Don't handle Function types separately when filtering overloaded methods

Along with fixing the regression, this also improves other cases,
one of which was added to the test.

Author: jchyb

compiler/src/dotty/tools/dotc/core/Definitions.scala

@@ -1118,7 +1118,7 @@ class Definitions {
         case ErasedFunctionOf(mt) =>
           Some(mt.paramInfos, mt.resType, mt.isContextualMethod)
         case _ =>
-          val tsym = ft.typeSymbol
+          val tsym = ft.dealias.typeSymbol
           if isFunctionSymbol(tsym) && ft.isRef(tsym) then
             val targs = ft.dealias.argInfos
             if (targs.isEmpty) None

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

@@ -2059,9 +2059,6 @@ trait Applications extends Compatibility {
           if isDetermined(alts2) then alts2
           else resolveMapped(alts1, _.widen.appliedTo(targs1.tpes), pt1)
 
-      case defn.FunctionOf(args, resultType, _) =>
-        narrowByTypes(alts, args, resultType)
-
       case pt =>
         val compat = alts.filterConserve(normalizedCompatible(_, pt, keepConstraint = false))
         if (compat.isEmpty)

tests/pos/i17245.scala

@@ -15,3 +15,6 @@ type OnChannel = Channel => Any
   val case2: OnChannel = Mockito.mock
   val case3 = Mockito.mock[OnChannel]
   val case4: OnChannel =  Mockito.mock[OnChannel](summon[ClassTag[OnChannel]])
+
+  // not a regressive case, but an added improvement with the fix for the above
+  val case5: Channel => Any = Mockito.mock[Channel => Any]