Avoid caching values that depend on typevar state.

TypeVars flip from the initial state, where underlying == origin to the final state
where underlying == inst. This flip can invalidate information that depends on the underlying
type of a TypeVar. Since we do not know when the flip occurs, we need to avoid keeping
any such information in a cache.

The commit makes three caches depend on a new value: typerState.ephemeral. The value is
set to `true` each time we follow the underlying type of a TypeVar, and this disables cached
information to be retained.

A test case for this commit is t2693.scala. This test passes typechecking with the previous commit,
but fails in -Ycheck:front because of stale cache info in an "#Apply" typeref. The present commit fixes that.

1 files changed, 6 insertions, 0 deletions

diff --git a/tests/pos/t2693.scala b/tests/pos/t2693.scala
new file mode 100644
index 000000000..5d4d0380c
--- /dev/null
+++ b/tests/pos/t2693.scala
@@ -0,0 +1,6 @@
+class A {
+  trait T[A]
+  def usetHk[T[_], A](ta: T[A]) = 0
+  usetHk(new T[Int]{}: T[Int])
+  usetHk(new T[Int]{}) // fails with: found: java.lang.Object with T[Int], required: ?T[ ?A ]
+}