Returning Array(Int32) errors if method should return Array(Int32 | String)

[akadusei]

While this works: https://play.crystal-lang.org/#/r/5ro7

def foo(x : Array(Int32 | String))
  typeof(x)
end

# I can pass a Array(Int32) to a method that expects Array(Int32 | String)
puts  foo([1]) #=> Array(Int32)

But this gets a compile error: https://play.crystal-lang.org/#/r/5rmy

def foo : Array(Int32 | String)
  [1]
end

puts foo

Output:

Error in line 5: instantiating 'foo()'

in line 18: type must be Array(Int32 | String), not Array(Int32)

I was kinda hoping returning Array(Int32) from a method that expects a return type of Array(Int32 | String) should pass.

[straight-shoota]

This is covariance vs contravariance. See #3803 and related issues.

[asterite]

As @straight-shoota says, it's indeed covariance and contravariance.

Right now there's a bit of inconsistency of what Array(Int32 | String) means depending on when you write it: a type restriction, a return value, the type of an instance variable. No decision has been taken yet.

But yeah, essentially a duplicate of #3803, so closing.