Explain why import scala.Predef.String fails build

Question

I found very strange issue with Scala import.

I wrote sample class:

package test_scala_predef

object App extends App {

  classOf[T]
  println( "Hello World!" )
}

class T {

}

This class compiles without any errors.

But, if I add

import scala.Predef.String

then I get compilation errors:

[INFO] Compiling 1 source files to /home/uthark/src/_/test_scala_predef/target/classes at 1374028063588
[ERROR] /home/uthark/src/_/test_scala_predef/src/main/scala/test_scala_predef/App.scala:10: error: not found: value classOf
[INFO]   classOf[T]
[INFO]   ^
[ERROR] /home/uthark/src/_/test_scala_predef/src/main/scala/test_scala_predef/App.scala:11: error: not found: value println
[INFO]   println( "Hello World!" )
[INFO]   ^
[ERROR] two errors found

I have an idea, that after I add specific import from scala.Predef, then implict import of scala.Predef._ is not added. But I can't find anything about it in the Scala documentation. Can anyone point me to the relevant section in documentation?

I checked Scala Language Specification (PDF), section 12.5 covering scala.Predef but also didn't find any related.

I use latest stable scala version available (2.10.2 at the moment)

Answer 1

I found the answer in the sources.

https://github.com/scala/scala/blob/master/src/compiler/scala/tools/nsc/typechecker/Contexts.scala:

  /** List of symbols to import from in a root context.  Typically that
   *  is `java.lang`, `scala`, and [[scala.Predef]], in that order.  Exceptions:
   *
   *  - if option `-Yno-imports` is given, nothing is imported
   *  - if the unit is java defined, only `java.lang` is imported
   *  - if option `-Yno-predef` is given, if the unit body has an import of Predef
   *    among its leading imports, or if the tree is [[scala.Predef]], `Predef` is not imported.
   */
  protected def rootImports(unit: CompilationUnit): List[Symbol] = {
    assert(definitions.isDefinitionsInitialized, "definitions uninitialized")

    if (settings.noimports) Nil
    else if (unit.isJava) RootImports.javaList
    else if (settings.nopredef || treeInfo.noPredefImportForUnit(unit.body)) {
      debuglog("Omitted import of Predef._ for " + unit)
      RootImports.javaAndScalaList
    }
    else RootImports.completeList
  }

This answers my question.

The key sentence is this:

Exceptions: ... if the unit body has an import of Predef among its leading imports

Also, in chat on #scala irc it was suggested to create bug in Scala issue tracking system, so I did it. https://issues.scala-lang.org/browse/SI-7672