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Type mismatch error when implementing method with return type dependent on member of parameterized type #9701

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scabug opened this issue Mar 12, 2016 · 3 comments
Closed

Type mismatch error when implementing method with return type dependent on member of parameterized type #9701

scabug opened this issue Mar 12, 2016 · 3 comments
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typer
Milestone
2.12.0-RC1

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@scabug
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scabug commented Mar 12, 2016

The following code:

trait A[X] {
  type M
}
class B[X]
trait C {
  def f[X](a: A[X]): B[a.M]
}
class D extends C {
  override def f[X](a: A[X]) = new B[a.M]
}

fails to compile, with the confusing error message:

[ERROR] C:\Users\Michael\workspace\paperdoll\src\main\scala\com\github\m50d\paperdoll\effect\Eff.scala:9: error: type mismatch;
[ERROR]  found   : B[a.M]
[ERROR]  required: B[a.M]
[ERROR]   override def f[X](a: A[X]) = new B[a.M]

As far as I can see this code should compile? If not then the error should be clearer.
@scabug
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scabug commented Mar 12, 2016

Imported From: https://issues.scala-lang.org/browse/SI-9701?orig=1
Reporter: Michael Donaghy (lmm)
Affected Versions: 2.11.8

@scabug
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scabug commented Mar 13, 2016

Michael Donaghy (lmm) said:
Paul Phillips pointed out that one can work around this by specifying the return type of the overriding method explicitly.

@scabug
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scabug commented Sep 29, 2016

@SethTisue said:
compiles in 2.12.0-RC1

@scabug scabug closed this as completed Sep 29, 2016
@scabug scabug added the typer label Apr 7, 2017
@scabug scabug added this to the 2.12.0-RC1 milestone Apr 7, 2017
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