Implicit value can no longer be accessed via implicit conversion

[WojciechMazur]

Community build regression for https://github.com/giiita/refuel (maintainer @giiita)
Based on Open Community Build #1884
Part of #15647 regressions tracker

Compiler version

Works in 3.1.1
Fails in 3.1.2 and 3.1.3
Fails in 3.2.0-RC* and 3.2.1-RC1-bin-20220711-6efd92d-NIGHTLY

Minimized code

trait JsonRowEntry {
  def readAs[E](implicit c: Read[E]): Option[E] = ???
}
trait Read[T]
trait Codec[T] extends Read[T]
trait CodecTypeProjection[C[_]]
object JsonTransform  {
  given SetCodec[T, C[_]: CodecTypeProjection]: scala.Conversion[C[T], C[Set[T]]] = ???
  given SetCodecExp[T, C[_]: CodecTypeProjection](using codec: C[T]): C[Set[T]] = codec
  given Codec[String] = ???
  given CodecTypeProjection[Read] = ???
}

@main def Test() = {
  import JsonTransform.given
  val tree = new JsonRowEntry {}
  tree.readAs[Set[String]]
}

Output

rror] ./test.scala:17:27: No given instance of type Read[Set[String]] was found for parameter c of method readAs in trait JsonRowEntry.
[error] I found:
[error] 
[error]     JsonTransform.SetCodecExp[String, C](
[error]       JsonTransform.given_CodecTypeProjection_Read
[error]     , /* missing */summon[C[String]])
[error] 
[error] But no implicit values were found that match type C[String].
[error]   tree.readAs[Set[String]]
[error]              

Expectation

Should be able to find correct implicit value by usage of implicit conversion

[griggt]

First bad commit is db5956b from #13886

[odersky]

That one is as expected. We now refuse to search implicits for type variables that are unknown, and we know that this will refuse to compile some code that compiled before. The reason for the restriction is that there is a latent danger of unboundedly long implicit searches, in particular as programs grow. That's more important than a small expressiveness win.

[smarter]

Related: #15160

[KacperFKorban]

I found a similar issue in earogov/ordset

Compiled for 3.0.0, ..., 3.1.1
Gives an error for 3.1.2, .., 3.2.0-RC1, 3.2.1-RC1-bin-20220715-d247c37-NIGHTLY

trait Box[E]

trait Domain

def fun[E, D[_] <: Domain](box: Box[E])(implicit domain: D[E]): Unit = {

  val newBox: Box[E] =  ???

  fun(newBox)
}

error:

[error] ./main.scala:11:14: No given instance of type D[E] was found for parameter domain of method fun
[error] 
[error] where:    D is a type variable with constraint <: [_] =>> Domain
[error]   fun(newBox)
[error]              ^

Is this behavior also expected? @odersky

[odersky]

Is this behavior also expected? @odersky

Probably. It looks similar. We know for sure if it is the same commit that broke it.

[KacperFKorban]

The commit that broke it is the same (db5956b), so it probably is also expected.