inference fails on overloaded methods despite non-conflicting signature

[cleishm]

Asked on StackOverflow: https://stackoverflow.com/questions/46652058/scala-type-inference-fails-on-overloaded-methods-despite-non-conflicting-signatu

% scala                                                                     
Welcome to Scala 2.12.3 (Java HotSpot(TM) 64-Bit Server VM, Java 1.8.0_111).
Type in expressions for evaluation. Or try :help.

scala>   trait Op[-Y, -Z, +A, +B] {
     |     def apply(other: (Y, Z)): (A, B)
     |   }
defined trait Op

scala>   implicit class RichTuple2[+A, +B](t: (A, B)) {
     |     def ~~~(other: Int): (A, B) = ???
     |     def ~~~[RA, RB](other: Op[A, B, RA, RB]): (RA, RB) = other.apply(t)
     |   }
defined class RichTuple2

scala>   def swap[A, B] = new Op[A, B, B, A] {
     |     override def apply(other: (A, B)) = (other._2, other._1)
     |   }
swap: [A, B]=> Op[A,B,B,A]

scala> (1, "foo") ~~~ swap
<console>:14: error: overloaded method value ~~~ with alternatives:
  [RA, RB](other: Op[Int,String,RA,RB])(RA, RB) <and>
  (other: Int)(Int, String)
 cannot be applied to (Op[Nothing,Nothing,Nothing,Nothing])
       (1, "foo") ~~~ swap

Removing the unused and non-conflicting method def ~~~(other: Int) results in successful inference and compilation.

Possibly related to #9523

[Jasper-M]

Actually you don't have to try so hard to get to see this beautiful error.

scala> object Foo {
     |   def foo(op: Op[Int,String,String,Int]) = op
     |   def foo(i: Int) = i
     | }
defined object Foo

scala> Foo.foo(swap)
<console>:15: error: overloaded method value foo with alternatives:
  (i: Int)Int <and>
  (op: Op[Int,String,String,Int])Op[Int,String,String,Int]
 cannot be applied to (Op[Nothing,Nothing,Nothing,Nothing])
       Foo.foo(swap)
           ^

This has always been a feature of overloaded methods.

The explanation probably contains terms like "expected type".

[cleishm]

@Jasper-M: you're right, that's a much easier example :)

[SethTisue]

works in Scala 3.3.1