Spurious, positionless overrides-nullary-method warning when extending under separate compilation

[SethTisue]

Scala version: 2.13.11

T.scala

trait T1 {
  def method: Int
}
trait T2 extends T1 {
  def method() = 0
}

C.scala

class C extends T2

Reproduction steps

scalac T.scala
scalac -classpath . C.scala

Result

compiling T.scala produces a correct warning:

T.scala:5: warning: method with a single empty parameter list overrides method without any parameter list
  def method() = 0
      ^
1 warning

but then compiling C.scala produces this:

warning: method with a single empty parameter list overrides method without any parameter list
1 warning

we shouldn't be issuing this warning at all IMO, but also note the lack of position information, which means it's difficult to figure out where it's even coming from

Context

This came up over at scala/scala#10484 , where after a recent change for 2.13.12 the attempt to issue the warning actually crashes the compiler.

[SethTisue]

@som-snytt @lrytz You two want to race to see who fixes it first...?

History: the warning originally landed in scala/scala#8846 (Scala 2.13.3) and then it was moved from namers to refchecks in scala/scala#10345 (Scala 2.13.11).

[som-snytt]

Thanks for the repro! I'll take a look, as I must refresh my memory for namePos of valdef and the Giants aren't playing today.

[som-snytt]

I'm glad I fixed crasher handling in partest at some point.

?! 1 - neg/t12851                                [caught UnsupportedOperationException - Position.point on NoPosition]

[SethTisue]

I thought the same problem might exist for nullary-overrides-nilary ("method without a parameter list overrides a method with a single empty one"), but it doesn't reproduce that way 'round 🤔

[som-snytt]

s/spurious/scurrilous