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Problems with polymorphic method, Nothing, Invariant return type #1570

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scabug opened this issue Dec 8, 2008 · 8 comments
Open

Problems with polymorphic method, Nothing, Invariant return type #1570

scabug opened this issue Dec 8, 2008 · 8 comments
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fixed in Scala 3 This issue does not exist in the Scala 3 compiler (https://github.com/lampepfl/dotty/) typelevel typer
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@scabug
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scabug commented Dec 8, 2008

Happening in Scala 2.7.1 and 2.7.2 at least:

scala> trait Test[A]
defined trait Test

scala> trait Test2[+A]
defined trait Test2

scala> def f[A](a:Test[A]):Test[A] = null
f: [A](Test[A])Test[A]

scala> def f2[A](a:Test[A]):Test2[A] = null
f2: [A](Test[A])Test2[A]

scala> val x:Test[Nothing] = null
x: Test[Nothing] = null

scala> f(x)
<console>:8: error: type mismatch;
 found   : Test[Nothing]
 required: Test[A]
      f(x)
        ^

scala> f[Nothing](x)
res2: Test[Nothing] = null

scala> f2(x)
res1: Test2[Nothing] = null

This happens only for A=Nothing. And only if the return type is invariant in a type variable being Nothing. There is probably not much sense in operating on Nothing types, but what is the rationale behind this behaviour?
@scabug
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scabug commented Dec 8, 2008

Imported From: https://issues.scala-lang.org/browse/SI-1570?orig=1
Reporter: Johannes Rudolph (jrudolph100)

@scabug
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scabug commented Dec 12, 2008

Mohsen Lesani [X] (lesani) said:

This is still a problem in Trunk.

@scabug
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scabug commented Feb 13, 2011

@soc said:
Slightly different error for f(x) in 2.9.0.r24145-b20110130020134

scala> f(x)
<console>:9: error: type mismatch;
 found   : Test[Nothing]
 required: Test[A]
Note: Nothing <: A, but trait Test is invariant in type A.
You may wish to define A as +A instead. (SLS 4.5)
       f(x)
         ^

@scabug
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scabug commented May 3, 2011

@adriaanm said:
see also #4509

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scabug commented Jul 23, 2014

Daniel Darabos (darabos) said:
What an old and annoying issue. I wouldn't know where to start fixing it. But if someone can give a rough outline of how this could be fixed, I can offer to work on the implementation. Thanks.

@scabug scabug added this to the Backlog milestone Apr 6, 2017
This was referenced Apr 6, 2017
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@dima-starosud
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Guys, should we try to adopt that as a quick-and-dirty fix for this?

This was referenced Mar 10, 2018
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@dwijnand dwijnand mentioned this issue Nov 22, 2019
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@SethTisue

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@smarter
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smarter commented Sep 30, 2020 •
edited by SethTisue

@SethTisue You must have done something weird in your repl session, I just tried it and all these examples typecheck in Dotty 0.27.0-RC1: https://scastie.scala-lang.org/smarter/aTB7lTdPTVa7wSI5dEW4wA/5

@SethTisue SethTisue added the fixed in Scala 3 This issue does not exist in the Scala 3 compiler (https://github.com/lampepfl/dotty/) label Sep 30, 2020
@durban durban mentioned this issue Mar 8, 2023
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