Better union-union subtyping

[soutaro]

Subtyping between union types can be improved by skipping common types. This happens on #filter_map.

Assume the following type definition:

class Array[Element]
  def filter_map: [T] () { (Element) -> (T | nil | false) } -> Array[T]
end

and a Ruby code

[1, 2, 3].filter_map do |i|
  if i.odd?
    i.to_s
  end
end

We want the type of the expression to be Array[String], but it returns Array[String | nil]. A subtyping check happens with String | nil <: T | nil | false, and was solved to T == String | nil.

The hack is to try types without type variables before types with type variables.

1. nil <: T | nil | false tries nil <: nil and nil <: false before nil <: T.
2. The nil <: nil holds, and the later nil <: T isn't tried.

And we have Array[String]! 🎉