Regression in type inference since 3.0.0

[senia-psm]

Compiler version

3.0.2, 3.0.1

Code below compiles in 3.0.0 (and 2.12-2.13), but fails compilation in 3.0.2 and 3.0.1

Minimized code

Note that even replacing

val a: ZIO[Int, Nothing, Symbol]  = Get ~> c(CA())

with

val a: ZIO[Int, Nothing, Symbol]  = ???

or moving suite from top level to object or removing useless WithTransformation will fix compilation.

trait ZIO[-A, +B, +C]

trait Spec[-A, +B]

def suite[R, E, T](specs: Spec[R, E]*): Spec[R, E] = ???

trait HttpRequest

trait RequestContext
trait RouteResult

case class CA()

object CA {
  implicit val aEv: Ev[CA] = ???
}

trait CB

trait Ev[T]

object CB {
  implicit def apply[T](a: T)(implicit ev: Ev[T]): CB = ???
}

def c(b: CB): RequestContext => Option[RouteResult] = ???

object RouteDefaultRunnableSpec {
  def assertM[R, E, A](effect: ZIO[R, E, A]): ZIO[R, E, String] = ???

  def testM[R, E](assertion: => ZIO[R, E, String]): Spec[R, E] = ???

  val Get: HttpRequest = ???

  implicit class WithTransformation(request: HttpRequest) {
    def ~>(f: HttpRequest => HttpRequest): HttpRequest = f(request)
  }

  implicit class WithRoute(request: HttpRequest) {
    def ~>(route: RequestContext => Option[RouteResult]): ZIO[Int, Nothing, Symbol] = ???
  }

  def spec =
    suite(
      testM {
        val a: ZIO[Int, Nothing, Symbol]  = Get ~> c(CA())
        val res: ZIO[Int, Nothing, String] = assertM(a)
        res
      },
    )
}

Output

[error] -- [E007] Type Mismatch Error: /file/path/something.scala:48:8 
[error] 48 |        res
[error]    |        ^^^
[error]    |        Found:    (res : ZIO[Int, Nothing, String])
[error]    |        Required: ZIO[Any, Any, String]

Expectation

Code should be compilable. Compilation of this code should not depend on val implementation if type of val is specified manually.

[senia-psm]

Originally found here: senia-psm/zio-test-akka-http#77

[smarter]

This regressed in #12519

[smarter]

... but it's actually already fixed in 3.1.0-RC1 (and -RC2), apparently by #13361