SI-2405 Confer implicit privileges to renamed imports.

Yin and yang would be pleased: A fix in two parts.

1. Use the name of the imported symbol, rather than the alias, in the generated `Select(qual, name)` tree.
2. Do the opposite in `isQualifyingImplicit`, which performs one part of the shadowing check.

But there is still work to do. The second part of the shadowing check, `nonImplicitSynonymInScope`,
fails to notice this case (irrespective of aliased imports).

  // Expecting shadowing #2. Alas, none is cast!
  object Test1 {
    object A { implicit val x: Int = 1 }
    import A.x
    def x: Int = 0
    implicitly[Int]
  }

I'm hitching the residual problem to SI-4270's wagon.

Author: retronym

src/compiler/scala/tools/nsc/typechecker/Contexts.scala

@@ -598,16 +598,16 @@ trait Contexts { self: Analyzer =>
      *  it is accessible, and if it is imported there is not already a local symbol
      *  with the same names. Local symbols override imported ones. This fixes #2866.
      */
-    private def isQualifyingImplicit(sym: Symbol, pre: Type, imported: Boolean) =
+    private def isQualifyingImplicit(name: Name, sym: Symbol, pre: Type, imported: Boolean) =
       sym.isImplicit &&
       isAccessible(sym, pre) &&
       !(imported && {
-        val e = scope.lookupEntry(sym.name)
+        val e = scope.lookupEntry(name)
         (e ne null) && (e.owner == scope)
       })
 
     private def collectImplicits(syms: List[Symbol], pre: Type, imported: Boolean = false): List[ImplicitInfo] =
-      for (sym <- syms if isQualifyingImplicit(sym, pre, imported)) yield
+      for (sym <- syms if isQualifyingImplicit(sym.name, sym, pre, imported)) yield
         new ImplicitInfo(sym.name, pre, sym)
 
     private def collectImplicitImports(imp: ImportInfo): List[ImplicitInfo] = {
@@ -621,7 +621,7 @@ trait Contexts { self: Analyzer =>
           var impls = collect(sels1) filter (info => info.name != from)
           if (to != nme.WILDCARD) {
             for (sym <- imp.importedSymbol(to).alternatives)
-              if (isQualifyingImplicit(sym, pre, imported = true))
+              if (isQualifyingImplicit(to, sym, pre, imported = true))
                 impls = new ImplicitInfo(to, pre, sym) :: impls
           }
           impls

src/compiler/scala/tools/nsc/typechecker/Implicits.scala

@@ -554,7 +554,11 @@ trait Implicits {
 
       val itree = atPos(pos.focus) {
         if (info.pre == NoPrefix) Ident(info.name)
-        else Select(gen.mkAttributedQualifier(info.pre), info.name)
+        else {
+          // SI-2405 Not info.name, which might be an aliased import
+          val implicitMemberName = info.sym.name
+          Select(gen.mkAttributedQualifier(info.pre), implicitMemberName)
+        }
       }
       printTyping("typedImplicit1 %s, pt=%s, from implicit %s:%s".format(
         typeDebug.ptTree(itree), wildPt, info.name, info.tpe)

test/files/neg/t2405.check

@@ -0,0 +1,8 @@
+t2405.scala:6: warning: imported `y' is permanently hidden by definition of method y
+		import A.{x => y}
+                          ^
+t2405.scala:8: error: could not find implicit value for parameter e: Int
+		implicitly[Int]
+                          ^
+one warning found
+one error found

test/files/neg/t2405.scala

@@ -0,0 +1,10 @@
+object A { implicit val x: Int = 1 }
+
+// Expecting shadowing #1
+object Test2 {
+	{
+		import A.{x => y}
+		def y: Int = 0
+		implicitly[Int]
+	}
+}

test/files/pos/t2405.scala

@@ -0,0 +1,23 @@
+object A { implicit val x: Int = 1 }
+
+// Problem as stated in the ticket.
+object Test1 {
+	import A.{x => y}
+	implicitly[Int]
+}
+
+// Testing for the absense of shadowing #1.
+object Test2 {
+	import A.{x => y}
+	val x = 2
+	implicitly[Int]
+}
+
+// Testing for the absense of shadowing #2.
+object Test3 {
+	{
+		import A.{x => y}
+		def x: Int = 0
+		implicitly[Int]
+	}
+}