[2.x] fix: inputFileChanges with nested task scopes (#8516)

Fixes #7489

When using nested task scopes like otherTask / testTask, the
inputFileChanges macro was returning the wrong scope. It checked
if the scope already had a task axis and returned it as-is, but
this meant it would use otherTask's scope instead of testTask's.

The fix always sets the task axis to the key being queried,
ensuring fileInputs settings are found at the correct scope.

main/src/main/scala/sbt/internal/FileChangesMacro.scala

@@ -98,8 +98,5 @@ object FileChangesMacro:
     '{ rescope[Seq[NioPath]]($ts, allOutputFiles).value }
 
   private def getTaskScope[A: Type](in: Expr[TaskKey[A]])(using Quotes): Expr[sbt.Scope] =
-    '{
-      if $in.scope.task.toOption.isDefined then $in.scope
-      else $in.scope.copy(task = Select($in.key))
-    }
+    '{ $in.scope.copy(task = Select($in.key)) }
 end FileChangesMacro

sbt-app/src/sbt-test/nio/nested-task-scope/build.sbt

@@ -0,0 +1,20 @@
+import sbt.nio.Keys._
+import sbt.internal.FileChangesMacro._
+
+val testTask = taskKey[Unit]("test task")
+val otherTask = taskKey[Unit]("dummy task")
+
+// Set fileInputs at nested task scope: otherTask / testTask
+otherTask / testTask / fileInputs := Seq(
+  baseDirectory.value.toGlob / "src" / "*.txt"
+)
+
+// Create a scoped task key to use with the inputFileChanges macro
+val scopedTestTask = otherTask / testTask
+
+// Test that inputFileChanges works with nested task scopes (fixes #7489)
+val checkChanges = taskKey[Unit]("check that file changes are detected")
+checkChanges := {
+  val files = scopedTestTask.inputFiles
+  assert(files.nonEmpty, s"inputFiles should not be empty, got: $files")
+}

sbt-app/src/sbt-test/nio/nested-task-scope/src/test.txt

@@ -0,0 +1 @@
+initial content

sbt-app/src/sbt-test/nio/nested-task-scope/test

@@ -0,0 +1,3 @@
+# Test that inputFileChanges works with nested task scopes (fixes #7489)
+
+> checkChanges