fix #2749: support implicit resolution of implicit function types

Author: liufengyun

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

@@ -123,7 +123,9 @@ object Implicits {
             record("discarded eligible")
             false
           }
-          else NoViewsAllowed.isCompatible(normalize(ref, pt), pt)
+          else
+            NoViewsAllowed.isCompatible(normalize(ref, pt), pt) ||
+              NoViewsAllowed.isCompatible(ref, pt)  // `pt` could be an implicit function type, check i2749
         }
       }
 

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

@@ -1640,6 +1640,7 @@ class Typer extends Namer with TypeAssigner with Applications with Implicits wit
         if (defn.isImplicitFunctionType(pt) &&
             xtree.isTerm &&
             !untpd.isImplicitClosure(xtree) &&
+            !ctx.mode.is(Mode.ImplicitShadowing) &&
             !ctx.isAfterTyper)
           makeImplicitFunction(xtree, pt)
         else xtree match {

tests/pos/i2749.scala

@@ -0,0 +1,7 @@
+object Test {
+  val f: implicit (implicit Int => Char) => Boolean = ???
+  implicit val n: Int = 3
+  implicit val g: implicit Int => Char = ???
+
+  f : Boolean
+}