SI-8177 specializeSym must use memberInfo on high side

When determining whether member `symLo` of `tpLo`
has a stronger type than member `symHi` of `tpHi`,
should we use memberType or memberInfo?

Well, memberType transforms (using `asSeenFrom`) `sym.tpe`,
whereas memberInfo performs the same transform on `sym.info`.
For term symbols, this ends up being the same thing (`sym.tpe == sym.info`).
For type symbols, however, the `.info` of an abstract type member
is defined by its bounds, whereas its `.tpe` is a `TypeRef` to that type symbol,
so that `sym.tpe <:< sym.info`, but not the other way around.

Thus, for the strongest (correct) result,
we should use `memberType` on the low side.

On the high side, we should use the result appropriate
for the right side of the `<:<` above (`memberInfo`).

I also optimized the method a little bit by avoiding calling memberType
if the symbol on the high side isn't eligble (e.g., it's a class).

PS: I had to add a workaround to reifyType, because
we now dealias a little less eagerly, which means
a type selection on refinement class symbols makes it to reify
this broke the t8104 tests.
I also had to update the run/t6992 test, which should now test the right thing.
Tests should be commented and/or use sensible names.
What is it testing? What is the expected outcome? We should not be left guessing.

1 files changed, 63 insertions, 0 deletions

diff --git a/test/files/neg/t0764b.scala b/test/files/neg/t0764b.scala
new file mode 100644
index 0000000000..14c623c67a
--- /dev/null
+++ b/test/files/neg/t0764b.scala
@@ -0,0 +1,63 @@
+// see neg/t0764 why this should probably be a pos/ test -- alas something's wrong with existential subtyping (?)
+
+// In all cases when calling "prepend" the receiver 'v'
+// has static type NodeAlias[A] or (equivalently) Node { type T = A }.
+// Since prepend explicitly returns the singleton type of the receiver,
+// the return type of prepend in all cases is "v.type", and so the call
+// to "new Main" can be parameterized with any of the following, in order
+// of decreasing specificity with a tie for second place:
+//
+//   new Main[v.type](v.prepend)
+//   new Main[NodeAlias[A]](v.prepend)
+//   new Main[Node { type T = A }](v.prepend)
+//   new Main(v.prepend)
+
+// the `fail` comments below denote what didn't compile before SI-8177 fixed all of them
+
+package p1 {
+  object t0764 {
+    type NodeAlias[A] = Node { type T = A }
+    trait Node { outer =>
+      type T <: Node
+      def prepend: Node { type T = outer.type } = ???
+    }
+
+    class Main1[A <: Node](v: NodeAlias[A]) {
+      private[this] def f1 = new Main1(v.prepend)                        // fail
+      private[this] def f2 = new Main1[NodeAlias[A]](v.prepend)          // fail
+      private[this] def f3 = new Main1[Node { type T = A }](v.prepend)   // fail
+      private[this] def f4 = new Main1[v.type](v.prepend)                // ok
+    }
+
+    class Main2[A <: Node](v: Node { type T = A }) {
+      private[this] def f1 = new Main2(v.prepend)                        // fail
+      private[this] def f2 = new Main2[NodeAlias[A]](v.prepend)          // fail
+      private[this] def f3 = new Main2[Node { type T = A }](v.prepend)   // fail
+      private[this] def f4 = new Main2[v.type](v.prepend)                // ok
+    }
+  }
+}
+
+package p2 {
+  object t0764 {
+    type NodeAlias[A] = Node { type T = A }
+    trait Node { outer =>
+      type T <: Node
+      def prepend: NodeAlias[outer.type] = ???
+    }
+
+    class Main1[A <: Node](v: NodeAlias[A]) {
+      private[this] def f1 = new Main1(v.prepend)                        // ok!  <<========== WOT
+      private[this] def f2 = new Main1[NodeAlias[A]](v.prepend)          // fail
+      private[this] def f3 = new Main1[Node { type T = A }](v.prepend)   // fail
+      private[this] def f4 = new Main1[v.type](v.prepend)                // ok
+    }
+
+    class Main2[A <: Node](v: Node { type T = A }) {
+      private[this] def f1 = new Main2(v.prepend)                        // fail
+      private[this] def f2 = new Main2[NodeAlias[A]](v.prepend)          // fail
+      private[this] def f3 = new Main2[Node { type T = A }](v.prepend)   // fail
+      private[this] def f4 = new Main2[v.type](v.prepend)                // ok
+    }
+  }
+}