false positive warning

[mr-git]

Compiler version

3.7.0

Minimized example

@main
def make: IndexedSeq[FalsePositive] =
  for {
    i <- 1 to 2
    given Int = i
    fp = FalsePositive()
  } yield fp

class FalsePositive(using Int):
  def usage(): Unit =
    println(summon[Int])

Output Error/Warning message

[warn] -- [E198] Unused Symbol Warning: /<path>/main/src/main/scala/FalsePositive.scala:5:4 
[warn] 5 |    given Int = i
[warn]   |    ^^^^^^^^^
[warn]   |    unused pattern variable
[warn] one warning found

Why this Error/Warning was not helpful

The message was unhelpful because the given Int is actually used - this is a "false positive".

Suggested improvement

The issue is with how code is structured in for-comprehension. If for-comprehension gets restructured to:

  for {
    i <- 1 to 2
    given Int = i
  } yield FalsePositive() // <=== notice that initialization happens without intermediate variable

The warning is not triggered any more!

[mr-git]

Forgot to mention - the issue is triggered starting with 3.7.0!

3.6.4 compiles both forms just fine, both with and without "-language:experimental.betterFors" flag.

Could it be related to some sugar-improvements for for-comprehensions in 3.7.0? I remember reading about reduction of .map calls...

[som-snytt]

3.6 doesn't track pattern variables. Thanks for the report!

Also thanks for the hint, I'll look at interactions with the different for desugaring; the lint just relies on correct positions.

[som-snytt]

betterFors is enabled under 3.7 with the flag -preview, TIL.

There is no false positive under -preview, curiously enough. I see that eliminates the tupling step, which appears to desugar the same as 3.6.4.

The problem is the desugaring of given Int = i which is not a simple assignment but should be the same as j: Int = i. (The reason it's not is you can't write case given s: Int by analogy to given val s: Int.)