Fix handling divergent implicits

If we hit a divergent implicit during a contextual search, we should
continue with a type-based search. Only ambiguities are globally fatal.
The removed test that prevented this logic was introduced when I experimented
with fatal divergence. It was left in by oversight.

Author: odersky

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

@@ -991,7 +991,7 @@ trait Implicits { self: Typer =>
         else implicitScope(wildProto).eligible
       searchImplicits(eligible, contextual).recoverWith {
         failure => failure.reason match {
-          case (_: AmbiguousImplicits) | (_: DivergingImplicit) => failure
+          case _: AmbiguousImplicits => failure
           case _ => if (contextual) bestImplicit(contextual = false) else failure
         }
       }

tests/pos/implicit-divergent.scala

@@ -0,0 +1,12 @@
+class Foo[T]
+object Foo {
+  implicit def intFoo: Foo[Int] = ???
+}
+
+object Test {
+  implicit def genFoo[T](implicit f: Foo[T => T]): Foo[T] = ???
+
+  def test: Unit = {
+    implicitly[Foo[Int]]
+  }
+}