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Incomplete substitution of type parameters in extension methods #11383

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joroKr21 opened this issue Jan 29, 2019 · 5 comments · Fixed by scala/scala#9172
Open

Incomplete substitution of type parameters in extension methods #11383

joroKr21 opened this issue Jan 29, 2019 · 5 comments · Fixed by scala/scala#9172
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fixed in Scala 3 This issue does not exist in the Scala 3 compiler (https://github.com/lampepfl/dotty/) typer valueclass
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Backlog

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@joroKr21
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This occurs when combining AnyVal extension methods and the Aux pattern for dependent types.

trait DepFn[A] { type B; def apply(a: A): B }
object DepFn { type Aux[A, C] = DepFn[A] { type B = C } }
class Syntax(val i: Int) extends AnyVal {
  def foo[A](e: Either[Int => A, DepFn.Aux[Int, A]]) = e.fold(_(i), _(i))
}

// Exiting paste mode, now interpreting.

  def foo[A](e: Either[Int => A, DepFn.Aux[Int, A]]) = e.fold(_(i), _(i))
                                                                     ^
<pastie>:4: error: type mismatch;
 found   : A
 required: A(in method foo$extension)

Note that surprisingly this compiles (the only difference is a type ascription):

class Syntax(val i: Int) extends AnyVal {
  def foo[A](e: Either[Int => A, DepFn.Aux[Int, A]]) = e.fold(_(i), _(i): A)
}

// Exiting paste mode, now interpreting.

defined class Syntax

And now the punchline (try it with a List[A]):

class Syntax(val i: Int) extends AnyVal {
  def foo[A](e: Either[Int => List[A], DepFn.Aux[Int, List[A]]]) = e.fold(_(i), _(i))
}

// Exiting paste mode, now interpreting.

  def foo[A](e: Either[Int => List[A], DepFn.Aux[Int, List[A]]]) = e.fold(_(i), _(i))
                                                                                 ^
<pastie>:4: error: type mismatch;
 found   : List[A]
 required: List[A]

huh? At this point the compiler looks silly 😆
@milessabin
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Works in Dotty.

@smarter smarter added the fixed in Scala 3 This issue does not exist in the Scala 3 compiler (https://github.com/lampepfl/dotty/) label Jan 29, 2019
@SethTisue SethTisue added this to the Backlog milestone Jan 29, 2019
@joroKr21
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The problem is that the second _(i) is typed as B where B is a type alias to A in foo. substituteSymbols is supposed to substitute A in foo with A in foo$extension but it does not follow the type alias. What should happen?

@joroKr21
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Maybe same issue as #9222?

@hrhino
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hrhino commented May 3, 2019

Found this today, which I suspect is a similar cause:

trait R[T] { def toX[U >: T]: X[U] }
trait X[A]
class Lose(val dumbo: Int) extends AnyVal {
  def fail[Q](foo: R[_ <: Q]): X[Q] = foo.toX
}

in which the Q in foo's type in fail$extension apparently doesn't get updated to point at the extension method's type parameter.

@hrhino hrhino mentioned this issue Jun 7, 2019
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@joroKr21 joroKr21 mentioned this issue Jan 16, 2020
Open
@joroKr21 joroKr21 mentioned this issue Jun 5, 2020
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@joroKr21 joroKr21 mentioned this issue Aug 16, 2020
Merged
@dwijnand dwijnand modified the milestones: Backlog, 2.13.4 Oct 7, 2020
@joroKr21 joroKr21 mentioned this issue Oct 13, 2020
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@dwijnand
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Reverted

@dwijnand dwijnand reopened this Oct 14, 2020
@SethTisue SethTisue modified the milestones: 2.13.4, Backlog Oct 14, 2020
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fixed in Scala 3 This issue does not exist in the Scala 3 compiler (https://github.com/lampepfl/dotty/) typer valueclass
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[reverted] Dealias when necessary to substitute symbols joroKr21/scala
6 participants
@smarter @milessabin @SethTisue @dwijnand @joroKr21 @hrhino