[2.x] fix: Resolve relative paths from -sbt-dir / sbt.global.base (#9001)

getFileProperty returned relative File objects when system properties
like sbt.global.base were set to relative paths (e.g. via -sbt-dir
../cache in .sbtopts). This caused an assertion failure in
IO.directoryURI: "Not absolute: ../cache/plugins".

Also fixes misleading scaladoc in SemanticSelector that claimed
`<=1.0` is equivalent to `<1.1.0` — they differ for pre-release
versions like 1.1.0-M1 (#4423).

Fixes #3729

---------

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

lm-core/src/main/contraband-scala/sbt/librarymanagement/SemanticSelector.scala

@@ -18,10 +18,10 @@ package sbt.librarymanagement
  * If no operator is specified, `=` is assumed.
  * 
  * If minor or patch versions are not specified, some numbers are assumed.
- * - `<=1.0` is equivalent to `<1.1.0`.
+ * - `<=1.0` matches `1.0.x` and below (but not `1.1.0-M1`).
  * - `<1.0` is equivalent to `<1.0.0`.
  * - `>=1.0` is equivalent to `>=1.0.0`.
- * - `>1.0` is equivalent to `>=1.1.0`.
+ * - `>1.0` matches `1.1.0-M1` and above.
  * - `=1.0` is equivalent to `>=1.0 <=1.0` (so `>=1.0.0 <1.1.0`).
  * 
  * Comparators can be combined by spaces to form the intersection set of the comparators.

lm-core/src/main/contraband/librarymanagement2.json

@@ -37,10 +37,10 @@
         "If no operator is specified, `=` is assumed.",
         "",
         "If minor or patch versions are not specified, some numbers are assumed.",
-        "- `<=1.0` is equivalent to `<1.1.0`.",
+        "- `<=1.0` matches `1.0.x` and below (but not `1.1.0-M1`).",
         "- `<1.0` is equivalent to `<1.0.0`.",
         "- `>=1.0` is equivalent to `>=1.0.0`.",
-        "- `>1.0` is equivalent to `>=1.1.0`.",
+        "- `>1.0` matches `1.1.0-M1` and above.",
         "- `=1.0` is equivalent to `>=1.0 <=1.0` (so `>=1.0.0 <1.1.0`).",
         "",
         "Comparators can be combined by spaces to form the intersection set of the comparators.",

main/src/main/scala/sbt/BuildPaths.scala

@@ -92,7 +92,7 @@ object BuildPaths:
           val tildePath = expandTildePrefix(path)
           Some(new File(tildePath))
         } else {
-          Some(new File(path))
+          Some(new File(path).getAbsoluteFile)
         }
       }
   }

main/src/test/scala/BuildPathsTest.scala

@@ -9,7 +9,7 @@
 package sbt
 
 import java.io.File
-import BuildPaths.{ expandTildePrefix, defaultGlobalBase, GlobalBaseProperty }
+import BuildPaths.{ expandTildePrefix, defaultGlobalBase, getFileProperty, GlobalBaseProperty }
 
 object BuildPathsTest extends verify.BasicTestSuite:
 
@@ -54,4 +54,22 @@ object BuildPathsTest extends verify.BasicTestSuite:
 
   test("it should expand ~-/foo/bar to $OLDPWD/foo/bar"):
     assertEquals(sys.env.getOrElse("OLDPWD", "") + "/foo/bar", expandTildePrefix("~-/foo/bar"))
+  test("getFileProperty resolves relative paths to absolute"):
+    val prop = "sbt.test.relative.path"
+    try
+      sys.props(prop) = "../relative/dir"
+      val result = getFileProperty(prop)
+      assert(result.isDefined)
+      assert(result.get.isAbsolute, s"Expected absolute path, got: ${result.get}")
+    finally sys.props.remove(prop)
+
+  test("getFileProperty preserves absolute paths"):
+    val prop = "sbt.test.absolute.path"
+    try
+      sys.props(prop) = "/absolute/path"
+      val result = getFileProperty(prop)
+      assert(result.isDefined)
+      assert(result.get.getPath == "/absolute/path")
+    finally sys.props.remove(prop)
+
 end BuildPathsTest