Cannot type_parameter a method that returns a Covariant generic #6745

Kache · 2023-02-16T19:46:13Z

Input

# typed: true

extend T::Sig

class CoBox
  extend T::Sig
  extend T::Generic

  Elem = type_member(:out)

  sig { params(value: Elem).void }
  def initialize(value)
    @value = value
  end
end

sig { type_parameters(:V).params(value: T.type_parameter(:V)).returns(CoBox[T.type_parameter(:V)]) }
def co_box(value)
  CoBox.new(value) # errors, but should not
end

# covariant usage
T.let(co_box(Bar.new), CoBox[Bar])
T.let(co_box(Baz.new), CoBox[Bar])


# invariant example follows, just for comparison
class Box
  extend T::Sig
  extend T::Generic

  Elem = type_member

  sig { params(value: Elem).void }
  def initialize(value)
    @value = value
  end
end

sig { type_parameters(:V).params(value: T.type_parameter(:V)).returns(Box[T.type_parameter(:V)]) }
def box(value)
  Box.new(value)
end

class Bar; end
class Baz < Bar; end

T.let(box(Bar.new), Box[Bar])

Observed output

editor.rb:19: Expected CoBox[T.type_parameter(:V) (of Object#co_box)] but found CoBox[<top>] for method result type https://srb.help/7005
    19 |  CoBox.new(value) # errors, but should not
          ^^^^^^^^^^^^^^^^
  Expected CoBox[T.type_parameter(:V) (of Object#co_box)] for result type of method co_box:
    editor.rb:18:
    18 |def co_box(value)
        ^^^^^^^^^^^^^^^^^
  Got CoBox[<top>] originating from:
    editor.rb:19:
    19 |  CoBox.new(value) # errors, but should not
          ^^^^^^^^^^^^^^^^
Errors: 1

Expected behavior

Perhaps I've misunderstood something, but I think methods like co_box shouldn't error.

My current workaround is to have them return CoBox[T.untyped]

The text was updated successfully, but these errors were encountered:

jez · 2023-02-16T19:54:15Z

You are missing a type annotation on CoBox.new:

→ corrected example

This annotation is secretly required. When it is not provided, Sorbet assumes:

T.untyped if the generic is invariant
<top> if the generic is covariant
T.noreturn if the generic is contravariant

The fix for this will be one of #3768 or #4450 (either: require a type argument for all generics, or infer a type argument from the arguments, and require the ones that can't be inferred).

(I've tried both in the past and neither is a simple fix so unfortunately they'll remain open for the time being.)

Closing this as a dupe of those other ones

Kache · 2023-02-16T20:02:40Z

thank you!

jez closed this as not planned Won't fix, can't repro, duplicate, stale Feb 16, 2023

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Cannot type_parameter a method that returns a Covariant generic #6745

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Cannot type_parameter a method that returns a Covariant generic #6745

Cannot type_parameter a method that returns a Covariant generic #6745

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