I found very confusing behaviour for implicit import of scala.Predef.

http://stackoverflow.com/questions/17690183/explain-why-import-scala-predef-string-fails-build

Spec, section 9.1 Compilation units states the following:

Implicitly imported into every compilation unit are, in that order : the package java.lang, the package scala, and the object scala.Predef (§12.5).

But if I open the implementation:

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

method rootImports and scaladoc:

/** 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

}

There is an exception in the comment: if the unit body has an import of Predef among its leading imports.

I think spec should mention this exception.