seq, array & openArray initialisers broken with polymorphic types

[johnnovak]

Currently, it is not possible to build a seq, array or openArray of the super type with the initialiser syntax when only a single subtype element is used. It works fine though when at least two different subtypes are passed into the initialiser.

type
  Base = ref object of RootObj
  A = ref object of Base
  B = ref object of Base

# Sequences
var s: seq[Base]

s = @[A(), B()]     # OK
#s = @[A()]         # FAILS TO COMPILE:
                    # Error: type mismatch: got (seq[A]) but expected 'seq[Base]'


# Arrays
var arr: array[0..1, Base]
arr = [A(), B()]    # OK
#arr = [A(), A()]   # FAILS TO COMPILE:
                    # Error: type mismatch: got (array[0..1, A]) but expected 'array[0..1, Base]'


# Open arrays
proc openArrayTest(oa: openArray[Base]) = discard

openArrayTest([A(), B()])   # OK
#openArrayTest([A()])       # FAILS TO COMPILE:
                            # Error: type mismatch: got (array[0..0, A])
                            # but expected one of:
                            #   proc openArrayTest(oa: openArray[Base])

openArrayTest(@[A(), B()])  # OK
#openArrayTest(@[A()])      # FAILS TO COMPILE:
                            # Error: type mismatch: got (seq[A])
                            # but expected one of:
                            #   proc openArrayTest(oa: openArray[Base])

[Araq]

That's not a bug. The subtype relation cannot be lifted from A <: B to C[A] <: C[B] it would be unsound. You need to cast your 'A' to 'Base' explicitly.