False "right-associative extension method cannot start with using clause" error

[TomasMikula]

Compiler version

3.7.4

Minimized code

https://scastie.scala-lang.org/BdGQYl82ROmShM8VigUKXQ

extension (s: String)
  def foo_::(using DummyImplicit): String = s

Output

right-associative extension method cannot start with using clause

Expectation

The restriction mentioned in the error message should not apply here. It should only apply to binary operators.

Quoted from https://docs.scala-lang.org/scala3/reference/contextual/right-associative-extension-methods.html (emphasis mine):

An extension method is treated as a right-associative operator (as in SLS §6.12.3) if it has a name ending in :, and is immediately followed by a single explicit term parameter

[som-snytt]

The documentation might be more clear:

An extension method that is a right-associative operator must have an initial term parameter list with a single parameter that is not a contextual parameter. Leading type parameters are allowed, but cannot be provided explicitly in an infix application.

In the current doc, "treated" has the force of "accepted". The words were previously tweaked, but without the intention of introducing wiggle room. In particular, "otherwise, it is an error" and not, "otherwise, it is not taken as a right-associative operator."

It's allowed to write def f_::[A] because it can't be written infix in Scala 3. (Scala 2 introduced x op [T]y, to illustrate.)

[som-snytt]

This is morally equivalent to writing an extension that cannot be invoked using extension syntax, where the message suggests not using an extension method.

OTOH, "multiarg infix" is still accepted for "normal" application:

X op ("hello", "world")

What is the word for normalizing left-associative? "Sinistronormative." Infix with using is outlawed.

What is X op_: ("hello", "world")? Then is there a more compelling case for accepting X.op_:? Well yes, it's ok for def op_:[A]. That is, it's OK for an argument supplied implicitly.