scala
Implicit conversion never applied but influences type inference
reproduction steps
https://stackoverflow.com/questions/62415172/mere-presence-of-implicit-conversion-makes-the-program-compile-despite-never-bei/62426914
using Scala 2.13.2,
scala> class Bar
class Bar
scala> def f[F[_], A](v: F[A]) = v
def f[F[_], A](v: F[A]): F[A]
scala> implicit def barToList(b: Bar): List[Int] = List(42)
def barToList(b: Bar): List[Int]
scala> f(new Bar)
val res1: Any = Bar@56881196
problem
I would expect implicit conversion would result in
scala> f[List, Int](new Bar)
val res2: List[Int] = List(42)
where the following is inferred
F[_] = List
A = Int
instead of actual
F[_] = Any
A = Nothing
Examining the output of -Xprint:typer shows the following implicit conversion never happend
f(barToList(new Bar()))
It works in Dotty 0.24.0-RC1:
➜ ~ dotr
Starting dotty REPL...
scala> class Bar
// defined class Bar
scala> def f[F[_], A](v: F[A]) = v
def f[F[_$1], A](v: F[A]): F[A]
scala> implicit def barToList(b: Bar): List[Int] = List(42)
def barToList(b: Bar): List[Int]
scala> f(new Bar)
val res1: List[Int] = List(42)
Just f("": Any) in scala 2 but not 3.
Inferring Any was almost linted under https://github.com/scala/bug/issues/9248. "It makes for a great test of how far we could push abide."
To explain a bit - just testing implicit conversions accumulated type constraints.
It also turns into an error with -Yno-predef
Dotty progressed. I think there is a ticket to improve more specific than F[A].
scala-cli compile --server=false -S 3 t12044.scala
-- [E007] Type Mismatch Error: /home/amarki/snips/t12044.scala:11:6 ------------
11 | f(new Bar)
| ^^^^^^^
|Found: Bar
|Required: F[A]
|
|where: A is a type variable
| F is a type variable with constraint <: [_] =>> Any
|
|Note that implicit conversions were not tried because the result of an implicit conversion
|must be more specific than F[A]
|
| longer explanation available when compiling with `-explain`
[[syntax trees at end of typer]] // /home/amarki/snips/t12044.scala
package <empty> {
import language.implicitConversions
class Bar() extends Object() {}
final lazy module val Test: Test = new Test()
final module class Test() extends Object(), App { this: Test.type =>
def f[F[_ >: Nothing <: Any] >: Nothing <: Any, A >: Nothing <: Any](v: F[A]
): F[A] = v
implicit def barToList(b: Bar): List[Int] = List.apply[Int]([42 : Int]*)
Console.println(
{
Test.f[[_] =>> Any, Any](new Bar())
}
)
}
}
1 error found
Compilation failed
With -Yno-predef, Scala 2 seems to be stuck.
scala-cli compile --server=false -S 2 t12044.scala
/home/amarki/snips/t12044.scala:11: error: type mismatch;
found : Bar
required: List[A]
f(new Bar)
^
[[syntax trees at end of typer]] // t12044.scala
package <empty> {
import scala.language.implicitConversions;
class Bar extends scala.AnyRef {
def <init>(): Bar = {
Bar.super.<init>();
()
}
};
object Test extends AnyRef with App {
def <init>(): Test.type = {
Test.super.<init>();
()
};
def f[F[_] >: [_]Nothing <: [_]Any, A](v: F[A]): F[A] = v;
implicit def barToList(b: Bar): List[Int] = scala.`package`.List.apply[Int](42);
scala.Console.println(f(new Bar()))
}
}
1 error
Compilation failed
Reopening: as pointed out by @joroKr21 this issue is about the limitation in inference.
the problem is that type variable bounds were collected from all implicit conversions instead of only the one that is applicable
I noticed, the "fixed in Scala 3" is true for 3.0, but current Scala 3 doesn't compile the code.
class Bar
class T {
def f[F[_], A](v: F[A]) = v
implicit def barToList(b: Bar): List[Int] = List(42)
def t = f(new Bar)
}
gives
-- [E007] Type Mismatch Error: /Users/luc/code/scala/scala13/sandbox/T.scala:5:12
5 | def t = f(new Bar)
| ^^^^^^^
|Found: Bar
|Required: F[A]
|
|where: A is a type variable
| F is a type variable with constraint <: [_] =>> Any
|
|Note that implicit conversions were not tried because the result of an implicit conversion
|must be more specific than F[A]
probably due to https://github.com/scala/scala3/issues/15670#issuecomment-1184279375
