Parameterless extension method without parentheses is not called unless explicitly qualified by this

[sjbiaga]

Compiler version

3.4.2-RC1

Minimized code

object Test:
  import C.f
  class C:
    def m(): Unit = println("m()")
    def test: Unit = for { i <- 1 to 2 } do { this.f; println(i); f }
 object C:
     extension(self: C) { def f: Unit = self.m() }

Output

m()
1
m()
2

Expectation

The second (or any) unqualified call to f should use C.this for self.

m()
1
m()
m()
2
m()

Nevertheless, if the first this.f is replaced with unqualified call to f, the compiler issues a warning:

missing argument list for value of type Test.C => Unit

[odersky]

Yes, that's as specced. Extension methods require an explicit receiver.

[sjbiaga]

Thanks,
I wasn't decided between using empty parentheses (which is unnecessary) or qualifying with this (which is ugly).

[som-snytt]

There was an old scala 2 ticket for the same intuition for implicits (by paulp).

I agree that it's too bad there is no warning about the mistake. The error in "statement position" is very helpful.

I had to remind myself why -Wnonunit-statement is also unhelpful. (The warning is ignored by IterableOnce et al.) As a mere lint, it could special-case foreach.