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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.
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