GC type lambdas that are not referenced from PolyTypes

In typerState,gc(), drop all type lambdas which are not referenced
from normal PolyTypes.

Author: odersky

src/dotty/tools/dotc/core/TyperState.scala

@@ -140,6 +140,30 @@ extends TyperState(r) {
     }
     for (poly <- toCollect)
       constraint = constraint.remove(poly)
+
+    lazy val keptLambdas: collection.Set[TypeLambda] = {
+      val toKeep = mutable.Set.empty[TypeLambda]
+      def keep(p: PolyParam) = p.binder match {
+        case tl: TypeLambda => toKeep += tl
+        case _ =>
+      }
+      for (poly <- constraint.domainPolys)
+        if (poly.isInstanceOf[PolyType])
+          for (param <- poly.paramRefs) {
+            constraint.lower(param).foreach(keep)
+            constraint.upper(param).foreach(keep)
+            constraint.entry(param).foreachPart {
+              case p: PolyParam if constraint contains p => keep(p)
+              case _ =>
+            }
+          }
+      toKeep
+    }
+    constraint.domainPolys.foreach {
+      case tl: TypeLambda if !keptLambdas.contains(tl) =>
+         constraint = constraint.remove(tl)
+      case _ =>
+    }
   }
 
   /** Try operation `op`; if it produces errors, execute `fallback` with constraint and