Build T::Class[AttachedClass]

[DarkDimius]

Input

→ View on sorbet.run

# typed: true
extend T::Sig

module M
  def some_instance_method; end
end

class A
  include M
end

T.let(A.new, M)

# There is no type to write on the RHS that's valid
T.let(A, T.class_of(M))

# We'd like to implement something to make this work
# T.let(A, T::Class[M])

# sig {params(klass: T::Class[M]).void}
sig {params(klass: Class).void}
def example1(klass)
  x = klass.new
  T.reveal_type(x)        # ideally: `M`
  x.some_instance_method  # ideally: no error
end

Observed output

editor.rb:24: Revealed type: Object https://srb.help/7014
    24 |  T.reveal_type(x)        # ideally: `M`
          ^^^^^^^^^^^^^^^^
  Got Object originating from:
    editor.rb:23:
    23 |  x = klass.new
              ^^^^^^^^^

editor.rb:25: Method some_instance_method does not exist on Object https://srb.help/7003
    25 |  x.some_instance_method  # ideally: no error
          ^^^^^^^^^^^^^^^^^^^^^^
  Got Object originating from:
    editor.rb:23:
    23 |  x = klass.new
              ^^^^^^^^^
  Autocorrect: Use `-a` to autocorrect
    editor.rb:25: Replace with singleton_method
    25 |  x.some_instance_method  # ideally: no error
            ^^^^^^^^^^^^^^^^^^^^

editor.rb:15: Argument does not have asserted type T.class_of(M) https://srb.help/7007
    15 |T.let(A, T.class_of(M))
        ^^^^^^^^^^^^^^^^^^^^^^^
  Got T.class_of(A) originating from:
    editor.rb:15:
    15 |T.let(A, T.class_of(M))
              ^
Errors: 3

Expected behavior

• The commented-out T.let holds
• The revealed type of x is M
• x.some_instance_method type checks.

[jez]

https://sorbet.run/#%23%20typed%3A%20true%0A%0Amodule%20MySingleton%0A%20%20module%20ClassMethods%0A%20%20%20%20Sorbet.sig%20%7Breturns(T.self_type)%7D%0A%20%20%20%20def%20instance%0A%20%20%20%20%20%20T.unsafe(nil)%0A%20%20%20%20end%0A%20%20end%0A%20%20mixes_in_class_methods(ClassMethods)%0Aend%0A%0Aclass%20A%0A%20%20include%20MySingleton%0Aend%0A%0AT.reveal_type(A.instance)

^ correctly modeling this requires T::Class[AttachedClass]. We used to use T.self_type, which is wrong, because it actually returns an instance of T.self_type (because it's a singleton class method)

[akshay-stripe]

subscribing. cc @axyjo

[hdoan741]

subscribing

[searls]

I left this comment yesterday on #3856. Is it a dupe of #62?

(Either way, this capability would be great for my mocking library, Mocktail! Without it there's not much value in typing)

[grncdr]

It's a bit unclear to me from the example given whether this actually covers the same use-case as #3856. Specifically this example does not make any use of type parameters.

Does fixing this issue imply that the following signature will be possible?

sig { type_parameters(:E).param(T::Class[T.type_parameter(:E)]).returns(T.type_parameter(:E)) }

If not, I think #3856 is a separate issue.