Harden constraint handling in IDE

I observed assertion failures, that a param ref is not
in the current constraint. Avoiding an assertion failure
if that case is encountered in the IDE.

Author: odersky

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

@@ -222,11 +222,16 @@ 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}")
-    inst
+    if (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}")
+      inst
+    }
+    else {
+      assert(ctx.mode.is(Mode.Interactive))
+      UnspecifiedErrorType
+    }
   }
 
   /** The instance type of `param` in the current constraint (which contains `param`).

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

@@ -3194,7 +3194,6 @@ object Types {
     private def instantiateWith(tp: Type)(implicit ctx: Context): Type = {
       assert(tp ne this, s"self instantiation of ${tp.show}, constraint = ${ctx.typerState.constraint.show}")
       typr.println(s"instantiating ${this.show} with ${tp.show}")
-      assert(ctx.typerState.constraint contains this) // !!! DEBUG
       if ((ctx.typerState eq owningState) && !ctx.typeComparer.subtypeCheckInProgress)
         inst = tp
       ctx.typerState.constraint = ctx.typerState.constraint.replace(origin, tp)