Unsound conformance from selection of member MatchType

[EugeneFlesselle]

Match type with no cases reduces to unreported ErrorType

Compiler version

3.4.2-RC1

Minimized code

trait T[N]:
  type M = N match
    case 0 => Any // or anything else

val t: T[Double] = new T[Double] {}
val x: t.M = "hello"
val y: Boolean = x // compiles + ClassCastException

We also have the same issue when using a projection:

val z: T[Double]#M = "hello"
val _: Boolean = z // compiles + ClassCastException

Minimised from @Gedochao's #19936 (comment)

[Gedochao]

Doesn't compile on 3.3.3, so it's also a regression...

[EugeneFlesselle]

Doesn't compile on 3.3.3, so it's also a regression...

It was introduced by #18016

[dwijnand]

Doesn't compile on 3.3.3, so it's also a regression...

It looks like it shouldn't compile. So not a regression, "just" a soundness bug.