Compiler ignores instance of typeclass and tries to derive it instead

Regression found in Open CB #4758 for wangzaixiang/wjson Part of the 3.2.1-RC1 regressions tracker https://github.com/lampepfl/dotty/issues/15949

There seem to be 2 reasons why code now fails to compile:

1. In Scala 3.2.0 compiler used defined instance of given [T:JsValueMapper]: JsValueMapper[Map[String, T]] in 3.2.1 it tries to derive type class instead using given [T: JsValueMapper, CC[x] <: IterableOps[x, CC, CC[x]]]: Conversion[CC[T], JsArray]. See AST below
2. While deriving Map[String, Int] TypeTree.of[T].symbol where T=Tuple2[String, Int] returns NoSymbol

Compiler version

3.2.1-RC1

Minimized code

// test.scala
import scala.collection.IterableOps

enum JsValue:
    case JsNumber(value: Double|Long)
    case JsString(value: String)
    case JsArray(elements: Seq[JsValue])
    case JsObject(fields: Map[String, JsValue])
    
object JsValue:
  given [T: JsValueMapper]: Conversion[T, JsValue] = ???
  given [T: JsValueMapper, CC[x] <: IterableOps[x, CC, CC[x]]]: Conversion[CC[T], JsArray] = ???

trait JsValueMapper[T]:
  def toJson(t: T): JsValue

object JsValueMapper:
  inline given[T](using deriving.Mirror.ProductOf[T]): JsValueMapper[T] = ${JsValueMapperMacro.generateImpl[T]}

  given JsValueMapper[Int] = ???
  given JsValueMapper[String] = ???
  given [T:JsValueMapper]: JsValueMapper[Map[String, T]] = ???

extension [T: JsValueMapper](obj: T)
  inline def toJson: JsValue = summon[JsValueMapper[T]].toJson(obj)

@main def Test = 
  val works = Map("a"->1,"b"->2,"c"->3).toJson
  val fails = Map("a"->1,"b"->2,"c"->3): JsValue // error

// macro.scala
import scala.quoted.*
import scala.deriving.*

object JsValueMapperMacro:
  def generateImpl[T: Type](using Quotes): Expr[JsValueMapper[T]] =
    import quotes.reflect.*
    import JsValueMapper.*
    val sym = TypeTree.of[T].symbol // no symbol for Tuple2[String,Int]
    val module = Ref(sym.companionModule)
    ???

Output

[error] java.lang.AssertionError: assertion failed
[error]         at scala.runtime.Scala3RunTime$.assertFailed(Scala3RunTime.scala:11)
[error]         at scala.quoted.runtime.impl.QuotesImpl$reflect$Ref$.apply(QuotesImpl.scala:435)
[error]         at scala.quoted.runtime.impl.QuotesImpl$reflect$Ref$.apply(QuotesImpl.scala:434)
[error]         at JsValueMapperMacro$.generateImpl(macro.scala:9)
[error] 
[error]   val fails = Map("a"->1,"b"->2,"c"->3): JsValue // error
[error]               ^

AST in Scala 3.2.0

val fails: wjson.JsValue = 
   wjson.JsValue.given_Conversion_T_JsValue[Map[String, Int]](
     wjson.JsValueMapper.given_JsValueMapper_Map[Int](
       wjson.JsValueMapper.given_JsValueMapper_Int
     )
   ).apply(
     Map.apply[String, Int](
       [ArrowAssoc[String]("a").->[Int](1),
         ArrowAssoc[String]("b").->[Int](2)
       ,ArrowAssoc[String]("c").->[Int](3) : (String, Int)]*
     )
   ):wjson.JsValue

AST in Scala 3.2.1-RC1

val fails: wjson.JsValue = 
  wjson.JsValue.given_Conversion_CC_JsArray[(String, Int), 
    scala.collection.immutable.Iterable
  ](
    wjson.JsValueMapper.given_JsValueMapper_T[(String, Int)](
      new scala.runtime.TupleMirror(2).$asInstanceOf[
        
          scala.deriving.Mirror.Product{
            MirroredMonoType = (String, Int); 
              MirroredType = (String, Int)
            ; MirroredLabel = ("Tuple2" : String); 
              MirroredElemTypes = (String, Int)
            ; MirroredElemLabels = (("_1" : String), ("_2" : String))
          }
        
      ]
    )
  ).apply(
    Map.apply[String, Int](
      [ArrowAssoc[String]("a").->[Int](1),
        ArrowAssoc[String]("b").->[Int](2)
      ,ArrowAssoc[String]("c").->[Int](3) : (String, Int)]*
    )
  ):wjson.JsValue

Expectation

Should compile

[markehammons]

@odersky I’m not really the expert here, but I prefer the behavior of 3.2.1 here compared to 3.2.0. implicit search precedence is already really complicated, and one more exception that can change precedence so drastically makes the rules even harder to grasp.

Just in my opinion.

[markehammons]

926d5bd1b1d11fdf39db7aaac53584c136aaf1cd is the commit that caused the change between 3.2.0 and 3.2.1. Double checked that its parent commit still behaves the way the author expects, and it does.