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, 11 insertions, 1 deletions

diff --git a/src/dotty/tools/dotc/core/TyperState.scala b/src/dotty/tools/dotc/core/TyperState.scala
index 8c742edab..fd8a534d4 100644
--- a/src/dotty/tools/dotc/core/TyperState.scala
+++ b/src/dotty/tools/dotc/core/TyperState.scala
@@ -23,6 +23,10 @@ class TyperState(r: Reporter) extends DotClass with Showable {
   /** The uninstantiated variables */
   def uninstVars = constraint.uninstVars
 
+  /** The ephemeral flag */
+  def ephemeral: Boolean = false
+  def ephemeral_=(x: Boolean): Unit = ()
+
   /** Gives for each instantiated type var that does not yet have its `inst` field
    *  set, the instance value stored in the constraint. Storing instances in constraints
    *  is done only in a temporary way for contexts that may be retracted
@@ -76,6 +80,12 @@ extends TyperState(r) {
   override def constraint = myConstraint
   override def constraint_=(c: Constraint) = myConstraint = c
 
+  private var myEphemeral: Boolean = previous.ephemeral
+
+  override def ephemeral = myEphemeral
+  override def ephemeral_=(x: Boolean): Unit = { myEphemeral = x }
+
+
   override def fresh(isCommittable: Boolean): TyperState =
     new MutableTyperState(this, new StoreReporter, isCommittable)
 
@@ -96,11 +106,11 @@ extends TyperState(r) {
     val targetState = ctx.typerState
     assert(isCommittable)
     targetState.constraint = constraint
-
     constraint foreachTypeVar { tvar =>
       if (tvar.owningState eq this)
         tvar.owningState = targetState
     }
+    targetState.ephemeral = ephemeral
     targetState.gc()
     reporter.flush()
   }