Handle case where typevar instance is fully determined

Need another check whether a constraint contains a typevariable
before going ahead with instantiating it.

Test case in collection strawman: `List.empty ++ List.empty`.

Author: odersky

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

@@ -611,7 +611,7 @@ class OrderingConstraint(private val boundsMap: ParamBounds,
   override def toString: String = {
     def entryText(tp: Type): String = tp match {
       case tp: TypeBounds => tp.toString
-      case _ =>" := " + tp
+      case _ => " := " + tp
     }
     val constrainedText =
       " constrained types = " + domainLambdas.mkString("\n")

compiler/src/dotty/tools/dotc/typer/Inferencing.scala

@@ -250,7 +250,10 @@ object Inferencing {
       //     val y: List[List[String]] = List(List(1))
       if (!hasUnreportedErrors)
         vs foreachBinding { (tvar, v) =>
-          if (v != 0) {
+          if (v != 0 && ctx.typerState.constraint.contains(tvar)) {
+            // previous interpolations could have already instantiated `tvar`
+            // through unification, that's why we have to check again whether `tvar`
+            // is contained in the current constraint.
             typr.println(s"interpolate ${if (v == 1) "co" else "contra"}variant ${tvar.show} in ${tp.show}")
             tvar.instantiate(fromBelow = v == 1)
           }