experimental.modularity breaks as-named infix type

[TomasMikula]

Compiler version

3.5.0

Minimized code

// used only as phantoms
sealed trait JsString
sealed trait JsNumber

infix sealed trait as[J, S]
object as:
  case object Str extends (JsString as String)
  case object Num extends (JsNumber as Double)

def test[J, S](
  repr: J as S, 
  value: S,
): Unit =
  ()

Output

Compiling without any compiler flags succeeds:

scala compile test.scala

Compiling with -source:future also succeeds:

scala compile -source:future test.scala

Compiling with -language:experimental.modularity fails:

% scala compile -source:future  -language:experimental.modularity test.scala
Compiling project (Scala 3.5.0, JVM (21))
[error] ./test.scala:7:37
[error] ',' or ')' expected, but identifier found
[error]   case object Str extends (JsString as String)
[error]                                     ^^
[error] ./test.scala:8:37
[error] ',' or ')' expected, but identifier found
[error]   case object Num extends (JsNumber as Double)
[error]                                     ^^
[error] ./test.scala:11:11
[error] ',' or ')' expected, but identifier found
[error]   repr: J as S, 
[error]           ^^
Error compiling project (Scala 3.5.0, JVM (21))
Compilation failed

Expectation

The modularity feature should not break existing syntax.

[Gedochao]

Let's track this under #21769