Inconsistency with partial application/currying between extension methods and functions

[Tylerwbrown]

Minimized example

extension (x: Int)
  def add(y: Int): Int = x + y

def newFunction: Int => Int = 4.add

object Main extends App:
  println(newFunction(1))

Output

compiler error:

value add is not a member of Int
An extension method was tried, but could not be fully constructed:
  {
    def $anonfun(y: Int): Int = extension_add(4)(y)
    closure($anonfun)
  }

Expectation

5

Explanation

There are 3 other similar cases in which this works as expected, so the above example seems like the odd one out.

extension (x: Int)
  def add: Int => Int = y => x + y

def newFunction: Int => Int = 4.add

def add: Int => Int = y => 4 + y

def newFunction: Int => Int = add

def add(x: Int): Int = x + 4

def newFunction: Int => Int = add

The first two are similar in that both add functions have a return type of Int => Int.

The interesting bit is that the third one works as expected (converting a function defined with (x: Int): Int into Int => Int, but this same conversion doesn't take place inside the context of an extension.

I don't know how extensions are actually implemented, and this might be expected behavior, which is why I didn't categorize this as a bug, just figured I'd mention since it seemed odd to me.