Another regression test for bad bounds

This one comes from #5854.

Author: odersky

compiler/test/dotty/tools/dotc/CompilationTests.scala

@@ -155,6 +155,7 @@ class CompilationTests extends ParallelTesting {
     compileFile("tests/neg-custom-args/overrideClass.scala", scala2Mode) +
     compileFile("tests/neg-custom-args/autoTuplingTest.scala", defaultOptions.and("-language:noAutoTupling")) +
     compileFile("tests/neg-custom-args/i1050.scala", defaultOptions.and("-strict")) +
+    compileFile("tests/neg-custom-args/nullless.scala", defaultOptions.and("-strict")) +
     compileFile("tests/neg-custom-args/nopredef.scala", defaultOptions.and("-Yno-predef")) +
     compileFile("tests/neg-custom-args/noimports.scala", defaultOptions.and("-Yno-imports")) +
     compileFile("tests/neg-custom-args/noimports2.scala", defaultOptions.and("-Yno-imports")) +

tests/neg/nullless.scala

@@ -0,0 +1,40 @@
+object nullless {
+  trait LowerBound[T] {
+    type M >: T;
+  }
+  trait UpperBound[U] {
+    type M <: U;
+  }
+  lazy val nothing : Nothing = nothing
+  class Box[V](val v: V)
+  lazy val box : Box[UpperBound[String] & LowerBound[Int]] = new Box(nothing)
+  def upcast(t : box.v.M) : box.v.M = t // error // error under -strict
+  def main(args : Array[String]) : Unit = {
+    val zero : String = upcast(0)
+    println("...")
+  }
+}
+object bar {
+  trait Sub {
+    type M
+    type L <: M
+    type U >: M
+    type M2 >: L <: U
+  }
+  class Box[V](val v: V)
+
+  class Caster[LL, UU] {
+    trait S2 {
+      type L = LL
+      type U = UU
+    }
+    final lazy val nothing: Nothing = nothing
+    final lazy val sub: S2 with Sub = nothing
+    final lazy val box : Box[S2 with Sub] = new Box(nothing)
+    def upcast(t: box.v.M2): box.v.M2 = t // error // error under -strict
+  }
+  def main(args : Array[String]) : Unit = {
+    val zero : String = (new Caster[Int,String]()).upcast(0)
+    println("...")
+  }
+}