Scala 2 regression: free-form names in backticks crash at runtime

[j-mie6]

Compiler version

With 3.0.0 (or 3.0.1-RC1 on scastie)

Minimized code

object foo {
  val `<x>` = "hello!"
  def main(args: Array[String]): Unit = println(`<x>`)
}

Output

In scastie this gives:

java.lang.ClassFormatError: Method "<x>" in class foo has illegal signature "()Ljava/lang/String;"
	at java.lang.ClassLoader.defineClass1(Native Method)
	at java.lang.ClassLoader.defineClass(ClassLoader.java:757)
	at java.security.SecureClassLoader.defineClass(SecureClassLoader.java:142)
	at java.net.URLClassLoader.defineClass(URLClassLoader.java:468)
	at java.net.URLClassLoader.access$100(URLClassLoader.java:74)
	at java.net.URLClassLoader$1.run(URLClassLoader.java:369)
	at java.net.URLClassLoader$1.run(URLClassLoader.java:363)
	at java.security.AccessController.doPrivileged(Native Method)
	at java.net.URLClassLoader.findClass(URLClassLoader.java:362)
	at sbt.internal.ManagedClassLoader.findClass(ManagedClassLoader.java:102)
	at sbt.internal.BottomClassLoader.lambda$findClass$0(BottomClassLoader.java:56)
	at sbt.internal.ClassLoadingLock.withLock(ClassLoadingLock.java:29)
	at sbt.internal.BottomClassLoader.findClass(BottomClassLoader.java:51)
	at sbt.internal.BottomClassLoader.loadClass(BottomClassLoader.java:66)
	at java.lang.ClassLoader.loadClass(ClassLoader.java:352)
	at java.lang.Class.forName0(Native Method)
	at java.lang.Class.forName(Class.java:348)
	at sbt.Run.getMainMethod(Run.scala:150)
	at sbt.Run.execute$1(Run.scala:81)
	at sbt.Run.$anonfun$runWithLoader$5(Run.scala:110)
	at scala.runtime.java8.JFunction0$mcV$sp.apply(JFunction0$mcV$sp.java:23)
	at sbt.util.InterfaceUtil$$anon$1.get(InterfaceUtil.scala:17)
	at sbt.TrapExit$App.run(TrapExit.scala:258)
	at java.lang.Thread.run(Thread.java:748)

I've also seen it give something like:

java.lang.ClassFormatError: Illegal method name "<expr>"

Expectation

Since (fun) names like this worked fine in 2.13, I'd expect them to work fine in 3 too... perhaps they used to be name mangled?

This form in particular using <name> is useful for giving names to grammar rules in parsers.

[smarter]

Huh, name encoding kicks in for val `<` , val `<>` , val `<a` , and val `a>` , but not val `<a>` , I don't know why.

[j-mie6]

perhaps it's similar to an xml literal?

[smarter]

Looks like a poor attempt at dealing with <init>/<clinit>: https://github.com/lampepfl/dotty/blob/f1252d8f83ace1561c8f53e132ef94c75be9d287/compiler/src/dotty/tools/dotc/core/Names.scala#L344-L346