Extend some more tests to include higher-kinded lambdas

I found these by systematically inspecting all usages of `widen`.

Author: odersky

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

@@ -238,8 +238,8 @@ object Inferencing {
       if (toTest.isEmpty) acc
       else tree match {
         case Apply(fn, _) =>
-          fn.tpe.widen match {
-            case mtp: MethodType =>
+          fn.tpe.widen.toLambda match {
+            case mtp: TermLambda =>
               val (occ, nocc) = toTest.partition(tvar => mtp.paramInfos.exists(tvar.occursIn))
               occurring(fn, nocc, occ ::: acc)
             case _ =>

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

@@ -1849,8 +1849,8 @@ class Typer extends Namer
   /** Interpolate and simplify the type of the given tree. */
   protected def simplify(tree: Tree, pt: Type, locked: TypeVars)(implicit ctx: Context): tree.type = {
     if (!tree.denot.isOverloaded) // for overloaded trees: resolve overloading before simplifying
-      if (!tree.tpe.widen.isInstanceOf[MethodOrPoly] // wait with simplifying until method is fully applied
-          || tree.isDef)                             // ... unless tree is a definition
+      if (!tree.tpe.widen.toLambda.isInstanceOf[LambdaType] // wait with simplifying until lambda is fully applied
+          || tree.isDef)                                    // ... unless tree is a definition
       {
         interpolateTypeVars(tree, pt, locked)
         tree.overwriteType(tree.tpe.simplified)