Incorrect "T.cast is useless" error on T.bind with rescue / ensure blocks

[alto-rlk]

Input

→ View on sorbet.run

# typed: strict
module M
  extend T::Sig

  sig {void}
  def good
    T.bind(self, C)
  end

  sig {void}
  def bad
    T.bind(self, C)
  rescue => e
  end

end

class C
  include M
end

Observed output

editor.rb:12: T.cast is useless because C is already a subtype of C https://srb.help/7015
    12 |    T.bind(self, C)
            ^^^^^^^^^^^^^^^
  Got C originating from:
    editor.rb:12:
    12 |    T.bind(self, C)
            ^^^^^^^^^^^^^^^
  Autocorrect: Use `-a` to autocorrect
    editor.rb:12: Replace with self
    12 |    T.bind(self, C)
            ^^^^^^^^^^^^^^^
Errors: 1

Runtime behavior: this version exercises the methods and runs with no errors.

Here's an additional sorbet.run example covering a few more cases.

Expected behavior

No errors.

[Morriar]

This usage of T.bind is strange. What would happen if I included the module in another class?

class D
  include M
end

I think what you're looking for here is requires_ancestor:

# typed: strict
module M
  extend T::Sig
  extend T::Helpers

  requires_ancestor { C }
end

class C
  include M
end

This also have the benefit of preventing M being included in something that is not a C.

[alto-rlk]

Thanks! I'd agree that requires_ancestor is the better solution for this use case in general, but it is still listed as experimental.

FWIW, I don't believe this bug is specific/related to M being a module.

[jez]

This is a perfectly valid use of bind, and this error should probably not reported. Not sure at a glance what the best solution is, but there are two options:

• silence the useless cast error for bind (not great, as that would cause sorbet to report some errors that it likely should)
• figure out why this extra cast instruction is getting into the method body:

<self>: C = cast(<castTemp>$6: C, C); in bb4

https://sorbet.run/?arg=--print=cfg-text#%23%20typed%3A%20strict%0Amodule%20M%0A%20%20extend%20T%3A%3ASig%0A%0A%20%20%23%20sig%20%7Bvoid%7D%0A%20%20%23%20def%20good%0A%20%20%23%20%20%20T.bind%28self%2C%20C%29%0A%20%20%23%20end%0A%0A%20%20sig%20%7Bvoid%7D%0A%20%20def%20bad%0A%20%20%20%20T.bind%28self%2C%20C%29%0A%20%20rescue%20%3D%3E%20e%0A%20%20end%0Aend%0A%0Aclass%20C%0A%20%20include%20M%0Aend

method ::M#bad {

bb0[rubyRegionId=0, firstDead=-1]():
    <self>: M = cast(<self>: NilClass, M);
    <magic>$7: T.class_of(<Magic>) = alias <C <Magic>>
    <exceptionValue>$3: T.untyped = <get-current-exception>
    <self>: C = cast(<castTemp>$6: NilClass, C);
    <returnMethodTemp>$2: C = <self>
    <exceptionValue>$3 -> (T.untyped ? bb3 : bb4)

# backedges
# - bb6(rubyRegionId=3)
# - bb9(rubyRegionId=0)
bb1[rubyRegionId=0, firstDead=-1]():
    <unconditional> -> bb1

# backedges
# - bb0(rubyRegionId=0)
# - bb4(rubyRegionId=1)
bb3[rubyRegionId=2, firstDead=-1](<returnMethodTemp>$2: C, <exceptionValue>$3: T.untyped, <magic>$7: T.class_of(<Magic>)):
    e: T.untyped = <exceptionValue>$3
    <cfgAlias>$10: T.class_of(StandardError) = alias <C StandardError>
    <isaCheckTemp>$11: T.untyped = e: T.untyped.is_a?(<cfgAlias>$10: T.class_of(StandardError))
    <isaCheckTemp>$11 -> (T.untyped ? bb7 : bb8)

# backedges
# - bb0(rubyRegionId=0)
bb4[rubyRegionId=1, firstDead=-1](<self>: C, <magic>$7: T.class_of(<Magic>)):
    <cfgAlias>$5: T.class_of(C) = alias <C C>
    keep_for_ide$4: T.class_of(C) = <cfgAlias>$5
    keep_for_ide$4: T.untyped = <keep-alive> keep_for_ide$4
    <castTemp>$6: C = <self>
    <self>: C = cast(<castTemp>$6: C, C);
    <returnMethodTemp>$2: C = <self>
    <exceptionValue>$3: T.untyped = <get-current-exception>
    <exceptionValue>$3 -> (T.untyped ? bb3 : bb5)

# backedges
# - bb4(rubyRegionId=1)
bb5[rubyRegionId=4, firstDead=-1](<returnMethodTemp>$2: C):
    <unconditional> -> bb6

# backedges
# - bb5(rubyRegionId=4)
# - bb7(rubyRegionId=2)
# - bb8(rubyRegionId=2)
bb6[rubyRegionId=3, firstDead=-1](<returnMethodTemp>$2: C, <gotoDeadTemp>$12: T.nilable(TrueClass)):
    <gotoDeadTemp>$12 -> (T.nilable(TrueClass) ? bb1 : bb9)

# backedges
# - bb3(rubyRegionId=2)
bb7[rubyRegionId=2, firstDead=-1](<returnMethodTemp>$2: C, <magic>$7: T.class_of(<Magic>)):
    <exceptionValue>$3: NilClass = nil
    <keepForCfgTemp>$8: Sorbet::Private::Static::Void = <magic>$7: T.class_of(<Magic>).<keep-for-cfg>(<exceptionValue>$3: NilClass)
    <unconditional> -> bb6

# backedges
# - bb3(rubyRegionId=2)
bb8[rubyRegionId=2, firstDead=-1](<returnMethodTemp>$2: C):
    <gotoDeadTemp>$12: TrueClass = true
    <unconditional> -> bb6

# backedges
# - bb6(rubyRegionId=3)
bb9[rubyRegionId=0, firstDead=1](<returnMethodTemp>$2: C):
    <finalReturn>: T.noreturn = return <returnMethodTemp>$2: C
    <unconditional> -> bb1

}

Anyways, thanks for the report. I'm not sure when I'll have time to get to this.

[jez]

(whoops wrong button 😅 sorry about the accidental close)