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Scalac cannot inference existential type #6604

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scabug opened this issue Nov 2, 2012 · 5 comments
Closed

Scalac cannot inference existential type #6604

scabug opened this issue Nov 2, 2012 · 5 comments
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@adriaanm
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enhancement existential infer minimized should compile typer usability
Milestone
2.11.0

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@scabug
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scabug commented Nov 2, 2012 

scala> scala.util.control.Exception.catching(classOf[ClassNotFoundException]).opt(Class.forName(""))
<console>:8: error: type mismatch;
 found   : java.lang.Class[_]
 required: java.lang.Class[?0(in value res0)] where type ?0(in value res0)
Note: Any >: ?0, but Java-defined class Class is invariant in type T.
You may wish to investigate a wildcard type such as `_ >: ?0`. (SLS 3.2.10)
              scala.util.control.Exception.catching(classOf[ClassNotFoundException]).opt(Class.forName(""))
                                                                                                      ^

If I set the type parameter explicitly, it compiles:

scala> scala.util.control.Exception.catching(classOf[ClassNotFoundException]).opt[Class[_]](Class.forName(""))
res0: Option[java.lang.Class[_]] = None
@scabug
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scabug commented Nov 2, 2012

Imported From: https://issues.scala-lang.org/browse/SI-6604?orig=1
Reporter: @Atry
Affected Versions: 2.9.1, 2.9.2, 2.10.0-M1, 2.10.0-M2, 2.10.0-M3, 2.10.0-M4, 2.10.0-M5, 2.10.0-M6, 2.10.0-M7
See #5886

@scabug
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scabug commented Nov 6, 2012

@adriaanm said:
I agree it would be nice to improve this. Sometimes you just have to add a type parameter, as type inference is limited. Avoid existential types whenever possible.

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scabug commented Jul 10, 2013

@adriaanm said:
Unassigning and rescheduling to M6 as previous deadline was missed.

@scabug scabug closed this as completed Dec 11, 2013
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scabug commented Feb 18, 2014

@retronym said:
Fixed along with #5886 in 01716c8d8553a027cb0583f90409652128286463 / v2.10.2~14

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scabug commented Feb 18, 2014

@retronym said:

scala-hash v2.10.2~15 -nc -e 'scala.util.control.Exception.catching(classOf[ClassNotFoundException]).opt(Class.forName(""))'
[info] downloading http://scala-webapps.epfl.ch/artifacts/70e2ead142f87ebd5f4c6859ce17c72fdff8bd3b/pack.tgz ...done.
[info] scala revision from 2013-05-15 15:59:12 -0700 downloaded to /Users/jason/usr/scala-v2.10.1-318-g70e2ead
/var/folders/s2/g6fgtpk52hl8njjvx8vyzr9r0000gn/T/scalacmd861028181716928737.scala:1: error: type mismatch;
 found   : Class[_]
 required: Class[?0(in value <local $anon>)] where type ?0(in value <local $anon>)
Note: Any >: ?0, but Java-defined class Class is invariant in type T.
You may wish to investigate a wildcard type such as `_ >: ?0`. (SLS 3.2.10)
scala.util.control.Exception.catching(classOf[ClassNotFoundException]).opt(Class.forName(""))
                                                                                        ^
  ~/code/scala scala-hash v2.10.2~14 -nc -e 'scala.util.control.Exception.catching(classOf[ClassNotFoundException]).opt(Class.forName(""))'

@scabug scabug added this to the 2.11.0 milestone Apr 7, 2017
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enhancement existential infer minimized should compile typer usability
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