`Tuple.new` isn't considered an `Enumerable`

[mattrberry]

foo : Enumerable(String) = Tuple.new # => Error: type must be Enumerable(String), not Tuple()

I'd expect an empty tuple to be considered an enumerable of any type. Compare this to the following, which works as expected

foo : Enumerable(String) = Tuple(String).new("")

Tested on Crystal 1.5.1

[HertzDevil]

Enumerable is invariant, so there is no relationship between Enumerable(String) and Enumerable(NoReturn), which Tuple() is a subtype of. See #3803. Similarly it can be shown that the subtyping relationship isn't transitive:

class Foo; end
class Bar < Foo; end

{% Tuple(Bar) <= Tuple(Foo) %}      # => true
{% Tuple(Foo) <= Enumerable(Foo) %} # => true
{% Tuple(Bar) <= Enumerable(Foo) %} # => false

This is actually quite problematic.

[asterite]

For Crystal 2.0 I'd like it if we could remove the special subtyping rules of Tuple and NamedTuple. They just make things too complex and inconsistent.

[HertzDevil]

How would unions between them work if they are no longer covariant?

[asterite]

They would be normal unions. Their types wouldn't merge.