Duplicate method name&signature involving overloading, generics, AnyVal & anonymous classes

[dwijnand]

class Bippy[A](val value: A) extends AnyVal

trait Foo[A] {
  def m(x: A): Int
}

trait Bar[A] extends Foo[A] {
  def m(x: A): Int
}

object Main {
  def create[A] = new Bar[Bippy[A]] { def m(x: Bippy[A]) = 0 }

  def main(args: Array[String]): Unit = create[Int]
}

java.lang.ClassFormatError: Duplicate method name&signature in class file Main$$anon$1
	at java.lang.ClassLoader.defineClass1(Native Method)
	at java.lang.ClassLoader.defineClass(ClassLoader.java:763)
	at java.security.SecureClassLoader.defineClass(SecureClassLoader.java:142)
	at java.net.URLClassLoader.defineClass(URLClassLoader.java:467)
	at java.net.URLClassLoader.access$100(URLClassLoader.java:73)
	at java.net.URLClassLoader$1.run(URLClassLoader.java:368)
	at java.net.URLClassLoader$1.run(URLClassLoader.java:362)
	at java.security.AccessController.doPrivileged(Native Method)
	at java.net.URLClassLoader.findClass(URLClassLoader.java:361)
	at java.lang.ClassLoader.loadClass(ClassLoader.java:424)
	at java.lang.ClassLoader.loadClass(ClassLoader.java:357)
	at Main$.create(f.scala:12)
	at Main$.main(f.scala:14)
	at Main.main(f.scala)

Scala 2.12.3

There's a number of changes that make the error go away. But making it non-anonymous makes it a compile error:

class BippyBar[A] extends Bar[Bippy[A]] {
  def m(x: Bippy[A]) = 0
}

error: bridge generated for member method m: (x: Bippy[A])Int in class BippyBar
which overrides method m: (x: A)Int in trait Bar
clashes with definition of the member itself;
both have erased type (x: Object)Int
  def m(x: Bippy[A]) = 0
      ^
one error found

[dwijnand]

The unminimised original is typelevel/cats#1802.

[dwijnand]

There's history on these types of issues in #6260.

[eed3si9n]

What's the expectation here? Are you asking that anonymous classes should be treated the same way as the named classes, and fail at compile time?

[dwijnand]

Yes.