Fix #1286: Warn on inexistent imports that are not used.

Author: nicolasstucki

src/dotty/tools/dotc/transform/FirstTransform.scala

@@ -160,7 +160,16 @@ class FirstTransform extends MiniPhaseTransform with InfoTransformer with Annota
     ast.Trees.flatten(reorderAndComplete(trees)(ctx.withPhase(thisTransformer.next)))
 
   override def transformOther(tree: Tree)(implicit ctx: Context, info: TransformerInfo) = tree match {
-    case tree: Import => EmptyTree
+    case tree: Import =>
+      val exprTpe = tree.expr.tpe
+      tree.selectors.foreach {
+        case ident @ Ident(name) if name != nme.WILDCARD && !exprTpe.member(name).exists =>
+            ctx.warning(s"$name is not a member of ${tree.expr.show}", ident.pos)
+        case Pair(ident @ Ident(name), _) if name != nme.WILDCARD && !exprTpe.member(name).exists =>
+            ctx.warning(s"$name is not a member of ${tree.expr.show}", ident.pos)
+        case _ =>
+      }
+      EmptyTree
     case tree: NamedArg => transform(tree.arg)
     case tree => tree
   }

tests/pos/i1286.scala

@@ -0,0 +1,12 @@
+import scala.idontexist // warn
+import scala.io.Idontexist // warn
+
+import scala.io
+import io.Idontexist2 // warn
+
+import scala.io.{ AnsiColor, Idontexist3 } // warn
+
+import scala.io.{ Idontexist4 => Foo } // warn
+import scala.io.{ Idontexist5 => _ } // warn
+
+object Test