Crash with FlexibleTypes when nullifying classes with Higher-kinded Type Parameters

[HarrisL2]

Compiler version

3.7.3-RC2

This bug was found while porting community build to compile with -Yexplicit-nulls and minimized.

Minimized code

package unsafe

class Foo[F[_]] {
  def accept[A](arg: F[A]): Nothing = ???
}

scalac local/unsafe.scala -d local/out -classpath "local/out"

import unsafe.*

val x: List[Map[String, Int]] = ???
val foo = Foo[List]()
val y = foo.accept[Any](x)

scalac local/safe.scala -Yexplicit-nulls -d local/out -classpath "local/out"

Output

Exception in thread "main" java.lang.AssertionError: assertion failed: Should not flexify HKTypeLambda(List(A), List(TypeBounds(TypeRef(ThisType(TypeRef(NoPrefix,module class scala)),class Nothing),TypeRef(ThisType(TypeRef(NoPrefix,module class scala)),class Any))), AppliedType(TypeRef(ThisType(TypeRef(NoPrefix,module class immutable)),class List),List(TypeRef(ThisType(TypeRef(NoPrefix,module class scala)),class Any))))

  exception occurred while typechecking local/safe.scala

  An unhandled exception was thrown in the compiler.
  Please file a crash report here:
  https://github.com/scala/scala3/issues/new/choose
  For non-enriched exceptions, compile with -Xno-enrich-error-messages.


     while compiling: local/safe.scala
        during phase: typer
                mode: Mode(ImplicitsEnabled,SafeNulls)
     library version: version 2.13.16
    compiler version: version 3.8.0-RC1-bin-SNAPSHOT-nonbootstrapped-git-f4df13e
            settings: -Yexplicit-nulls true 

Expectation

The compiler should be able to handle Higher Kinded Types in when nullifying unsafe files