Allow constraining a parameter to Nothing (#19397)

Fixes #19359

Author: odersky

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

@@ -858,7 +858,8 @@ trait ConstraintHandling {
           addParamBound(bound)
         case _ =>
           val pbound = avoidLambdaParams(bound)
-          kindCompatible(param, pbound) && addBoundTransitively(param, pbound, !fromBelow)
+          (pbound.isNothingType || kindCompatible(param, pbound))
+          && addBoundTransitively(param, pbound, !fromBelow)
       finally
         canWidenAbstract = saved
         addConstraintInvocations -= 1

tests/warn/i19359.scala

@@ -0,0 +1,23 @@
+sealed trait Plan[+E <: Err, +F[+e <: E] <: Fal[e]]
+
+case class Err()                                   extends Plan[Err, Nothing]
+case class Fal[+E <: Err]()                        extends Plan[E,   Fal]
+case class Btw[+E <: Err, +F[+e <: E] <: Fal[e]]() extends Plan[E,   F]
+
+class Tests:
+  def test1(plan: Plan[Err, Nothing]): Unit = plan match
+    case Err() =>
+    case Btw() =>
+
+  def main1 = test1(Btw())
+
+/*
+
+Previously, Plan[Err, Nothing] dropped Btw,
+because it didn't instantiate ?F to Nothing
+
+    <== decompose(Plan[Err, Nothing], [Btw[Err, Fal]]) = [Not, Err]
+  <== decompose(Btw[Err, Fal] & Plan[Err, Nothing]) = []
+<== simplify(Prod(Btw())) = Empty
+
+*/