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2.12 Regression: covariant type alias can't be used as type ctor param #10168

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scabug opened this issue Jan 31, 2017 · 4 comments
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2.12 Regression: covariant type alias can't be used as type ctor param #10168

scabug opened this issue Jan 31, 2017 · 4 comments
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backend duplicate regression should compile

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@scabug
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scabug commented Jan 31, 2017

The attached file compiles fine with 2.11.8 but results in 4 errors using 2.12.0 and 2.12.1:

[error] /home/golly/crap/bug212/Blah.scala:8: covariant type Id occurs in invariant position in supertype Blah.X[Blah.WithIdTypeAlias.Id] of object Y1
[error]     object Y1 extends X[Id]   // FAILURE
[error]            ^
[error] /home/golly/crap/bug212/Blah.scala:9: covariant type Id occurs in invariant position in type => Blah.X[Blah.WithIdTypeAlias.Id] of value Y2
[error]     val Y2 = new X[Id]        // FAILURE
[error]         ^
[error] /home/golly/crap/bug212/Blah.scala:10: covariant type Id occurs in invariant position in type => Blah.X[Blah.WithIdTypeAlias.Id] of method Y3
[error]     def Y3 = new X[Id]        // FAILURE
[error]         ^
[error] /home/golly/crap/bug212/Blah.scala:13: covariant type Id occurs in invariant position in type => Blah.X[Blah.WithIdTypeAlias.Id] of method EX
[error]     def EX: X[Id] = new X[Id] // FAILURE
[error]         ^
[error] four errors found

WithIdClass is a copy-paste of WithIdTypeAlias with only the Id definition changed. When Id is a class and not a type alias, it compiles without error in 2.12.x.
@scabug
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scabug commented Jan 31, 2017

Imported From: https://issues.scala-lang.org/browse/SI-10168?orig=1
Reporter: David Barri (japgolly)
Affected Versions: 2.12.0, 2.12.1
See #10140
Attachments:

  • Blah.scala (created on Jan 31, 2017 7:33:20 AM UTC, 635 bytes)

@scabug
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scabug commented Feb 14, 2017

David Barri (japgolly) said:
Hello. This regression prevents me from releasing the upcoming redesign of scalajs-react for Scala 2.12.
I'd really, really like to get it fixed in 2.12.2. Is there anything I can do to help that happen?

@scabug
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scabug commented Feb 14, 2017

@SethTisue said:
this is a duplicate of #10140, isn't it?

@scabug
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scabug commented Feb 14, 2017

David Barri (japgolly) said:
Oh well spotted, I missed that. Yes it is!

It also provides a simple workaround by moving the type alias out into its own class/trait. Fantastic!
Urgency *= 0.1.
Thank you.

I'll close this now.

@scabug scabug closed this as completed Feb 14, 2017
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