Extension method with default argument fails to construct

[soronpo]

Compiler version

v3.1.1-RC2

Minimized code

https://scastie.scala-lang.org/MiSpvFT0SKygDNEJdezOzQ

class Foo

extension (foo : Foo)
  def patch(arg : List[Int], arg2 : Int = 0) : Unit = {}
  def patch(arg : Int) : Unit = patch(List(arg))

Output

value patch is not a member of Playground.Foo.
An extension method was tried, but could not be fully constructed:

    Playground.patch()

Expectation

No error.

[odersky]

The problem comes from a combination of overloading, currying, and default arguments. It has nothing to do with extension methods. For instance:

class Foo

object Test {
  def patch2(foo: Foo)(arg : List[Int], arg2 : Int = 0) : Unit = {}
  def patch2(foo: Foo)(arg : Int) : Unit = patch2(foo)(List(arg))
}

fails with

-- [E134] Type Error: i14278a.scala:5:43 ---------------------------------------
5 |  def patch2(foo: Foo)(arg : Int) : Unit = patch2(foo)(List(arg))
  |                                           ^^^^^^
  |None of the overloaded alternatives of method patch2 in object Test with types
  | (foo: Foo)(arg: Int): Unit
  | (foo: Foo)(arg: List[Int], arg2: Int): Unit
  |match arguments ((foo : Foo))
1 error found

Scala 2.13 also fails here:

14278a.scala:5: error: ambiguous reference to overloaded definition,
both method patch2 in object Test of type (foo: Foo)(arg: Int): Unit
and  method patch2 in object Test of type (foo: Foo)(arg: List[Int], arg2: Int): Unit
match argument types (Foo)
  def patch2(foo: Foo)(arg : Int) : Unit = patch2(foo)(List(arg))
                                           ^
1 error

Maybe the advice (requirement?) should be that overloaded curried methods (which includes extension methods) cannot have default arguments.

[soronpo]

Or the advice could be to use implicit classes, since we don't have the problem there. It's incorrect to compare Scala 2.13 according to how extension methods are desugared. Scala 2.13 has implicit classes and that is what extension methods are meant to replace. From user perspective I do not care about the desugaring. I care about expressiveness.

[som-snytt]

I don't think default args should break this case. If one alternative is applicable, then it is selected; if more than one is applicable, then alternatives requiring default args are discarded. Edit: changed in #14775