Weird compilation error about an implicit type is not a valid singleton type, since it is not an immutable path #9290
Description
Minimized code
See: https://github.com/NicolasRouquette/lunatech-scala-2-to-scala3-course
// file: package.scala
// If both lines below are commented, then either form of the implicit/extension compiles OK.
// If both lines are uncommented, then the implicit variant fails to compile.
package org.lunatechlabs.dotty
package object sudoku {}// file: ToplevelDefinitions.scala
package org.lunatechlabs.dotty.sudoku
final case class SudokuField(sudoku: Int)
// This form compiles only when package.scala is commented.
// This form compiles with error when package.scala is uncommented.
implicit class SudokuFieldOps(val sudokuField: SudokuField) extends AnyVal {
def foo: Int = ???
}
// This form compiles OK regardless of whether package.scala is commented or not.
//extension on (sudokuField: SudokuField) {
// def foo: Int = ???
//}
## Output
```scala
[error] -- [E083] Type Error: /opt/local/github.dotty/scala2-to-scala3-course.bug/exercises/src/main/scala/org/lunatechlabs/dotty/sudoku/ToplevelDefinitions.scala:7:0
[error] 7 |implicit class SudokuFieldOps(val sudokuField: SudokuField) extends AnyVal {
[error] |^
[error] |org.lunatechlabs.dotty.sudoku.SudokuFieldOps.type is not a valid singleton type, since it is not an immutable path
[error] one error foundExpectation
All 4 variants should compile without any errors.