"polymorphic expression cannot be instantiated to expected type": foo: List[Int] inferred, foo: ListInt not.

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ticket/6745-3 ~/code/scala scalac210 sandbox/test.scala
sandbox/test.scala:7: error: polymorphic expression cannot be instantiated to expected type;
 found   : [M[_], A]Test.Arb[M[A]]
 required: Test.Arb[Test.ListInt]
  foo: Arb[ListInt]
  ^
one error found
ticket/6745-3 ~/code/scala scalac29 sandbox/test.scala
ticket/6745-3 ~/code/scala cat !$
cat sandbox/test.scala
object Test {
  class Arb[_]
  implicit def foo[M[_], A]: Arb[M[A]] = null
  foo: Arb[List[Int]]

  type ListInt = List[Int]
  foo: Arb[ListInt]
}

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Imported From: https://issues.scala-lang.org/browse/SI-6846?orig=1
Reporter: @retronym
Affected Versions: 2.10.0-RC5

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@paulp said:
Regressed in r25322 aka 658ba1b4e6.

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@paulp said:
To make up for this, I threw in this one:

object Test2 {
  import scala.collection.immutable.List

  class Carb[+_]
  implicit def bar[M[_], A]: Carb[M[A]] = null

  type ListInt = List[Int]

  val x: List[Int] = List(1)
  val y: ListInt = List(1)

  type ListSingletonX = x.type
  type ListSingletonY = y.type

  bar: Carb[List[Int]]
  bar: Carb[ListInt]
  bar: Carb[ListSingletonX]
  bar: Carb[ListSingletonY]
}

Compiles with my imminent pull request, in 2.9 did this:

scalac29 -d /tmp test/files/pos/t6846.scala 
test/files/pos/t6846.scala:25: error: polymorphic expression cannot be instantiated to expected type;
 found   : [M[_], A]Test2.Carb[M[A]]
 required: Test2.Carb[Test2.ListSingletonX]
  bar: Carb[ListSingletonX]
  ^
test/files/pos/t6846.scala:26: error: polymorphic expression cannot be instantiated to expected type;
 found   : [M[_], A]Test2.Carb[M[A]]
 required: Test2.Carb[Test2.ListSingletonY]
  bar: Carb[ListSingletonY]
  ^
two errors found

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@paulp said:
Oh, but I see now that type constructor inference is useless when combined variance - it appears to always choose Any/Nothing for the type argument if it can. Oh, in that case the invariant version is actually failing in the correct way. The 2.9 version also fails, but for the wrong reason.

test/files/pos/t6846.scala:25: error: type mismatch;
 found   : Test2.Carb[List[Int]]
 required: Test2.Carb[Test2.ListSingletonX]
Note: List[Int] >: Test2.ListSingletonX, but class Carb is invariant in type _.
You may wish to define _ as -_ instead. (SLS 4.5)
  bar: Carb[ListSingletonX]
  ^
test/files/pos/t6846.scala:26: error: type mismatch;
 found   : Test2.Carb[List[Int]]
 required: Test2.Carb[Test2.ListSingletonY]
Note: List[Int] >: Test2.ListSingletonY, but class Carb is invariant in type _.
You may wish to define _ as -_ instead. (SLS 4.5)
  bar: Carb[ListSingletonY]
  ^
two errors found

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@adriaanm said:
scala/scala#1869