Do not reset the GivenOrImplicit flags on implicits without explicit types

Also fix a neg test for i3067. After this modification to
the completion algorithm, implicit definitions without
explicit types stay visible in the implicit scope and hence,
the error of the missing implicit is not reporter. This
is fine, however, since the real error to be fixed is the
lack of an explicit type of the implicit value, not the
lack of the implicit value itself.

Author: anatoliykmetyuk

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

@@ -746,10 +746,8 @@ class Namer { typer: Typer =>
       else bound
     }
 
-  def missingType(sym: Symbol, modifier: String)(implicit ctx: Context): Unit = {
+  def missingType(sym: Symbol, modifier: String)(implicit ctx: Context): Unit =
     ctx.error(s"${modifier}type of implicit definition needs to be given explicitly", sym.sourcePos)
-    sym.resetFlag(GivenOrImplicit)
-  }
 
   /** The completer of a symbol defined by a member def or import (except ClassSymbols) */
   class Completer(val original: Tree)(implicit ctx: Context) extends LazyType with SymbolLoaders.SecondCompleter {

tests/neg/i3067.scala

@@ -10,6 +10,6 @@ object o {
 
   implicit def y = "abc"   // error
 
-  implicit object a extends Test(_ map identity)  // error // error: no implicit argument found
+  implicit object a extends Test(_ map identity)  // error
   implicit object b extends Test(_ map identity) // error // error: cyclic reference
 }