`(using ?)` causes internal error in the incremental compiler

[sasha2048]

Compiler version

3.4.0-RC1

Minimized code

def f(using ?): Unit = {}

Output

C:\somepath> sbt clean
[info] welcome to sbt 1.9.7 (Eclipse Adoptium Java 11.0.21)
[info] loading project definition from C:\somepath\project
[info] loading settings for project root from build.sbt ...
[info] set current project to err (in build file:/C:/somepath/)
[success] Total time: 0 s, completed 3 січ. 2024 р., 18:45:09
C:\somepath> sbt compile
[info] welcome to sbt 1.9.7 (Eclipse Adoptium Java 11.0.21)
[info] loading project definition from C:\somepath\project
[info] loading settings for project root from build.sbt ...
[info] set current project to err (in build file:/C:/somepath/)
[info] Executing in batch mode. For better performance use sbt's shell
[info] compiling 1 Scala source to C:\somepath\target\scala-3.4.0-RC1\classes ...
[error] Internal error in the incremental compiler while compiling C:\somepath\src\main\scala\Main.scala: Unhandled type  of class class dotty.tools.dotc.core.Types$RealTypeBounds
[error] one error found
[error] (Compile / compileIncremental) Compilation failed
[error] Total time: 3 s, completed 3 січ. 2024 р., 18:45:28

Expectation

A meaningful error message.

[nicolasstucki]

This fails here https://github.com/lampepfl/dotty/blob/main/compiler/src/dotty/tools/dotc/sbt/ExtractAPI.scala#L591

[nicolasstucki]

This code currently compiles, but it should probably fail while typing.

The definition is typed as

   def f(using x$1:  >: Nothing <: Any): Unit =
      {
        ()
      }

and it does fail Ycheck after typer.

[bishabosha]

similar to #18649