SI-5294 Use bounds of abstract prefix in asSeenFrom

ASF was failing to recognize the correspondence between a
prefix if it has an abstract type symbol, even if it is bounded by
the currently considered class.

Distilling the test cases, this led to incorrect typechecking of the
RHS of `G` in:

```
trait T {
  type A
  trait HasH { type H[U] <: U }
  type F[N <: HasH] = N#H[T]
  type G[N <: HasH] = F[N]#A // RHS was incorrectly reduced to T.this.A
}
```

In the fuller examples (included as test cases), this meant that
type level functions written as members of `HList` could not be
implemented in terms of each other, e.g. defining `Apply[N]` as
`Drop[N]#Head` had the wrong semantics.

This commit checks checks if the prefix has the candidate class
as a base type, rather than checking if its type symbol has this
as a base class. The latter formulation discarded information about
the instantation of the abstract type.

Using the example above:

```
scala> val F = typeOf[T].member(TypeName("F")).info
F: $r.intp.global.Type = [N <: T.this.HasH]N#H[T]

scala> F.resultType.typeSymbol.baseClasses // old approach
res14: List[$r.intp.global.Symbol] = List(class Any)

scala> F.resultType.baseClasses // new approach
res13: List[$r.intp.global.Symbol] = List(trait T, class Object, class Any)
```

It is worth noting that dotty rejects some of these programs,
as it introduces the rule that:

> // A type T is a legal prefix in a type selection T#A if
> // T is stable or T contains no abstract types except possibly A.
> final def isLegalPrefixFor(selector: Name)(implicit ctx: Context)

However, typechecking the program above in this comment in dotty
yields:

    <trait> trait T() extends Object {
      type A
      <trait> trait HasH() extends Object {
        type H <: [HK$0] =>  <: HK$0
      }
      type F = [HK$0] => HK$0#H{HK$0 = T}#Apply
      type G = [HK$0] => HK$0#H{HK$0 = T}#Apply#A
    }

As the equivalent code [1] in dotc's `asSeenFrom` already looks for a base type
of the prefix, rather than looking for a superclass of the prefix's
type symbol.

[1] https://github.com/lampepfl/dotty/blob/d2c96d02fccef3a82b88ee1ff31253b6ef17f900/src/dotty/tools/dotc/core/TypeOps.scala#L62

4 files changed, 88 insertions, 0 deletions

diff --git a/test/files/pos/t5294b.scala b/test/files/pos/t5294b.scala
new file mode 100644
index 0000000000..038d2ff806
--- /dev/null
+++ b/test/files/pos/t5294b.scala
@@ -0,0 +1,36 @@
+class Test {
+  def test = {
+
+    val l1 = null: Int #: String #: Boolean #: String #: HNil.type
+
+    type _2 = Succ[Succ[Zero.type]]
+
+    val t1: Boolean = null.asInstanceOf[ l1.type#Drop[_2]#Head ]
+
+    val t2: Boolean = null.asInstanceOf[ l1.type#Apply[_2] ]
+  }
+}
+
+
+sealed trait Nat {
+  type Fold[U, F[_ <: U] <: U, Z <: U] <: U
+}
+
+final object Zero extends Nat {
+  type Fold[U, F[_ <: U] <: U, Z <: U] = Z
+}
+
+final class Succ[N <: Nat] extends Nat {
+  type Fold[U, F[_ <: U] <: U, Z <: U] = F[N#Fold[U, F, Z]]
+}
+
+trait HList {
+  type Head
+  type Tail <: HList
+  type Drop[N <: Nat] = N#Fold[HList, ({ type L[X <: HList] = X#Tail })#L, this.type]
+  type Apply[N <: Nat] = Drop[N]#Head
+}
+
+class #: [H, T <: HList] extends HList { type Head = H; type Tail = T }
+
+object HNil extends HList { type Head = Nothing;  type Tail = Nothing }
diff --git a/test/files/pos/t5294c.scala b/test/files/pos/t5294c.scala
new file mode 100644
index 0000000000..2709098988
--- /dev/null
+++ b/test/files/pos/t5294c.scala
@@ -0,0 +1,30 @@
+sealed trait Nat {
+  type IsZero[U, A <: U, B <: U] <: U
+}
+
+final object Zero extends Nat {
+  type IsZero[U, T <: U, F <: U] = T
+}
+
+final class Succ[N <: Nat] extends Nat {
+  type IsZero[U, T <: U, F <: U] = F
+}
+
+trait HList {
+  type Head
+  type Tail <: HList
+  type Drop[N <: Nat] = N#IsZero[HList, this.type, Tail]
+  type Apply[N <: Nat] = Drop[N]#Head // typechecks as HList.this.Head
+}
+
+object Test {
+  type ::[H, T <: HList] = HList { type Head = H; type Tail = T}
+  type HNil <: HList
+  type T = Int :: String :: HNil
+
+  type U = T#Drop[Succ[Zero.type]]#Head
+  type V = T#Apply[Succ[Zero.type]]
+  var u: U = ???
+  var v: V = ???
+  u = v
+}
diff --git a/test/pending/pos/t6161.scala b/test/files/pos/t6161b.scala
index 5783cc85f2..5783cc85f2 100644
--- a/test/pending/pos/t6161.scala
+++ b/test/files/pos/t6161b.scala
diff --git a/test/files/run/t5294.scala b/test/files/run/t5294.scala
new file mode 100644
index 0000000000..2551ae89a6
--- /dev/null
+++ b/test/files/run/t5294.scala
@@ -0,0 +1,22 @@
+import scala.language.higherKinds
+
+package p {
+  trait T[+A, +CC] {
+    def t: CC
+  }
+  class C {
+    def test[CC[X] <: T[X,String] with T[X,Int]](from: CC[_]): Unit = ()
+  }
+}
+
+object Test {
+  def main(args: Array[String]): Unit = {
+    val symtab = reflect.runtime.universe.asInstanceOf[reflect.internal.SymbolTable]
+    val CTpe = reflect.runtime.universe.typeOf[p.C].asInstanceOf[symtab.Type]
+    val TClass = reflect.runtime.universe.symbolOf[p.T[_, _]].asInstanceOf[symtab.Symbol]
+    import symtab._
+    val from = CTpe.member(TermName("test")).paramss.head.head
+    assert(from.baseClasses contains TClass)
+    assert(from.info.baseTypeIndex(TClass) != -1) // was failing!
+  }
+}