Type inference problem with dependent method types

[odersky]

Minimized example

The test is a version of i8802.scala, with an explicit implicit argument.

trait Foo[A1, B1] {
  type Out
}

object Test {

  type Bar[A2]

  def unit: Bar[Unit] = ???
  def product[A3, B3](fst: Bar[A3], snd: Bar[B3])(implicit foo: Foo[A3, B3]): Bar[foo.Out] = ???

  implicit def foo[A4]: Foo[A4, Unit] { type Out = A4 } = ???

  def check[A5](bar: Bar[A5])(a: A5): Unit = {}

  check(product(unit, unit)) // ok  
  check(product(unit, unit)(summon[Foo[Unit, Unit]]))(()) // error if implicit argument is given explicitly
}

Output

-- [E007] Type Mismatch Error: ../pos/i8802a.scala:17:27 -----------------------
17 |  check(product(unit, unit)(summon[Foo[Unit, Unit]]))(()) // error
   |        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |        Found:    Test.Bar[Unit]
   |        Required: Test.Bar[(Foo[Unit, Unit] & Singleton)#Out]

longer explanation available when compiling with `-explain`
-- [E007] Type Mismatch Error: ../pos/i8802a.scala:17:54 -----------------------
17 |  check(product(unit, unit)(summon[Foo[Unit, Unit]]))(()) // error
   |                                                      ^^
   |                             Found:    Unit
   |                             Required: (Foo[Unit, Unit] & Singleton)#Out

Expectation

Should compile