Implicit argument list modifier affects overload resolution with explicit arguments

[milessabin]

The following fails with an ambiguous overload,

object Test extends App {
  class Foo
  class Bar extends Foo

  def overload(implicit foo: Foo): Unit = {}
  def overload(implicit bar: Bar): Unit = {}

  overload(new Bar)
}
/*
implicit-overload.scala:8: error: ambiguous reference to overloaded definition,
both method overload in object Test of type (implicit bar: Test.Bar)Unit
and  method overload in object Test of type (implicit foo: Test.Foo)Unit
match argument types (Test.Bar)
  overload(new Bar)
  ^
one error found
*/

However, remove the implicit argument list modifier and it compiles as expected,

object Test extends App {
  class Foo
  class Bar extends Foo

  def overload(foo: Foo): Unit = {}
  def overload(bar: Bar): Unit = {}

  overload(new Bar)
}

This is because the logic in Inferencer#isAsSpecific only compares the result types of method types with an implicit argument list.

I think that at least part of this logic is intended to handle implicit method selection rather than overload resolution per se, but is misplaced and should actually be moved to ImplicitSearch#improves where, as well as fixing this bug, we have opportunities to improve implicit method selection based on implicit argument lists.

This bug is also present in Dotty.

[milessabin]

I think that the logic is there in an attempt to bring the behaviour with implicit arguments into line with the behaviour with default arguments.

[odersky]

Note that this will be likely affected by the future requirement to pass implicit arguments using explicitly. I.e. in the future you have to write:

overload.explicitly(new Bar)

I believe that doing this change will pave the way to a natural fix: If f is of a type consisting of one or more alternatives with implicit method types, then f.explicitly is of the type where all implicit method alternatives are turned to explicit methods. Afterwards, normal overloading resolution will do its thing.

[milessabin]

This appears to be fixed. Needs a test case to close.

[milessabin]

Whoops! The issue is fixed in Dotty, not here: scala/scala3#3190.