Ambiguity between extension methods and methods reached through conversions

[odersky]

Compiler version

3.4.1

Minimized example

class Tup():
  def app(n: Int): String = "a"

class NT(t: Tup):
  def toTup = t
object NT:
  extension (x: NT)
    def app(n: Int): Boolean = true
  given Conversion[NT, Tup] = _.toTup

def test =
  val nt = new NT(Tup())
  val x = nt.app(3)
  val _: Boolean = x

Output

16 |  val x = nt.app(3)
   |          ^^
   |Found:    (nt : NT)
   |Required: ?{ app: ? }
   |Note that implicit extension methods cannot be applied because they are ambiguous;
   |both given instance given_Conversion_NT_Tup in object NT and object NT provide an extension method `app` on (nt : NT)
   |
   | longer explanation available when compiling with `-explain`
1 error found

Expectation

This should compile, and the method in object NT should be chosen. An extension method available directly should be preferable over a member reachable through implicit conversions. But right now it reports an ambiguity.

[odersky]

Note that we already prefer extensions over conversions in the case where an implicit reference is overloaded.