[2.x] fix: Detect alias/task key name conflicts (#8659)

Problem
When a user defines an alias with a name that matches an existing task or setting key (e.g., `alias c = compile` when a custom task `c` exists), the alias silently wins and shadows the task.

Solution
Detect conflicts at alias creation time and fail with an error message:
```
Alias 'c' conflicts with a task or setting key of the same name. Use a different alias name to avoid ambiguity.
```

main-command/src/main/scala/sbt/BasicCommands.scala

@@ -471,19 +471,40 @@ object BasicCommands {
     }(runAlias)
 
   def runAlias(s: State, args: Option[(String, Option[Option[String]])]): State =
+    runAlias(s, args, _ => Set.empty)
+
+  def runAlias(
+      s: State,
+      args: Option[(String, Option[Option[String]])],
+      definedKeyNames: State => Set[String]
+  ): State =
     args match {
       case Some(x ~ None) if !x.isEmpty   => printAlias(s, x.trim); s
       case Some(name ~ Some(None))        => removeAlias(s, name.trim)
-      case Some(name ~ Some(Some(value))) => addAlias(s, name.trim, value.trim)
+      case Some(name ~ Some(Some(value))) => addAlias(s, name.trim, value.trim, definedKeyNames)
       case _                              => printAliases(s); s
     }
   def addAlias(s: State, name: String, value: String): State =
-    if Command.validID(name) then
-      val removed = removeAlias(s, name)
-      if value.isEmpty then removed else addAlias0(removed, name, value)
-    else
+    addAlias(s, name, value, _ => Set.empty)
+
+  def addAlias(
+      s: State,
+      name: String,
+      value: String,
+      definedKeyNames: State => Set[String]
+  ): State =
+    if !Command.validID(name) then
       System.err.println("Invalid alias name '" + name + "'.")
       s.fail
+    else if definedKeyNames(s).contains(name) then
+      System.err.println(
+        s"Alias '$name' conflicts with a task or setting key of the same name. " +
+          "Use a different alias name to avoid ambiguity."
+      )
+      s.fail
+    else
+      val removed = removeAlias(s, name)
+      if value.isEmpty then removed else addAlias0(removed, name, value)
   private def addAlias0(s: State, name: String, value: String): State =
     s.copy(definedCommands = newAlias(name, value) +: s.definedCommands)
 

main/src/main/scala/sbt/Main.scala

@@ -305,6 +305,16 @@ import sbt.internal.util.complete.DefaultParsers.*
 object BuiltinCommands {
   def initialAttributes = AttributeMap.empty
   import BasicCommands.exit
+
+  def aliasWithKeyConflictCheck: Command =
+    Command(AliasCommand, Help.more(AliasCommand, AliasDetailed)) { s =>
+      val name = token(OpOrID.examples(aliasNames(s)*))
+      val assign = token(OptSpace ~ '=' ~ OptSpace)
+      val sfree = removeAliases(s)
+      val to = matched(sfree.combinedParser, partial = true).failOnException | any.+.string
+      OptSpace ~> (name ~ (assign ~> to.?).?).?
+    }((s, args) => runAlias(s, args, Project.definedKeyNames))
+
   def ConsoleCommands: Seq[Command] =
     Seq(ignore, exit, IvyConsole.command, setLogLevel, early, act, nop)
 
@@ -360,8 +370,9 @@ object BuiltinCommands {
       waitCmd,
       promptChannel,
       TestCommand.testOnly,
+      aliasWithKeyConflictCheck,
     ) ++
-      allBasicCommands ++
+      allBasicCommands.filterNot(_.nameOption.contains(AliasCommand)) ++
       ContinuousCommands.value ++
       BuildServerProtocol.commands
 

main/src/main/scala/sbt/ProjectExtra.scala

@@ -237,6 +237,18 @@ trait ProjectExtra extends Scoped.Syntax:
     def isProjectLoaded(state: State): Boolean =
       (state has Keys.sessionSettings) && (state has Keys.stateBuildStructure)
 
+    def definedKeyNames(state: State): Set[String] =
+      if !isProjectLoaded(state) then Set.empty
+      else
+        val keyIndex = structure(state).index.keyIndex
+        val globalKeys = keyIndex.keys(None)
+        val projectKeys = for
+          uri <- keyIndex.buildURIs
+          project <- keyIndex.projects(uri)
+          key <- keyIndex.keys(Some(ProjectRef(uri, project)))
+        yield key
+        globalKeys ++ projectKeys
+
     def extract(state: State): Extracted = {
       val se = Project.session(state)
       val st = Project.structure(state)
@@ -281,11 +293,10 @@ trait ProjectExtra extends Scoped.Syntax:
         .put(sessionSettings, session)
         .put(Keys.onUnload.key, onUnload)
       val newState = unloaded.copy(attributes = newAttrs)
-      // TODO: Fix this
       onLoad(
         preOnLoad(
           updateCurrent(newState)
-        ) /*LogManager.setGlobalLogLevels(updateCurrent(newState), structure.data)*/
+        )
       )
     }
 

sbt-app/src/sbt-test/actions/alias-key-conflict/build.sbt

@@ -0,0 +1,2 @@
+val c = taskKey[Unit]("A custom task named 'c'")
+c := { println("Custom task 'c' executed") }

sbt-app/src/sbt-test/actions/alias-key-conflict/test

@@ -0,0 +1,6 @@
+# Test that alias creation fails when it conflicts with a task key name
+# The task 'c' is defined in build.sbt, so 'alias c = compile' should fail
+-> alias c = compile
+
+# Verify the task 'c' still works
+> c