Using the “Prolog in Scala” to find available type class instances

This can be done (at least in some cases) with compiler internals

import scala.language.experimental.macros
import scala.reflect.internal.util
import scala.reflect.macros.{blackbox, contexts}

object Macros {

  def allImplicits[A]: List[String] = macro impl[A]

  def impl[A: c.WeakTypeTag](c: blackbox.Context): c.Tree = {
    import c.universe._

    val context = c.asInstanceOf[contexts.Context]
    val global: context.universe.type = context.universe
    val analyzer: global.analyzer.type = global.analyzer
    val callsiteContext = context.callsiteTyper.context

    val tpA = weakTypeOf[A]

    val search = new analyzer.ImplicitSearch(
      tree = EmptyTree.asInstanceOf[global.Tree],
      pt = tpA.asInstanceOf[global.Type],
      isView = false,
      context0 = callsiteContext.makeImplicit(reportAmbiguousErrors = false),
      pos0 = c.enclosingPosition.asInstanceOf[util.Position]
    )

    q"${search.allImplicits.map(_.tree.symbol.toString).distinct}"
  }
}

allImplicits[CanFoo[_, String]]
// List(value canFooBoolString, value canFooDblString, value canFooIntString)

Tested in 2.13.0.

Still working in 2.13.10.

Scala 3 implementation is similar

import dotty.tools.dotc.typer.{Implicits => dottyImplicits}
import scala.quoted.{Expr, Quotes, Type, quotes}

inline def allImplicits[A]: List[String] = ${impl[A]}

def impl[A: Type](using Quotes): Expr[List[String]] = {
  import quotes.reflect.*
  given c: dotty.tools.dotc.core.Contexts.Context =
    quotes.asInstanceOf[scala.quoted.runtime.impl.QuotesImpl].ctx

  val typer = c.typer

  val search = new typer.ImplicitSearch(
    TypeRepr.of[A].asInstanceOf[dotty.tools.dotc.core.Types.Type],
    dotty.tools.dotc.ast.tpd.EmptyTree,
    Position.ofMacroExpansion.asInstanceOf[dotty.tools.dotc.util.SourcePosition].span
  )

  def eligible(contextual: Boolean): List[dottyImplicits.Candidate] =
    if contextual then
      if c.gadt.isNarrowing then
        dotty.tools.dotc.core.Contexts.withoutMode(dotty.tools.dotc.core.Mode.ImplicitsEnabled) {
          c.implicits.uncachedEligible(search.wildProto)
        }
      else c.implicits.eligible(search.wildProto)
    else search.implicitScope(search.wildProto).eligible


  def implicits(contextual: Boolean): List[dottyImplicits.SearchResult] =
    eligible(contextual).map(search.tryImplicit(_, contextual))

  val contextualImplicits = implicits(true)
  val nonContextualImplicits = implicits(false)
  val contextualSymbols = contextualImplicits.map(_.tree.symbol)
  val filteredNonContextual = nonContextualImplicits.filterNot(sr => contextualSymbols.contains(sr.tree.symbol))

  val implicitStrs = (contextualImplicits ++ filteredNonContextual).collect {
    case success: dottyImplicits.SearchSuccess => success.tree.asInstanceOf[ImplicitSearchSuccess].tree.show
  }

  Expr(implicitStrs)
}

trait CanFoo[T, U]

object CanFoo {
  given canFooIntString: CanFoo[Int, String] with {}
  given canFooDblString: CanFoo[Double, String] with {}
  given canFooBoolString: CanFoo[Boolean, String] with {}
  given canFooIntSym: CanFoo[Int, Symbol] with {}
  given canFooDblSym: CanFoo[Double, Symbol] with {}
  given canFooBoolSym: CanFoo[Boolean, Symbol] with {}
}

allImplicits[CanFoo[_, String]] 
//"CanFoo.canFooIntString", "CanFoo.canFooBoolString", "CanFoo.canFooDblString"

Scala 3.2.0.