Fix handling of Path.relativze on Windows (#3299)

* WindowsPath: Correct detection of the path type and the extraction of a root for DriveRelative and DirectoryRelative paths

* WindowsPath: Correct parent extraction, consider DirectoryRelative

* WindowsPath: Correct path creation in `relativize`

* WindowsPath: Empty path's type should be Relative

* [WIP] `relativize` should throw exceptions on invalid input

* WindowsPath: add unit tests

Tests are based on UnixPathTest with modifications and additions to
for Windows paths.
Tests were verified by running them vs JVM on Windows.
Some additional tests are commented out to mark known issues and help
with future development.

* WindowsPath: better switching of tests on JVM 8; add assertion IDs

Added IDs, so it is possible to identify which assertion is failing
just from the test output.

javalib/src/main/scala/scala/scalanative/nio/fs/windows/WindowsPath.scala

@@ -42,10 +42,10 @@ class WindowsPath private[windows] (
           case Array(host, share) => share + "\\"
           case _                  => ""
         }
-      case (PathType.Absolute, Some(root))          => root
-      case (PathType.DirectoryRelative, Some(root)) => root + "\\"
-      case (PathType.DriveRelative, _)              => "\\"
-      case _                                        => ""
+      case (PathType.Absolute, Some(root))      => root
+      case (PathType.DirectoryRelative, _)      => "\\"
+      case (PathType.DriveRelative, Some(root)) => root
+      case _                                    => ""
     }
     drivePrefix + segments.mkString(seperator)
   }
@@ -69,7 +69,9 @@ class WindowsPath private[windows] (
 
   override def getParent(): Path = {
     val nameCount = getNameCount()
-    if (nameCount == 0 || (nameCount == 1 && !isAbsolute()))
+    if (nameCount == 0)
+      null
+    else if (nameCount == 1 && pathType != PathType.Absolute && pathType != PathType.DirectoryRelative)
       null
     else if (root.isDefined)
       new WindowsPath(pathType, root, segments.init)
@@ -164,20 +166,26 @@ class WindowsPath private[windows] (
     resolveSibling(WindowsPathParser(other))
 
   override def relativize(other: Path): Path = {
-    if (isAbsolute() ^ other.isAbsolute()) {
+    val otherType = other match {
+      case null           => throw new NullPointerException()
+      case p: WindowsPath => p.pathType
+      case _ =>
+        throw new IllegalArgumentException("'other' is different Path class")
+    }
+    if (pathType != otherType) {
       throw new IllegalArgumentException("'other' is different type of Path")
     } else {
-      val normThis = new WindowsPath(WindowsPath.normalized(this))
+      val normThis = WindowsPathParser(WindowsPath.normalized(this))
       if (normThis.toString.isEmpty()) {
         other
       } else if (other.startsWith(normThis)) {
         other.subpath(getNameCount(), other.getNameCount())
       } else if (normThis.getParent() == null) {
-        new WindowsPath("../" + other.toString())
+        WindowsPathParser("../" + other.toString())
       } else {
         val next = normThis.getParent().relativize(other).toString()
         if (next.isEmpty()) new WindowsPath("..")
-        else new WindowsPath("../" + next)
+        else WindowsPathParser("../" + next)
       }
     }
   }

javalib/src/main/scala/scala/scalanative/nio/fs/windows/WindowsPathParser.scala

@@ -27,20 +27,23 @@ object WindowsPathParser {
       rawPath.size > n && pred(rawPath.charAt(n))
     }
 
-    val (tpe, root) = if (charAtIdx(0, isSlash)) {
-      if (charAtIdx(1, isSlash))
-        UNC -> Some(getUNCRoot(rawPath))
-      else if (charAtIdx(1, isASCIILetter) && charAtIdx(2, _ == ':'))
-        // URI specific, absolute path starts with / followed by absolute path
-        Absolute -> Some(rawPath.substring(1, 4))
-      else
-        DriveRelative -> None
-    } else if (charAtIdx(0, isASCIILetter) && charAtIdx(1, _ == ':')) {
-      if (charAtIdx(2, isSlash))
-        Absolute -> Some(rawPath.substring(0, 3))
-      else
-        DirectoryRelative -> Some(rawPath.substring(0, 2))
-    } else Relative -> None
+    val (tpe, root) =
+      if (rawPath.isEmpty)
+        Relative -> None
+      else if (charAtIdx(0, isSlash)) {
+        if (charAtIdx(1, isSlash))
+          UNC -> Some(getUNCRoot(rawPath))
+        else if (charAtIdx(1, isASCIILetter) && charAtIdx(2, _ == ':'))
+          // URI specific, absolute path starts with / followed by absolute path
+          Absolute -> Some(rawPath.substring(1, 4))
+        else
+          DirectoryRelative -> Some(rawPath.substring(0, 1))
+      } else if (charAtIdx(0, isASCIILetter) && charAtIdx(1, _ == ':')) {
+        if (charAtIdx(2, isSlash))
+          Absolute -> Some(rawPath.substring(0, 3))
+        else
+          DriveRelative -> Some(rawPath.substring(0, 2))
+      } else Relative -> None
 
     val relativePath = root
       .map(r => rawPath.substring(r.length))