New warning with 2.13.17: a type was inferred to be kind-polymorphic `Nothing` to conform to `F[_]`

[durban]

Reproduction steps

Scala version: 2.13.17

package com.example

trait Thing[F[_]]

object Repro {
  def foo[F[_]](i: Int)(implicit F: Thing[F]): F[Unit] = ???
  def bar[F[_] : Thing](i: Int): F[Unit] = foo(i) // <- warning here
}

[warn] .../Repro.scala:...:44: a type was inferred to be kind-polymorphic `Nothing` to conform to `F[_]`
[warn] Applicable -Wconf / @nowarn filters for this warning: msg=<part of the message>, cat=lint-infer-any, site=com.example.Repro.bar
[warn]   def bar[F[_] : Thing](i: Int): F[Unit] = foo(i)
[warn]                                            ^

Problem

I don't see a reason for this warning, but in case there is a good reason, I would definitely like to understand it.

Note: if I do foo[F](i) instead of foo(i), the warning disappears.

Note, that this is probably the same warning as reported here: https://contributors.scala-lang.org/t/scala-2-13-17-release-planning/6994/20

[durban]

A similar but different warning is also new with 2.13.17:

package com.example

trait Base1
class MyClass1[F[_]](val obj: MyClass2[F], val fa: F[Unit]) extends Base1

sealed trait Base2
class MyClass2[F[_]] extends Base2 {
  def unit: F[Unit] = ???
}

object Repro {
  def foo[F[_]](obj: Base2): Base1 = obj match {
    case obj: MyClass2[_] =>
      new MyClass1(obj, obj.unit) // <- warning here
  }
}

[warn] .../Repro.scala:...:7: a type was inferred to be kind-polymorphic `Any` to conform to `F[_]`
[warn] Applicable -Wconf / @nowarn filters for this warning: msg=<part of the message>, cat=lint-infer-any, site=com.example.Repro.foo
[warn]       new MyClass1(obj, obj.unit)
[warn]       ^

Note, that if I remove the second argument of MyClass1's constructor (i.e., if I do new MyClass1(obj)), the warning disappears.

I don't know, if the reason for this is the same as for the original report.

[gaeljw]

I have the same error with a concrete case when upgrading to 2.13.17, minified below:

//> using scala 2.13.17
//> using dep com.softwaremill.sttp.client::core:2.3.0

import sttp.client._

implicit val sttpBackend: SttpBackend[Identity, Nothing, NothingT] = ???

val someUrl = "http://example.com"
val response: Identity[Response[Either[String, String]]] = basicRequest.post(uri"$someUrl").send()

$ scala-cli -Xlint:infer-any test.sc

[warn] a type was inferred to be kind-polymorphic `Nothing` to conform to `F[_]`
[warn] val response: Identity[Response[Either[String, String]]] = basicRequest.post(uri"$someUrl").send()
[warn]                                                            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

[gaeljw]

For the record, here are the changes I had to make in my codebase:

- val response: Identity[Response[Either[String, String]]] = basicRequest.post(uri"$credentialsWsUrl").send()
+ val response: Identity[Response[Either[String, String]]] = basicRequest.post(uri"$credentialsWsUrl").send[Identity]()

- implicit val sttpBackendMocked: SttpBackendStub[Identity, Nothing, Nothing] = SttpBackendStub.synchronous
+ implicit val sttpBackendMocked: SttpBackendStub[Identity, Nothing, Nothing] = SttpBackendStub.synchronous[Nothing]

[som-snytt]

@gaeljw It's a lint. I'd expect people to either -Xlint:-infer-any or equivalent @nowarn, rather than change code -- unless the change is warranted or desired.

[gaeljw]

That's a possibility as well, for sure. I personally prefer to have as less as possible nowarn. I assume warnings (treated as errors in our codebases) are here for a good reason and prefer to make the compiler happy with additional code in such cases.

I'll admit that like OP I don't really understand what's behind the warning in this case though.