Fix broken dotty.tools.io.Path#parent

The previous implementation wrongly assumed that if
`jpath.normalize.getParent` returns null, then `jpath` must be a root.

However the null in this case really only means that no name elements
remain after normalization and the call to `getParent`. Since redundant
name elements such as "." and ".." may be removed by `normalize`, and
`getParent` may simply remove the last name element, there are cases
other than roots to consider, such as the current directory.

Some examples of broken behavior prior this commit:

  scala> Path("./foo.txt").parent
  val res0: dotty.tools.io.Directory = ./foo.txt  // should be Directory(.)

  scala> Path("foo.txt").parent
  val res1: dotty.tools.io.Directory = foo.txt    // should be Directory(.)

  scala> Path(".").parent
  val res4: dotty.tools.io.Directory = .          // should be Directory(..)

The changes here are based in part on the Scala 2.13 implementation of
scala.reflect.io.Path#parent

Fixes scala#11644

Author: griggt

compiler/src/dotty/tools/io/Path.scala

@@ -127,7 +127,15 @@ class Path private[io] (val jpath: JPath) {
    * @return The path of the parent directory, or root if path is already root
    */
   def parent: Directory = jpath.normalize.getParent match {
-    case null => Directory(jpath)
+    case null =>
+      if (isAbsolute)  // it should be a root
+        toDirectory
+      else if (segments.nonEmpty && segments.last == "..")
+        (this / "..").toDirectory
+      else path match {
+        case "" | "." => Directory("..")
+        case _        => Directory(".")
+      }
     case parent => Directory(parent)
   }
   def parents: List[Directory] = {

compiler/test/dotty/tools/io/PathTest.scala

@@ -0,0 +1,25 @@
+package dotty.tools.io
+
+import org.junit.Assert._
+import org.junit.Test
+
+class PathTest {
+  // Ref https://github.com/lampepfl/dotty/issues/11644#issuecomment-792457275
+  @Test def parent(): Unit = {
+    assertEquals(Directory(".."), Path("").parent)
+    assertEquals(Directory(".."), Path(".").parent)
+    assertEquals(Directory("..") / "..", Path("..").parent)
+    assertEquals(Directory("."), Path("foo.txt").parent)
+    assertEquals(Directory("."), (Path(".") / "foo.txt").parent)
+    assertEquals(Directory("."), (Path("bar") / ".").parent)
+    assertEquals(Directory("."), (Path("foo") / ".." / "." / "bar").parent)
+    assertEquals(Directory("baz") / "bar", (Path(".") / "baz" / "bar" / "foo.txt").parent)
+
+    for (root <- Path.roots) {
+      val rootDir = Directory(root.path)
+      assertEquals(rootDir, root.parent)
+      assertEquals(rootDir, (root / "foo.txt").parent)
+      assertEquals(rootDir / "baz" / "bar", (root / "baz" / "bar" / "foo.txt").parent)
+    }
+  }
+}

project/scripts/bootstrapCmdTests

@@ -58,3 +58,9 @@ grep -qe "def main(args: scala.Array\[scala.Predef.String\]): scala.Unit =" "$tm
 echo "testing ./bin/scaladoc"
 clear_out "$OUT1"
 ./bin/scaladoc -project Hello -d "$OUT1" "$OUT/out.jar"
+
+# check compilation when the class/tasty files already exist in the current directory
+echo "testing i11644"
+cwd=$(pwd)
+clear_out "$OUT"
+(cd "$OUT" && "$cwd/bin/scalac" "$cwd/tests/pos/i11644.scala" && "$cwd/bin/scalac" "$cwd/tests/pos/i11644.scala")

tests/pos/i11644.scala

@@ -0,0 +1 @@
+@main def hello = println("hello, world")