assertion failure when method defined twice

[scabug]

abstract class M
{
    def f: List[M] = Nil
}

object M1 extends M

object M2 extends M
{
    override def f = List(M1)
}

object M3 extends M
{
    override def f = List(M1)
    override def f = List(M1)
}

object M4 extends M
{
    override def f = List(
        M3,
        M2
    )
}

error: method f is defined twice
    override def f = List(M1)
                 ^
Exception in thread "main" java.lang.AssertionError: assertion failed: List(method f, method f)
	at scala.tools.nsc.symtab.Symbols$$Symbol.suchThat(Symbols.scala:1061)
	at scala.tools.nsc.symtab.Types$$$$anonfun$$32.apply(Types.scala:4546)
	at scala.tools.nsc.symtab.Types$$$$anonfun$$32.apply(Types.scala:4545)
	at scala.collection.TraversableLike$$$$anonfun$$map$$1.apply(TraversableLike.scala:240)
	at scala.collection.TraversableLike$$$$anonfun$$map$$1.apply(TraversableLike.scala:240)
	at scala.collection.LinearSeqLike$$class.foreach(LinearSeqLike.scala:88)
	at scala.collection.immutable.List.foreach(List.scala:45)
	at scala.collection.TraversableLike$$class.map(TraversableLike.scala:240)
	at scala.collection.immutable.List.map(List.scala:45)
	at scala.tools.nsc.symtab.Types$$class.lubsym$$1(Types.scala:4545)
	at scala.tools.nsc.symtab.Types$$$$anonfun$$34.apply(Types.scala:4576)
	at scala.tools.nsc.symtab.Types$$$$anonfun$$34.apply(Types.scala:4571)
	at scala.collection.LinearSeqLike$$class.foreach(LinearSeqLike.scala:88)
	at scala.collection.immutable.List.foreach(List.scala:45)
	at scala.tools.nsc.symtab.Types$$class.lub0$$1(Types.scala:4571)
	at scala.tools.nsc.symtab.Types$$class.lub(Types.scala:4595)
	at scala.tools.nsc.symtab.SymbolTable.lub(SymbolTable.scala:13)
	at scala.tools.nsc.symtab.Types$$class.solveOne$$1(Types.scala:4342)
...

[scabug]

Imported From: https://issues.scala-lang.org/browse/SI-2779?orig=1
Reporter: Eric Willigers (ewilligers)

[scabug]

@hubertp said:
Throws assertion in r20082 and 2.7.7.

[scabug]

@odersky said:
(In r20230) Closed #2779. review by community.