False positive [E198] Unused Symbol Warning

[durban]

Compiler version

3.3.7

Minimized code

class Foo {
  def foo(): Any = {
    var i = 0 // <- warning here
    val f = () => i += 1
    f
  }
}

Output

[warn] -- [E198] Unused Symbol Warning: .../ThunkSuite.scala:3:8
[warn] 3 |    var i = 0
[warn]   |        ^
[warn]   |        unused local definition

Expectation

No warning (as i is not unused; in fact, if I remove i, that results in an error).

Related: typelevel/cats-effect#4519

[som-snytt]

Probably duplicates #24265 which also uses a variable i. Maybe try a different name. Just kidding, I'll verify duplication soon. Thanks for the report!

[som-snytt]

The current warning now says:

-- [E198] Unused Symbol Warning: i24280.scala:5:8 ------------------------------
5 |    var i = 0 // <- warning here
  |        ^
  |        local variable was mutated but not read
1 warning found

which is accurate. That change from last summer is on 3.7.4-RC2.

The ticket has some discussion. #23704

It does not warn for i = i + 1 (as a limitation, I think).