Keep classpath used to run the compiler separate from compilation classpath.

[adriaanm]

A program should not be able to see the classes of the compiler that's compiling it unless the compiler's classes are explicitly added to the compilation (user) classpath. Currently, they all leak through by default (you have one chance to opt-out by not setting usejavacp): https://github.com/lampepfl/dotty/blob/master/src/dotty/tools/dotc/config/PathResolver.scala#L227

This is not a problem in sbt (it manages the classpath itself), and you might consider it a feature in the REPL, but it's pretty complicated for build tools to navigate the maze of settings. Would be good to clean this up early on.

[gkossakowski]

An example of a problem: https://issues.scala-lang.org/browse/SI-8358

[smarter]

@adriaanm : is this fixed/planned to be fixed in Scala 2.12 ?

[adriaanm]

No, but we were just talking about tackling this in 2.13 earlier this week!

[SethTisue]

the context being, this is important if we want to support alternate and/or slimmer “standard” libraries as in Martin's proposal at scala/slip#31

[smarter]

Starting with Java 9, we cannot inspect the Java bootclasspath anymore, so I think the problem basically solved itself :). (usejavacp still exists, but I think that's OK since it's not enabled by default)