Fix handling of null in qualifiers

Author: mbovel

compiler/src/dotty/tools/dotc/qualified_types/ENode.scala

@@ -461,6 +461,8 @@ object ENode:
         case tpd.Literal(_) | tpd.Ident(_) | tpd.This(_)
             if tree.tpe.isInstanceOf[SingletonType] && tpd.isIdempotentExpr(tree) =>
           Some(Atom(substParamRefs(tree.tpe, paramSyms, paramTps).asInstanceOf[SingletonType]))
+        case tpd.Literal(Constant(null)) => // null does not have a SingletonType
+          Some(Atom(ConstantType(Constant(null))))
         case tpd.Select(tpd.New(_), nme.CONSTRUCTOR) =>
           constructorNode(tree.symbol)
         case tree: tpd.Select if isCaseClassApply(tree.symbol) =>

tests/pos-custom-args/qualified-types/class_constraints.scala

@@ -4,4 +4,6 @@ class Multi(x: Int with x > 0)(y: String with y.length > 2, z: Double with z >=
 
 case class MultiCase(x: Int with x > 0)(val y: String with y.length > 2, val z: Double with z >= 0.0)
 
+class TParam[T](x: T with x != null)
+
 @main def Test = ()