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Allow poly-kinded GADT vars to infer constraints #19916

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Allow poly-kinded GADT vars to infer constraints #19916

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3 changes: 3 additions & 0 deletions compiler/src/dotty/tools/dotc/core/SymDenotations.scala
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Original file line number Diff line number Diff line change
Expand Up @@ -2287,6 +2287,9 @@ object SymDenotations {
case CapturingType(parent, refs) =>
tp.derivedCapturingType(recur(parent), refs)

case tp: HKLambda =>
NoType

case tp: TypeProxy =>
def computeTypeProxy = {
val superTp = tp.superType
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9 changes: 3 additions & 6 deletions compiler/src/dotty/tools/dotc/core/TypeComparer.scala
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Expand Up @@ -580,12 +580,9 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
if (cls2.typeParams.isEmpty) {
if (cls2 eq AnyKindClass) return true
if (isBottom(tp1)) return true
if (tp1.isLambdaSub) return false
// Note: We would like to replace this by `if (tp1.hasHigherKind)`
// but right now we cannot since some parts of the standard library rely on the
// idiom that e.g. `List <: Any`. We have to bootstrap without scalac first.
if cls2 eq AnyClass then return true
if cls2 == defn.SingletonClass && tp1.isStable then return true
if tp1.hasSimpleKind then
if cls2 eq AnyClass then return true
if cls2 == defn.SingletonClass && tp1.isStable then return true
return tryBaseType(cls2)
Comment on lines +583 to 586
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Was this change actually necessary? Isn't tryBaseType always going to fail anyway when tp1.hasSimpleKind is false? (given that you added the case HKTypeLambda => false in baseType, which makes sense)

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@dwijnand dwijnand Mar 15, 2024 •
edited
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Yes, because then it cascades to more "tries", including allowing a gadt variable bounded by AnyKind inferring a gadt bound.

}
else if (cls2.is(JavaDefined)) {
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21 changes: 21 additions & 0 deletions tests/pos/i19904.orig.scala
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@@ -0,0 +1,21 @@
sealed trait Wrap[A <: AnyKind]

trait Foo[T <: AnyKind, X[_ <: T]]

trait Bar[T <: AnyKind, X[_ <: T]] {

def foo: Foo[
Wrap[? <: T],
[x <: Wrap[? <: T]] =>> x match { case Wrap[a] => X[a] }
]

}

object Qux extends Bar[Any, [_] =>> Unit] {

override def foo: Foo[
Wrap[? <: Any],
[x <: Wrap[? <: Any]] =>> x match { case Wrap[a] => Unit }
] = ???

}
15 changes: 15 additions & 0 deletions tests/pos/i19904.scala
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@@ -0,0 +1,15 @@
sealed trait Bx[AK1 <: AnyKind]
trait C1[AK2 <: AnyKind, X[_ <: AK2]]
trait C2[AK3 <: AnyKind, Y[_ <: AK3]]:
def mm: C1[Bx[? <: AK3], [x <: Bx[? <: AK3]] =>> x match { case Bx[a] => Y[a] }]
class C3 extends C2[Any, [_] =>> Unit]:
override def mm: C1[Bx[? <: Any], [y <: Bx[? <: Any]] =>> y match { case Bx[b] => Unit }] = ???

/*
C1[Bx[?], [y <: Bx[?]] =>> y match { case Bx[b] => Unit } <: Unit] <: C1[Bx[?], [x <: Bx[?]] =>> x match { case Bx[a] => Unit } <: Unit]
[x <: Bx[?]] =>> x match { case Bx[a] => Unit } <: Unit <: [y <: Bx[?]] =>> y match { case Bx[b] => Unit } <: Unit
x match { case Bx[a] => Unit } <: Unit <: x match { case Bx[b] => Unit } <: Unit
[a] =>> case Bx[a] => Unit <: [b <: AnyKind] =>> case Bx[b] => Unit
[ <: AnyKind] <: []
AnyKind <: Any = false
*/