Do not choose implicits that are ambiguous references

Before this commit, implicit_ambiguous_ref.scala failed to compile
because the implict search returned `x`, we now always try to typecheck
a reference to an identifier before returning it from the implicit search.

Author: smarter

src/dotty/tools/dotc/typer/Implicits.scala

@@ -490,7 +490,7 @@ trait Implicits { self: Typer =>
             untpd.Apply(untpd.TypedSplice(generated), untpd.TypedSplice(argument) :: Nil),
             pt)
         val generated1 = adapt(generated, pt)
-        lazy val shadowing =
+        val shadowing =
           typed(untpd.Ident(ref.name) withPos pos.toSynthetic, funProto)
                (nestedContext.addMode(Mode.ImplicitShadowing).setExploreTyperState)
         def refMatches(shadowing: Tree): Boolean =

tests/run/implicit_ambiguous_ref.scala

@@ -0,0 +1,19 @@
+object A {
+  val x: Int = 1
+}
+
+abstract class LowPriorityB {
+  implicit val y: String = "y"
+}
+
+object B extends LowPriorityB {
+  implicit val x: String = "x"
+}
+
+object Test {
+  import A._, B._
+  def main(args: Array[String]): Unit = {
+    // B.x is an ambiguous ref, so LowPriorityB.y should be used
+    println(implicitly[String])
+  }
+}