Unused implicit conversion imports are not detected under certain circumstances

[jdrphillips]

Compiler version

3.6.4

Minimized code

package testing

object Syntax {
  implicit def conversion(a: DummyClass): DummyClass = new DummyClass
}

class DummyClass

import Syntax.*

case class CaseClass(field: Boolean)

Note: I could not reproduce this in the REPL

Output

Expectation

The above code should return the following warning:

[warn] -- [E198] Test2.scala:9:14 
[warn] 9 |import Syntax.*
[warn]   |              ^
[warn]   |              unused import
[warn] one warning found

As far as I can tell, this warning behaves correctly for:

• trait
• class
• abstract class
• object
• def

The warning behaves incorrectly for:

• case class
• case object (even trivial ones)

Note also this occurs even if the implicit conversion is completely irrelevant to any types involved, as in the above example

[som-snytt]

3.7 warns.

-- [E198] Unused Symbol Warning: /home/amarki/snips/i22976.scala:6:26 ----------
6 |  implicit def conversion(a: DummyClass): DummyClass = new DummyClass
  |                          ^
  |                          unused explicit parameter
-- [E198] Unused Symbol Warning: /home/amarki/snips/i22976.scala:12:14 ---------
12 |import Syntax.conversion
   |              ^^^^^^^^^^
   |              unused import
there was 1 feature warning; re-run with -feature for details
3 warnings found

[Gedochao]

Was fixed in c65d45d (#20894).
The backport to 3.3 LTS is in progress:

• Backport "In selector check, prefix of reference must match import qualifier" to 3.3 LTS scala3-lts#223

cc @tgodzik @som-snytt