3.9.0 regression: new Cyclic reference error in the code that worked before

[MateuszKubuszok]

Compiler version

3.9.0

Minimized code

lib.scala:

package repro

trait Outer {
  trait InnerSupport { this: Companion.type =>
    def foo: Unit = ()
  }
  val Companion: CompanionTrait
  trait CompanionTrait extends InnerSupport { this: Companion.type => }
}

user.test.scala:

package repro
class User(c: Outer)

scala-cli compile --test --scala 3.9.0 .   # Cyclic reference involving trait CompanionTrait
scala-cli compile --test --scala 3.8.4 .   # Compiles fine

Output

Compiling project (test, Scala 3.9.0, JVM (22))
Error compiling project (test, Scala 3.9.0, JVM (22))
Error: Unexpected error when compiling repro-final_088de9e3c3-test: dotty.tools.dotc.core.CyclicReference: Cyclic reference involving trait CompanionTrait

 Run with -explain-cyclic for more details.

Compilation failed

Expectation

Compiling project (Scala 3.9.0, JVM (22))
Compiled project (Scala 3.9.0, JVM (22))
Compiling project (test, Scala 3.9.0, JVM (22))
Compiled project (test, Scala 3.9.0, JVM (22))

Context: I bumped Scala 3 version to 3.9.0 on Kindlings in this PR. The code worked before, as far as I can tell it is correct - if I compile the macro, I can run it against 3.9.0 (e.g. I bumped Chimney to use that newer version of Hearth yesterday, and moved Chimney to 3.9.0 and it works). Kindlings however fails, and apparently it fails in its macro-extensions definition. This is minimization of
the pattern that occurs there which should work since 3.3.8 till 3.8.4 and fail on 3.9.0.

[belamenso]

Bisecting the reproduction (needs two-pass separate compilation: compile lib.scala first, then user.test.scala against its TASTy), I found that

• on 3.9.0 it was introduced in e8b3e2d (backport of Erase more aliases of Unit as result type to void. #26252)
• on main it was introduced in Erase more aliases of Unit as result type to void. #26252 and disappeared (first clean commit) on 3adfcbd

The "fix" from 3adfcbd seems accidental.

[Gedochao]

Thanks for the bisection!