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Fix #9061: Treat type F <: [X] => G like type F[X] <: G. #9084

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Merged
merged 2 commits into from
May 30, 2020
Merged

Fix #9061: Treat type F <: [X] => G like type F[X] <: G. #9084

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Fix #9061: Treat `type F <: [X] => G` like `type F[X] <: G`.
odersky May 30, 2020
769e108
Also fix variances of curried bound parameters
odersky May 30, 2020
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2 changes: 2 additions & 0 deletions compiler/src/dotty/tools/dotc/core/Variances.scala
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Original file line number Diff line number Diff line change
Expand Up @@ -156,4 +156,6 @@ object Variances {
if (v > 0) "+"
else if (v < 0) "-"
else ""

val alwaysInvariant: Any => Invariant.type = Function.const(Invariant)
}
20 changes: 18 additions & 2 deletions compiler/src/dotty/tools/dotc/typer/Namer.scala
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Expand Up @@ -15,6 +15,7 @@ import util.Spans._
import util.Property
import collection.mutable
import tpd.ListOfTreeDecorator
import Variances.alwaysInvariant
import config.{Config, Feature}
import config.Printers.typr
import Annotations._
Expand Down Expand Up @@ -968,7 +969,10 @@ class Namer { typer: Typer =>
override final def typeSig(sym: Symbol): Type =
val tparamSyms = completerTypeParams(sym)(ictx)
given ctx as Context = nestedCtx
def abstracted(tp: TypeBounds): TypeBounds = HKTypeLambda.boundsFromParams(tparamSyms, tp)

def abstracted(tp: TypeBounds): TypeBounds =
HKTypeLambda.boundsFromParams(tparamSyms, tp)

val dummyInfo1 = abstracted(TypeBounds.empty)
sym.info = dummyInfo1
sym.setFlag(Provisional)
Expand Down Expand Up @@ -998,8 +1002,20 @@ class Namer { typer: Typer =>
}
sym.info = dummyInfo2

// If this type does not have type parameters itself, treat the parameters
// of a type lambda on the RHS as non-variant. E.g.
// type F <: [X] =>> G and type F[X] <: G
// are treated alike.
def addVariances(tp: Type): Type = tp match
case tp @ TypeBounds(lo, hi) =>
tp.derivedTypeBounds(addVariances(lo), addVariances(hi))
case tp: HKTypeLambda if tparamSyms.isEmpty && !tp.isDeclaredVarianceLambda =>
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This won't catch curried type lambdas like:

F[X] <: [A] =>> Any

which are also problematic:

case class Contra[-A](f: A => Int)

case class Covarify[+F[X] <: ([A] =>> Any), +A](fa: F[Int][A])

@main def main = {
  val x = Covarify[[X] =>> Contra, Int](Contra[Int](_ + 5))
  val y: Covarify[[X] =>> Contra, Any] = x
  println(y.fa.f("abc"))
}

tp.withVariances(tp.paramNames.map(alwaysInvariant))
case _ =>
tp

val rhs1 = typedAheadType(rhs)
val rhsBodyType: TypeBounds = rhs1.tpe.toBounds
val rhsBodyType: TypeBounds = addVariances(rhs1.tpe).toBounds
val unsafeInfo = if (isDerived) rhsBodyType else abstracted(rhsBodyType)

def opaqueToBounds(info: Type): Type =
Expand Down
8 changes: 4 additions & 4 deletions compiler/src/dotty/tools/dotc/util/common.scala
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Expand Up @@ -6,8 +6,8 @@ import core.Types.WildcardType
/** Common values hoisted out for performance */
object common {

val alwaysTrue: Any => Boolean = Function.const(true) _
val alwaysFalse: Any => Boolean = Function.const(false) _
val alwaysZero: Any => Int = Function.const(0) _
val alwaysWildcardType: Any => WildcardType.type = Function.const(WildcardType) _
val alwaysTrue: Any => Boolean = Function.const(true)
val alwaysFalse: Any => Boolean = Function.const(false)
val alwaysZero: Any => Int = Function.const(0)
val alwaysWildcardType: Any => WildcardType.type = Function.const(WildcardType)
}
9 changes: 9 additions & 0 deletions tests/neg/i9061.scala
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@@ -0,0 +1,9 @@
case class Contra[-A](f: A => Int)

case class Covarify[+F <: ([A] =>> Any), +A](fa: F[A]) // error: covariant type A occurs in invariant position in type F[A] of value fa

@main def main = {
val x = Covarify[Contra, Int](Contra[Int](_ + 5))
val y: Covarify[Contra, Any] = x
println(y.fa.f("abc"))
}