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

Author: nicolasstucki

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

@@ -274,6 +274,18 @@ class PostTyper extends MacroTransform with IdentityDenotTransformer  { thisTran
               case tpe => tpe
             }
           )
+        case Import(expr, selectors) =>
+          val exprTpe = expr.tpe
+          def notExists(name: Name): Boolean =
+            name != nme.WILDCARD && !exprTpe.member(name).exists
+          selectors.foreach {
+            case ident @ Ident(name) if notExists(name) =>
+              ctx.warning(s"$name is not a member of ${expr.show}", ident.pos)
+            case Pair(ident @ Ident(name), _) if notExists(name) =>
+              ctx.warning(s"$name is not a member of ${expr.show}", ident.pos)
+            case _ =>
+          }
+          super.transform(tree)
         case tree =>
           super.transform(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