Consecutive calls of MapLike::filterKeys causes StackOverflowError

[scabug]

I'm not sure if this even a bug, or just a feature. However:

This code will throw StackOverflowError

var filtered  = Map("1" -> 2)
for(i <- 1 to 10000) {
  filtered = filtered.filterKeys(s => s != "2")
}
println(filtered.get("1"))

This would not:

 var filtered  = Map("1" -> 2)
 val add = Map("2" -> 1)
 for(i <- 1 to 10000) {
    filtered = filtered.filterKeys(_ != "2") ++ add
 }
println(filtered.get("1"))

This will throw as well:

var filtered  = Map("1" -> 2)
val add = Map()
for(i <- 1 to 10000) {
  filtered = filtered.filterKeys(_ != "2") ++ add
}
println(filtered.get("1"))

[scabug]

Imported From: https://issues.scala-lang.org/browse/SI-9864?orig=1
Reporter: Dmitry Roenko
See #4776, #751

[scabug]

@SethTisue said:
This is a consequence of filterKeys being lazy, so the lambdas just stack up. I would suggest using filter here since there is no need for the lazy behavior.

If it's surprising that filterKeys is lazy, that's covered by #4776 (and, a while earlier, #751). As for the problem that stacking up lazy operations tends to result in eventual stack overflows, that's a pervasive issue in Scala collections and in Scala generally; ultimately we have the lack of tail calls on the JVM to thank.