Fix #4720: support smarter type inference

Fix regression in eeb5965. A type variable can
already be solved in addToConstraint, so don't assume it is unsolved (hence its
constraint entry is a TypeBounds) when first processing it.

Fix #4720.

Author: Blaisorblade

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

@@ -346,13 +346,19 @@ trait ConstraintHandling {
       checkPropagated(i"initialized $tl") {
         constraint = constraint.add(tl, tvars)
         tl.paramRefs.forall { param =>
-          val bounds = constraint.nonParamBounds(param)
-          val lower = constraint.lower(param)
-          val upper = constraint.upper(param)
-          if (lower.nonEmpty && !bounds.lo.isRef(defn.NothingClass) ||
-            upper.nonEmpty && !bounds.hi.isRef(defn.AnyClass)) constr.println(i"INIT*** $tl")
-          lower.forall(addOneBound(_, bounds.hi, isUpper = true)) &&
-            upper.forall(addOneBound(_, bounds.lo, isUpper = false))
+          constraint.entry(param) match {
+            case bounds: TypeBounds =>
+              val lower = constraint.lower(param)
+              val upper = constraint.upper(param)
+              if (lower.nonEmpty && !bounds.lo.isRef(defn.NothingClass) ||
+                upper.nonEmpty && !bounds.hi.isRef(defn.AnyClass)) constr.println(i"INIT*** $tl")
+              lower.forall(addOneBound(_, bounds.hi, isUpper = true)) &&
+                upper.forall(addOneBound(_, bounds.lo, isUpper = false))
+            case _ =>
+              // Happens if param was already solved while processing earlier params of the same TypeLambda.
+              // See #4720.
+              true
+          }
         }
       }
     }

tests/pos/i4720.scala

@@ -0,0 +1,4 @@
+object Test {
+  def main(args: Array[String]):Unit = m(1)
+  def m[Y<:Int, Z>:Int, W>:Z<:Y](d:Y):Unit={}
+}