SI-8046 Only use fast TypeRef#baseTypeSeq with concrete base types

We can only compute the base type sequence (BTS) of a `TypeRef` by
element-wise transforming the BTS of the referenced symbol if
there are no abstract types in its BTS type symbols.

In the now-working test case, `pos/t8046.scala`, the difference
between the old and new calculation of the BTS is:

    this                 = Three.this.Alias[Int]
    sym.info.baseTypeSeq = BTS(One.this.Op[A],Any)
    mapped BTS           = BTS(Three.this.Op[Int],Any)
    full BTS             = BTS(Three.this.Op[Int],Int => Int,Object,Any)

The change to account for PolyType in ArgsTypeRef#transform is now
needed to avoid the full BTS of:

    BTS(Three.this.Op[A],A => A,Object,Any)

Interestingly, everything actually works without that change.

1 files changed, 13 insertions, 0 deletions

diff --git a/test/files/run/t8046/t8046c.scala b/test/files/run/t8046/t8046c.scala
new file mode 100644
index 0000000000..0b484da530
--- /dev/null
+++ b/test/files/run/t8046/t8046c.scala
@@ -0,0 +1,13 @@
+import language._
+
+trait One {
+  type Op[A]
+  type Alias[A] = Op[A]
+}
+
+trait Three extends One {
+  trait Op[A] extends (A => A)
+
+  def f4[T <: Alias[Int]](f: T) = 0
+}
+