Don't let caseLambda bindings accumulate in constraint

Remove them when a `matchCases` has finished.

Author: odersky

compiler/src/dotty/tools/dotc/core/TypeComparer.scala

@@ -2815,6 +2815,7 @@ class TrackingTypeComparer(initctx: Context) extends TypeComparer(initctx) {
         NoType
 
     inFrozenConstraint {
+      val savedConstraint = constraint
       // Empty types break the basic assumption that if a scrutinee and a
       // pattern are disjoint it's OK to reduce passed that pattern. Indeed,
       // empty types viewed as a set of value is always a subset of any other
@@ -2831,7 +2832,8 @@ class TrackingTypeComparer(initctx: Context) extends TypeComparer(initctx) {
       if (provablyEmpty(scrut))
         NoType
       else
-        recur(cases)
+        try recur(cases)
+        finally constraint = savedConstraint // caseLambda additions are dropped
     }
   }
 }