Less eager removal of type parameters from constraint

Previously, a unified or otherwise instantiated type parameter
would be removed from the constraint, i.e. if it was the
last parameter of its polytype to be instantiated, the polytype
would be dropped. This is a potential problem since it means
that the alias `param = instance` is forgetten whereas we might
still need it in the same subtype test sequence.

The solution is to wait with cleaning up polytypes until
all associated type variables are fully instantiated.

This change uncovered another bug, where we failed
to follow an existing instantiation when adding to
a constraint. This manifested itself in deep subtype
errors for run/colltest1 and some others.

1 files changed, 11 insertions, 4 deletions

diff --git a/src/dotty/tools/dotc/core/ConstraintHandling.scala b/src/dotty/tools/dotc/core/ConstraintHandling.scala
index ace441566..dfce9317b 100644
--- a/src/dotty/tools/dotc/core/ConstraintHandling.scala
+++ b/src/dotty/tools/dotc/core/ConstraintHandling.scala
@@ -164,6 +164,7 @@ trait ConstraintHandling {
         }
       }
     }
+    assert(constraint.contains(param))
     val bound = if (fromBelow) constraint.fullLowerBound(param) else constraint.fullUpperBound(param)
     val inst = avoidParam(bound)
     typr.println(s"approx ${param.show}, from below = $fromBelow, bound = ${bound.show}, inst = ${inst.show}")
@@ -282,10 +283,16 @@ trait ConstraintHandling {
           else NoType
         case bound: TypeVar if constraint contains bound.origin =>
           prune(bound.underlying)
-        case bound: PolyParam if constraint contains bound =>
-          if (!addParamBound(bound)) NoType
-          else if (fromBelow) defn.NothingType
-          else defn.AnyType
+        case bound: PolyParam =>
+          constraint.entry(bound) match {
+            case NoType => bound
+            case _: TypeBounds =>
+              if (!addParamBound(bound)) NoType
+              else if (fromBelow) defn.NothingType
+              else defn.AnyType
+            case inst =>
+              prune(inst)
+          }
         case _ =>
           bound
       }