Hint about forbidden combination of implicit values and conversions

compiler/src/dotty/tools/dotc/reporting/messages.scala

@@ -2554,7 +2554,8 @@ class MissingImplicitArgument(
     pt: Type,
     where: String,
     paramSymWithMethodCallTree: Option[(Symbol, tpd.Tree)] = None,
-    ignoredInstanceNormalImport: => Option[SearchSuccess]
+    ignoredInstanceNormalImport: => Option[SearchSuccess],
+    ignoredConvertibleImplicits: => Iterable[TermRef]
   )(using Context) extends TypeMsg(MissingImplicitArgumentID), ShowMatchTrace(pt):
 
   arg.tpe match
@@ -2743,8 +2744,18 @@ class MissingImplicitArgument(
         // show all available additional info
         def hiddenImplicitNote(s: SearchSuccess) =
           i"\n\nNote: ${s.ref.symbol.showLocated} was not considered because it was not imported with `import given`."
+        def showImplicitAndConversions(imp: TermRef, convs: Iterable[TermRef]) =
+          i"\n- ${imp.symbol.showDcl}${convs.map(c => "\n    - " + c.symbol.showDcl).mkString}"
+        def noChainConversionsNote(ignoredConvertibleImplicits: Iterable[TermRef]): Option[String] =
+          Option.when(ignoredConvertibleImplicits.nonEmpty)(
+            i"\n\nNote: implicit conversions are not automatically applied to arguments of using clauses. " +
+            i"You will have to pass the argument explicitly.\n" +
+            i"The following implicits in scope can be implicitly converted to ${pt.show}:" +
+            ignoredConvertibleImplicits.map { imp => s"\n- ${imp.symbol.showDcl}"}.mkString
+          )
         super.msgPostscript
         ++ ignoredInstanceNormalImport.map(hiddenImplicitNote)
+            .orElse(noChainConversionsNote(ignoredConvertibleImplicits))
             .getOrElse(ctx.typer.importSuggestionAddendum(pt))
 
   def explain(using Context) = ""

compiler/src/dotty/tools/dotc/typer/Implicits.scala

@@ -923,7 +923,34 @@ trait Implicits:
           // example where searching for a nested type causes an infinite loop.
           None
 
-    MissingImplicitArgument(arg, pt, where, paramSymWithMethodCallTree, ignoredInstanceNormalImport)
+    def allImplicits(currImplicits: ContextualImplicits): List[ImplicitRef] =
+      if currImplicits.outerImplicits == null then currImplicits.refs
+      else currImplicits.refs ::: allImplicits(currImplicits.outerImplicits)
+
+    /** Whether the given type is for an implicit def that's a Scala 2 implicit conversion */
+    def isImplicitDefConversion(typ: Type): Boolean = typ match {
+      case PolyType(_, resType) => isImplicitDefConversion(resType)
+      case mt: MethodType => !mt.isImplicitMethod && !mt.isContextualMethod
+      case _ => false
+    }
+
+    def ignoredConvertibleImplicits = arg.tpe match
+      case fail: SearchFailureType =>
+        if (fail.expectedType eq pt) || isFullyDefined(fail.expectedType, ForceDegree.none) then
+          // Get every implicit in scope and try to convert each
+          allImplicits(ctx.implicits)
+            .view
+            .map(_.underlyingRef)
+            .distinctBy(_.denot)
+            .filter { imp =>
+              !isImplicitDefConversion(imp.underlying)
+                && imp.symbol != defn.Predef_conforms
+                && viewExists(imp, fail.expectedType)
+            }
+        else
+          Nil
+
+    MissingImplicitArgument(arg, pt, where, paramSymWithMethodCallTree, ignoredInstanceNormalImport, ignoredConvertibleImplicits)
   }
 
