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

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

@@ -275,6 +275,19 @@ class PostTyper extends MacroTransform with IdentityDenotTransformer  { thisTran
               case tpe => tpe
             }
           )
+        case Import(expr, selectors) =>
+          val exprTpe = expr.tpe
+          def checkIdent(ident: Ident): Unit = {
+            val name = ident.name.asTermName.encode
+            if (name != nme.WILDCARD && !exprTpe.member(name).exists && !exprTpe.member(name.toTypeName).exists)
+              ctx.error(s"${ident.name} is not a member of ${expr.show}", ident.pos)
+          }
+          selectors.foreach {
+            case ident: Ident                 => checkIdent(ident)
+            case Thicket((ident: Ident) :: _) => checkIdent(ident)
+            case _                            =>
+          }
+          super.transform(tree)
         case tree =>
           super.transform(tree)
       }

tests/neg/i1286.scala

@@ -0,0 +1,16 @@
+import scala.idontexist // error
+import scala.io.Idontexist // error
+
+import scala.io
+import io.Idontexist2 // error
+
+import scala.io.{ AnsiColor, Idontexist3 } // error
+
+import scala.io.{ Idontexist4 => Foo } // error
+import scala.io.{ Idontexist5 => _ } // error
+
+import scala.language.dynamics
+import scala.language.noAutoTupling
+import scala.language.idontexist // error
+
+object Test

tests/repl/imports.check

@@ -15,4 +15,14 @@ scala> buf += xs
    |       
 scala> buf ++= xs
 res1: scala.collection.mutable.ListBuffer[Int] = ListBuffer(1, 2, 3)
+scala> import util.foo
+-- Error: <console> ----------------------------------------------------------------------------------------------------
+8 |import util.foo
+  |            ^^^
+  |            foo is not a member of util
+scala> import util.foo.bar
+-- [E008] Member Not Found Error: <console> ----------------------------------------------------------------------------
+8 |import util.foo.bar
+  |       ^^^^^^^^
+  |       value `foo` is not a member of util.type - did you mean `util.Left`?
 scala> :quit