Extension method fails to construct with `<overloaded XX> does not take parameters`

[soronpo]

When an extension method is defined for a typeclass more than once but with different arguments, it fails to construct if the method argument has type arguments. If we were to remove the type arguments, the method construction works.

Compiler version

v3.1.0-RC2

Minimized code

class MyType()
trait Candidate[R]
given Candidate[MyType] with {}
class Fuzzy[W]()
class Fuzzy1()
class Bear()

extension [L](lhs: L)(using Candidate[L])
  def +[RW](rhs: Fuzzy[RW]): Unit = {}
  def +(rhs: Bear): Unit = {}
  def -(rhs: Fuzzy1): Unit = {}
  def -(rhs: Bear): Unit = {}

val works = MyType() - Fuzzy1()
val fails = MyType() + Fuzzy[1]()

Output

[error] 15 |val fails = MyType() + Fuzzy[1]()
[error]    |            ^^^^^^^^^^
[error]    |      value + is not a member of MyType.
[error]    |      An extension method was tried, but could not be fully constructed:
[error]    |
[error]    |          +()    failed with
[error]    |
[error]    |              value +: <overloaded +> does not take parameters
[error] one error found

Expectation

No error.

[soronpo]

Workaround: move the using clause to the method parts:

extension [L](lhs: L) 
  def +[RW](rhs: Fuzzy[RW])(using Candidate[L]): Unit = {}
  def +(rhs: Bear)(using Candidate[L]): Unit = {}
  def -(rhs: Fuzzy1)(using Candidate[L]): Unit = {}
  def -(rhs: Bear)(using Candidate[L]): Unit = {}

[ValeriePe]

This issue was picked for the Issue Spree 18 of July 5th which takes place in a week from now. @griggt @gagandeepkalra will be working on it. If you have any insight into the issue or guidance on how to fix it, please leave it here.