False positive extension method will never be selected

[abosshard]

Compiler version

3.5.0 and later

Minimized code

//> using scala "3.6.4"

class A:
  def f(i: Int) = ()

extension (self: A)
  def f() = println("extension") // [warn]: Extension method f will never be selected from type A

A().f()

Output

[warn] ./Example.sc:7:6
[warn] Extension method f will never be selected from type A
[warn] because A already has a member with the same name and compatible parameter types.
[warn]  def f() = println("extension") // [warn]: Extension method f will never be selected from type A
[warn]      ^
extension

Expectation

No warning, because the extension method is actually selected in this case.

[abosshard]

I think the issue should have the labels "area:linting" and "area:extension-methods", but it seems I don't have the right to assign them. Thanks!

[som-snytt]

There is a subtle distinction between "lints" "turned on" by -W and the rare warnings issued unconditionally. (Migration warnings are conditional because dependent on user config.) (Not sure if the compiler team wants to maintain the subtle distinction.)

Probably this one should be a lint, but a scalafix rule could warn more about "collisions" due to extension methods.

There is an existing test for a "nilary" extension method but not for "nullary". Thanks for the report.