Adopt scala's scheme for root import hiding

scalac hides a root import from Predef if there is an eplicit Predef import.
We now do the same (previously we did this only if the overriding import undefined
something, using a `x => _` syntax). To avoid cycles and races one had to be very careful
not to force import symbols too early, so we now compare the name before the symbol proper.

All this is likely temporary - the comment of ImportInfo#unimported points to a different,
more systematic solution.

Author: odersky

compiler/src/dotty/tools/dotc/core/Definitions.scala

@@ -747,6 +747,7 @@ class Definitions {
     else if (ctx.settings.YnoPredef.value) StaticRootImportFns
     else StaticRootImportFns ++ PredefImportFns
 
+  lazy val ShadowableImportNames = Set("Predef", "DottyPredef").map(_.toTermName)
   lazy val RootImportTypes = RootImportFns.map(_())
 
   /** Modules whose members are in the default namespace and their module classes */

compiler/src/dotty/tools/dotc/typer/ImportInfo.scala

@@ -15,17 +15,20 @@ object ImportInfo {
     val selectors = untpd.Ident(nme.WILDCARD) :: Nil
     def expr = tpd.Ident(refFn())
     def imp = tpd.Import(expr, selectors)
-    new ImportInfo(imp.symbol, selectors, isRootImport = true)
+    new ImportInfo(imp.symbol, selectors, None, isRootImport = true)
   }
 }
 
 /** Info relating to an import clause
- *  @param   sym        The import symbol defined by the clause
- *  @param   selectors  The selector clauses
- *  @param   rootImport true if this is one of the implicit imports of scala, java.lang
- *                      or Predef in the start context, false otherwise.
+ *  @param   sym          The import symbol defined by the clause
+ *  @param   selectors    The selector clauses
+ *  @param   symNameOpt   Optionally, the name of the import symbol. None for root imports.
+ *                        Defined for all explicit imports from ident or select nodes.
+ *  @param   isRootImport true if this is one of the implicit imports of scala, java.lang,
+ *                        scala.Predef or dotty.DottyPredef in the start context, false otherwise.
  */
-class ImportInfo(symf: => Symbol, val selectors: List[untpd.Tree], val isRootImport: Boolean = false)(implicit ctx: Context) {
+class ImportInfo(symf: => Symbol, val selectors: List[untpd.Tree],
+                 symNameOpt: Option[TermName], val isRootImport: Boolean = false)(implicit ctx: Context) {
 
   lazy val sym = symf
 
@@ -105,11 +108,11 @@ class ImportInfo(symf: => Symbol, val selectors: List[untpd.Tree], val isRootImp
    */
   lazy val unimported: Symbol = {
     lazy val sym = site.termSymbol
-    val hasMaskingSelector = selectors exists {
-      case Thicket(_ :: Ident(nme.WILDCARD) :: Nil) => true
-      case _ => false
+    def maybeShadowsRoot = symNameOpt match {
+      case Some(symName) => defn.ShadowableImportNames.contains(symName)
+      case None => false
     }
-    if (hasMaskingSelector && defn.RootImportTypes.exists(_.symbol == sym)) sym
+    if (maybeShadowsRoot && defn.RootImportTypes.exists(_.symbol == sym)) sym
     else NoSymbol
   }
 

compiler/src/dotty/tools/dotc/typer/Namer.scala

@@ -373,8 +373,13 @@ class Namer { typer: Typer =>
   }
 
   /** A new context that summarizes an import statement */
-  def importContext(sym: Symbol, selectors: List[Tree])(implicit ctx: Context) =
-    ctx.fresh.setImportInfo(new ImportInfo(sym, selectors))
+  def importContext(imp: Import, sym: Symbol)(implicit ctx: Context) = {
+    val impNameOpt = imp.expr match {
+      case ref: RefTree => Some(ref.name.asTermName)
+      case _            => None
+    }
+    ctx.fresh.setImportInfo(new ImportInfo(sym, imp.selectors, impNameOpt))
+  }
 
   /** A new context for the interior of a class */
   def inClassContext(selfInfo: DotClass /* Should be Type | Symbol*/)(implicit ctx: Context): Context = {
@@ -423,7 +428,7 @@ class Namer { typer: Typer =>
         setDocstring(pkg, stat)
         ctx
       case imp: Import =>
-        importContext(createSymbol(imp), imp.selectors)
+        importContext(imp, createSymbol(imp))
       case mdef: DefTree =>
         val sym = enterSymbol(createSymbol(mdef))
         setDocstring(sym, origStat)

compiler/src/dotty/tools/dotc/typer/Typer.scala

@@ -1588,7 +1588,7 @@ class Typer extends Namer with TypeAssigner with Applications with Implicits wit
       case (imp: untpd.Import) :: rest =>
         val imp1 = typed(imp)
         buf += imp1
-        traverse(rest)(importContext(imp1.symbol, imp.selectors))
+        traverse(rest)(importContext(imp, imp1.symbol))
       case (mdef: untpd.DefTree) :: rest =>
         mdef.removeAttachment(ExpandedTree) match {
           case Some(xtree) =>