What does "reflective access of structural type member method should be enabled..." warning mean in Scala?

The warning actually tells where to look in the documentation for an explanation:

Test.scala:9: warning: reflective access of structural type member method y should be enabled
by making the implicit value language.reflectiveCalls visible.
This can be achieved by adding the import clause 'import scala.language.reflectiveCalls'
or by setting the compiler option -language:reflectiveCalls.
See the Scala docs for value scala.language.reflectiveCalls for a discussion
why the feature should be explicitly enabled.

Referenced Scaladoc entry (be sure to click the |> arrow to the left to expand the documentation entry).