Fix #4721: don't assert constraints are satisfiable

Author: Blaisorblade

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

@@ -341,24 +341,22 @@ trait ConstraintHandling {
    *  and propagate all bounds.
    *  @param tvars   See Constraint#add
    */
-  def addToConstraint(tl: TypeLambda, tvars: List[TypeVar]): Unit =
-    assert {
-      checkPropagated(i"initialized $tl") {
-        constraint = constraint.add(tl, tvars)
-        tl.paramRefs.forall { param =>
-          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
-          }
+  def addToConstraint(tl: TypeLambda, tvars: List[TypeVar]): Boolean =
+    checkPropagated(i"initialized $tl") {
+      constraint = constraint.add(tl, tvars)
+      tl.paramRefs.forall { param =>
+        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/neg/i4721.scala

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

tests/neg/i4721a.scala

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