.com/nim-lang/Nim/issues/6528

The following does not compile:

type
  MetadataArray* = object
    data*: array[8, int]
    len*: int


proc foo(a: openarray[int] or MetadataArray)=
  echo "barbaz"

let a = @[1,2]

foo(a)

Error: type mismatch: got (seq[int])
but expected one of:
proc foo(a: openArray[int] or MetadataArray)

varargs have the same issue

[Araq]

There is a conversion from seq[int] to openArray[int] but conversions are not lifted to | type constructors. Maybe they should, not sure if that's safe to do in general though. Type systems are tricky. But since you live with the generic instantiations already, use a concept instead?

[zah]

Duplicate of #7432