Unused warning for NotGiven is not useful

[eejbyfeldt]

Compiler version

3.7.0-RC1

Minimized example

import scala.util.NotGiven

object Test {
  def f[T](a: Int)(using NotGiven[T <:< Int]) = a + 2
}

Output Error/Warning message

-- [E198] Unused Symbol Warning: unused_notgiven.scala:4:25 ----------------------------------
4 |  def f[T](a: Int)(using NotGiven[T <:< Int]) = a + 2
  |                         ^
  |                         unused implicit parameter
1 warning found

Why this Error/Warning was not helpful

The unused warning is not useful as NotGiven is just used for evidence and not expected to be used explicitly.

Suggested improvement

It could be made more helpful if no warning was produced. I belive NotGiven could be excluded here

scala3/compiler/src/dotty/tools/dotc/transform/CheckUnused.scala

Line 607 in d112546

case dd.DummyImplicitClass | dd.SubTypeClass | dd.SameTypeClass => true

with the same motivation as <:< and =:=.

[odersky]

How about other implicits that are only used for type evidence? Things like CanEqual, CanThrow, <:<, =:= ? It seems the unused warning should be disabled for these as well. But how do you tell that? It's impossible to be precise.

Also, what about implicit parameters that are unused in a given method but still need to be present since we foresee an override that would use them?

[som-snytt]

Workaround is

def f[T](@unused a: Int)(using @unused x: NotGiven[T <:< Int]) = println()

or turn on

-Wunused:explicits

but not

-Wunused:implicits

Scala 3 does not parse -Wunused:-implicits to mean "turn off".

CanEqual is already excluded; it excludes context bounds that are "marker" traits, so probably it should exclude any implicit param that is a marker trait.

Also it should dealias more for these checks.

[eejbyfeldt]

Workaround is

def f[T](@unused a: Int)(using @unused x: NotGiven[T <:< Int]) = println()

There reason I just did immediately went for the @unused was that it did not for my particular case and it was not clear why. But now I minimized it and reported it here #23033