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"diverging implicits" when searching for a covariant typeclass #8877

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scabug opened this issue Oct 2, 2014 · 1 comment
Open

"diverging implicits" when searching for a covariant typeclass #8877

scabug opened this issue Oct 2, 2014 · 1 comment

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@scabug
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scabug commented Oct 2, 2014

Discovered this bug in the discussion of scodec/scodec#26

here is a minimal test case that fails:

    trait Decoder[+A]
    trait Codec[A] extends Decoder[A]
    trait F[+A]
    
    object Foo {
      implicit def foo[A:Codec]: Codec[F[A]] = null
      val x = implicitly[Decoder[F[Int]]]
    }

trying to compile this gives me:

    [info] /Users/v724000/devel/variancebad/src/main/scala/foo.scala:7: foo is not a valid implicit value for Codec[A] because:
    [info] hasMatchingSymbol reported error: diverging implicit expansion for type Codec[A]
    [info] starting with method foo in object Foo
    [info]       val x = implicitly[Decoder[F[Int]]]

If I add the variance annotation to Codec, or if I remove the variance annotation from either F or Decoder then it does the right thing (which is to give me implicit not found instead of diverging implicits).

If I add an implicit Codec for Int:

          implicit val int: Codec[Int] = null

it compiles in 2.10.4 but in 2.11.2 I still get diverging implicits.
@scabug
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scabug commented Oct 2, 2014

Imported From: https://issues.scala-lang.org/browse/SI-8877?orig=1
Reporter: Stew O'Connor (stew)
Affected Versions: 2.10.4, 2.11.2

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@scabug