Fix #6362: Add regression test

Author: OlivierBlanvillain

tests/pos/6362.scala

@@ -0,0 +1,8 @@
+object Test {
+  type LeafElem[X] = X match {
+    case String => Char
+    case Array[t] => LeafElem[t]
+    case Iterable[t] => LeafElem[t]
+    case AnyVal => X
+  }
+}