Fix #6571: Add regression test

Author: OlivierBlanvillain

tests/neg/6571.scala

@@ -0,0 +1,28 @@
+object Test {
+  class Inv[T]
+  type M[t] = t match {
+    case Inv[u] => u
+  }
+  def foo: M[Inv[Int] & Inv[String]] = "" // error
+  //                                   ^^
+  // Found:    ("" : String)
+  // Required: Test.M[Test.Inv[Int] & Test.Inv[String]]
+  //
+  // Note: a match type could not be fully reduced:
+  //
+  //   trying to reduce  Test.M[Test.Inv[Int] & Test.Inv[String]]
+  //   failed since selector  Test.Inv[Int] & Test.Inv[String]
+  //   is uninhabited (there are no values of that type).
+
+
+  def bar: M[Inv[String] & Inv[Int]] = 0 // error
+  //                                   ^
+  // Found:    (0 : Int)
+  // Required: Test.M[Test.Inv[String] & Test.Inv[Int]]
+
+  // Note: a match type could not be fully reduced:
+
+  //   trying to reduce  Test.M[Test.Inv[String] & Test.Inv[Int]]
+  //   failed since selector  Test.Inv[String] & Test.Inv[Int]
+  //   is uninhabited (there are no values of that type).
+}