I figured this out. It's another effect of a new behavior in 2.12: Eta-expansion of zero-argument method values is deprecated Imagine:
def f(): String = "foo" def g(fn: () => String) = ??? g(f) works in 2.11 without warning. It generates a warning in 2.12, because it wants you to explicitly make a function from the method reference: g(() => f). It will maybe be an error in 2.13. But, this affects implicit resolution. Some of the implicits that power SparkContext.sequenceFile() need to change to be vals of type () => WritableConverter[T], not methods that return WritableConverter[T]. I'm working through this and other deprecated items in 2.12 and preparing more 2.11-compatible changes that allow these to work cleanly in 2.12. On Fri, Sep 15, 2017 at 11:21 AM Sean Owen <so...@cloudera.com> wrote: > I'm working on updating to Scala 2.12, and, have hit a compile error in > Scala 2.12 that I'm strugging to design a fix to (that doesn't modify the > API significantly). If you "./dev/change-scala-version.sh 2.12" and > compile, you'll see errors like... > > [error] > /Users/srowen/Documents/Cloudera/spark/core/src/test/scala/org/apache/spark/FileSuite.scala:100: > could not find implicit value for parameter kcf: () => > org.apache.spark.WritableConverter[org.apache.hadoop.io.IntWritable] > [error] Error occurred in an application involving default arguments. > [error] val output = sc.sequenceFile[IntWritable, Text](outputDir) > > Clearly implicit resolution changed a little bit in 2.12 somehow. I > actually don't recall seeing this error before, so might be somehow related > to 2.12.3, but not sure. > > As you can see the implicits that have always existed and been imported > and should apply here don't seem to be found. > > If anyone is a Scala expert and could glance at this, you might help save > me a lot of puzzling. >