   /** A string indicating the formal parameter corresponding to a  missing argument */

tests/neg/i16453.check

@@ -0,0 +1,45 @@
+-- [E172] Type Error: tests/neg/i16453.scala:21:19 ---------------------------------------------------------------------
+21 |  summon[List[Int]] // error
+   |                   ^
+   |                 No given instance of type List[Int] was found for parameter x of method summon in object Predef
+-- [E172] Type Error: tests/neg/i16453.scala:23:21 ---------------------------------------------------------------------
+23 |  summon[Option[Int]] // error
+   |                     ^
+   |No given instance of type Option[Int] was found for parameter x of method summon in object Predef
+   |
+   |Note: implicit conversions are not automatically applied to arguments of using clauses. You will have to pass the argument explicitly.
+   |The following implicits in scope can be implicitly converted to Option[Int]:
+   |- final lazy given val baz3: Char
+   |- final lazy given val bar3: Int
+-- [E172] Type Error: tests/neg/i16453.scala:24:26 ---------------------------------------------------------------------
+24 |  implicitly[Option[Char]] // error
+   |                          ^
+   |No given instance of type Option[Char] was found for parameter e of method implicitly in object Predef
+   |
+   |Note: implicit conversions are not automatically applied to arguments of using clauses. You will have to pass the argument explicitly.
+   |The following implicits in scope can be implicitly converted to Option[Char]:
+   |- final lazy given val baz3: Char
+-- [E172] Type Error: tests/neg/i16453.scala:25:20 ---------------------------------------------------------------------
+25 |  implicitly[String] // error
+   |                    ^
+   |No given instance of type String was found for parameter e of method implicitly in object Predef
+   |
+   |Note: implicit conversions are not automatically applied to arguments of using clauses. You will have to pass the argument explicitly.
+   |The following implicits in scope can be implicitly converted to String:
+   |- final lazy given val baz3: Char
+-- [E172] Type Error: tests/neg/i16453.scala:35:16 ---------------------------------------------------------------------
+35 |  summon[String] // error
+   |                ^
+   |No given instance of type String was found for parameter x of method summon in object Predef
+   |
+   |Note: implicit conversions are not automatically applied to arguments of using clauses. You will have to pass the argument explicitly.
+   |The following implicits in scope can be implicitly converted to String:
+   |- implicit val baz2: Char
+-- [E172] Type Error: tests/neg/i16453.scala:36:25 ---------------------------------------------------------------------
+36 |  implicitly[Option[Int]] // error
+   |                         ^
+   |No given instance of type Option[Int] was found for parameter e of method implicitly in object Predef
+   |
+   |Note: implicit conversions are not automatically applied to arguments of using clauses. You will have to pass the argument explicitly.
+   |The following implicits in scope can be implicitly converted to Option[Int]:
+   |- implicit val bar2: Int

tests/neg/i16453.scala

@@ -0,0 +1,37 @@
+import scala.language.implicitConversions
+
+trait Foo { type T }
+
+// This one is irrelevant, shouldn't be included in error message
+given irrelevant: Long = ???
+
+/** Use Scala 3 givens/conversions */
+def testScala3() = {
+  given c1[T]: Conversion[T, Option[T]] = ???
+  given c2[F <: Foo](using f: F): Conversion[f.T, Option[f.T]] = ???
+  given Conversion[Char, String] = ???
+  given Conversion[Char, Option[Int]] = ???
+
+  given foo: Foo with
+    type T = Int
+  given bar3: Int = 0
+  given baz3: Char = 'a'
+
+  // This should get the usual error
+  summon[List[Int]] // error
+
+  summon[Option[Int]] // error
+  implicitly[Option[Char]] // error
+  implicitly[String] // error
+}
+
+/** Use Scala 2 implicits */
+def testScala2() = {
+  implicit def toOpt[T](t: T): Option[T] = ???
+  implicit def char2Str(c: Char): String = ???
+  implicit val bar2: Int = 1
+  implicit val baz2: Char = 'b'
+
+  summon[String] // error
+  implicitly[Option[Int]] // error
+}