Attempt to fix scala#4147: constrain type constructors less

Unfortunately this is currently broken because of the poly-kinded
Nothing, we may end up doing something like:
  ?F[A] <:< Foo[A, B]
    ?F <:< Foo
      ?F := Nothing // OK, even though ?F has the wrong kind

Author: smarter

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

@@ -662,7 +662,7 @@ class TypeComparer(initctx: Context) extends DotClass with ConstraintHandling {
           tycon1.dealias match {
             case tycon1: TypeParamRef =>
               (tycon1 == tycon2 ||
-              canConstrain(tycon1) && tryInstantiate(tycon1, tycon2)) &&
+              canConstrain(tycon1) && isSubType(tycon1, tycon2)) &&
               isSubArgs(args1, args2, tp1, tparams)
             case tycon1: TypeRef =>
               tycon2.dealias match {
@@ -801,7 +801,7 @@ class TypeComparer(initctx: Context) extends DotClass with ConstraintHandling {
         case param1: TypeParamRef =>
           def canInstantiate = tp2 match {
             case AppliedType(tycon2, args2) =>
-              tryInstantiate(param1, tycon2.ensureHK) && isSubArgs(args1, args2, tp1, tycon2.typeParams)
+              isSubType(param1, tycon2.ensureHK) && isSubArgs(args1, args2, tp1, tycon2.typeParams)
             case _ =>
               false
           }

tests/pos/i4147.scala

@@ -0,0 +1,12 @@
+trait Higher[F[_]]
+trait Super[A]
+trait Sub[A] extends Super[A]
+
+object Test {
+  implicit def higherSub: Higher[Sub] = ???
+  implicit def deriv[F[_]](implicit bla: Higher[F]): F[String] = ???
+
+  implicit def deriv[F <: [X] => Super[X]]: F[Int] = ???
+
+  val x: Super[String] = deriv
+}

tests/pos/i4147b.scala

@@ -0,0 +1,14 @@
+trait Bar[F[_]]
+trait Foo[A]
+trait Qux[A] extends Foo[A]
+object Qux {
+  implicit def string: Qux[String] = ???
+  implicit def bar: Bar[Qux] = ???
+}
+
+case class Problem[A](a: A)
+object Problem {
+  import Qux._
+  implicit def deriv[F[_]](implicit bla: Bar[F], baz: F[String]): F[Problem[String]] = ???
+  implicitly[Foo[Problem[String]]](deriv)
+}