SI-7967 Account for type aliases in self-type checks

These eluded the check for "illegal inheritance; self-type does not
conform" as AliasTypeRef doesn't forward `typeOfThis`.

This commit dealiases before calling `typeOfThis`. That seems to be
the most localised change.

Without the dealias, we had:

    parent.tpe            = TypeRef(pre, sym = AliasTypeSymbol("CC"), Nil)
    parent.tpe.typeOfThis = parent.tpe.transform(sym.typeOfThis) = CC

After:

    parent.tpe.dealias            = TypeRef(pre, sym = ClassSymbol("C"), Nil)
    parent.tpe.dealias.typeOfThis = C with B

1 files changed, 9 insertions, 0 deletions

diff --git a/test/files/neg/t7967.scala b/test/files/neg/t7967.scala
new file mode 100644
index 0000000000..4f13347948
--- /dev/null
+++ b/test/files/neg/t7967.scala
@@ -0,0 +1,9 @@
+
+trait B
+trait C {self: B =>}
+
+object Test {
+  new C {} // fails
+  type CC = C
+  new CC {} // should fail, doesn't
+}