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Improve subtyping check for not yet eta-expanded higher kinded types #17139

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Apr 22, 2023
Merged

Improve subtyping check for not yet eta-expanded higher kinded types #17139

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2 changes: 1 addition & 1 deletion compiler/src/dotty/tools/dotc/core/TypeComparer.scala
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Original file line number Diff line number Diff line change
Expand Up @@ -580,7 +580,7 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
val base = nonExprBaseType(tp1, cls2)
if (base.typeSymbol == cls2) return true
}
else if tp1.isLambdaSub && !tp1.isAnyKind then
else if tp1.typeParams.nonEmpty && !tp1.isAnyKind then
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Not sure about this, this could be quite a drastic change that needs more motivation. Can you push to staging, then I can try some things?

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@odersky pushed.
The problem I'm trying to fix here is that when the compiler tries to compare two higher-kinded types like Foo1 and Foo2 in the test example, it returns false too early without trying to eta-expand the types. In the other cases from the linked issue, where everything compiles successfully, for some reason at least one of the compared types gets already eta-expanded at some earlier stage, which leads the compiler in the right direction to prove that the subtyping relation does occur.
To be more precise, the 2 alternative working workarounds are:

  • remove the theoretically redundant package prefix, changing
implicit val bar: Bar[pkg.Foo2]

into

implicit val bar: Bar[Foo2]
  • compile both files in a single compiler run (e.g. by renaming Test_2.scala to Test_1.scala when compiling with vulpix)

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@odersky have you managed to experiment with these changes? If there any chance to push this forward?

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This does avoid the fourthTry logic for the cases where tp1 is a generic class without type parameters. But I checked that this logic does not do anything in this case, so I think the change is OK.

return recur(tp1, EtaExpansion(tp2))
fourthTry
}
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14 changes: 14 additions & 0 deletions tests/pos/i16183/Lib_1.scala
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@@ -0,0 +1,14 @@
package pkg

trait Foo1[A]
trait Foo2[A] extends Foo1[A]

trait Bar[F[_]]
object Bar {
implicit val bar: Bar[pkg.Foo2] = ???
}

trait Qux
object Qux {
implicit def qux[F[_]](implicit bar: Bar[F]): F[Qux] = ???
}
6 changes: 6 additions & 0 deletions tests/pos/i16183/Test_2.scala
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@@ -0,0 +1,6 @@
import pkg._

object Test {
implicitly[Foo2[Qux]]
implicitly[Foo1[Qux]]
}