Finding implicit Tag fails sometimes for generic type argument

[andrzejressel]

I couldn't find a pattern why these errors are happening.

ZIO: 2.0 and 2.1
Scala: 3.3 and 3.4

Example1

https://scastie.scala-lang.org/YYk2KAU0T8e51QTozYJZeg

import zio.ZIOApp
import zio.*

trait SourceOfC

trait ABC[MyC: Tag] {

  def run: ZIO[Any, Any, Any] = {
    val t = (for {
      _ <- ZIO.service[MyC]
      _ <- ZIO.service[SourceOfC] // Comment 1
      p <- useMyC() // Comment 2
    } yield ())
    
    val o = t.provide(sourceOfCLayer, obtainC)

    o
  }

  val sourceOfCLayer: ZLayer[Any, Throwable, SourceOfC] = ???
  val obtainC: ZLayer[SourceOfC, Any, MyC] = ???
  def useMyC(): ZIO[MyC & SourceOfC, Nothing, Unit] = ???
}

Commenting either line 11 or 12 fixes the issue. Replacing MyC with empty trait also fixes it.

Example 2

https://scastie.scala-lang.org/cuewZKmkT8mE0lOldDBcZw

import zio.ZIOApp
import zio.*

trait ABC[MyC: Tag] {

  def run2() = {
    (for {
      _ <- ZIO.service[MyC]
    } yield ()).provide(provideC, anotherLayer) // Removing anotherLayer fixes
  }

  val provideC: ZLayer[Any, Any, MyC] = ???
  val anotherLayer: ZLayer[Any, Any, Unit] = ???

}

Here the existence of Unit layer (like Logging ones) breaks stuff. Making anotherLayer return type non-Unit or extracting MyC to empty trait fixes.

At the beginning I thought it has to do with number of abstract arguments, but that's not the case: https://scastie.scala-lang.org/6h5rekHNSZKybKnOEMYO2A

Error for SEO:

ABC.this.MyC is a type parameter without an implicit Tag!
  could not find implicit value for izumi.reflect.Tag[ABC.this.MyC]. Did you forget to put on a Tag, TagK or TagKK context bound on one of the parameters in ABC.this.MyC? e.g. def x[T: Tag, F[_]: TagK] = ...

[jdegoes]

/bounty $100

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Attempt	Started (GMT+0)	Solution
🟢 @andrzejressel	May 20, 2024, 6:17:56 PM	#8883

[andrzejressel]

Index: core/shared/src/main/scala-3/zio/internal/macros/LayerMacroUtils.scala
IDEA additional info:
Subsystem: com.intellij.openapi.diff.impl.patch.CharsetEP
<+>UTF-8
===================================================================
diff --git a/core/shared/src/main/scala-3/zio/internal/macros/LayerMacroUtils.scala b/core/shared/src/main/scala-3/zio/internal/macros/LayerMacroUtils.scala
--- a/core/shared/src/main/scala-3/zio/internal/macros/LayerMacroUtils.scala	(revision b9f54f5211389cf2fb807c3774f400e7ed336339)
+++ b/core/shared/src/main/scala-3/zio/internal/macros/LayerMacroUtils.scala	(date 1716157106781)
@@ -1,8 +1,9 @@
 package zio.internal.macros
 
-import zio._
-import scala.quoted._
-import scala.compiletime._
+import zio.*
+
+import scala.quoted.*
+import scala.compiletime.*
 import zio.internal.macros.StringUtils.StringOps
 import zio.internal.ansi.AnsiStringOps
 
@@ -61,7 +62,11 @@
         case '{$lhs: ZLayer[i, e, o]} =>
           rhs match {
             case '{$rhs: ZLayer[i2, e2, o2]} =>
-              '{$lhs.++($rhs)}
+              Expr.summon[Tag[o2]] match {
+                case Some(t) =>
+                  '{$lhs.++($rhs)(using $t)}
+                case _ => throw new Error("OptionalImplicit should be always available")
+              }
           }
       }

Seems to fix it, but I hate it. Maybe it's compiler problem?

EDIT: Yeah - it seems that compiler will take the most outer implicit instead of closest one

EDIT2: I've asked on users forum about that: https://users.scala-lang.org/t/scala-3-when-calling-method-with-implicit-value-within-quotes-implicit-in-scope-is-not-found/10003

EDIT3: I believe the message is not from implicitnotfound annotation, but from macro that generates Izumi tag - I'll try to verify it today

[andrzejressel]

/attempt #8834

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