Move AmbiguousNamedTupleAssignment check to Typer

Co-Authored-By: Nicolas Stucki <3648029+nicolasstucki@users.noreply.github.com>
Co-Authored-By: Oliver Bračevac <bracevac@users.noreply.github.com>

compiler/src/dotty/tools/dotc/ast/Desugar.scala

@@ -1639,8 +1639,6 @@ object desugar {
       if ctx.mode.is(Mode.Type) then
         AppliedTypeTree(ref(defn.NamedTupleTypeRef), namesTuple :: tup :: Nil)
       else
-        if names.length == 1 && ctx.scope.lookup(names.head).is(Flags.Mutable) then
-          report.migrationWarning(AmbiguousNamedTupleAssignment(names.head, elemValues.head), tree.srcPos)
         Apply(
           Apply(
             TypeApply(

compiler/src/dotty/tools/dotc/typer/Typer.scala

@@ -3398,6 +3398,7 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
   /** Translate tuples of all arities */
   def typedTuple(tree: untpd.Tuple, pt: Type)(using Context): Tree =
     val tree1 = desugar.tuple(tree, pt)
+    checkAmbiguousNamedTupleAssignment(tree)
     if tree1 ne tree then typed(tree1, pt)
     else
       val arity = tree.trees.length
@@ -3423,6 +3424,18 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
           val resTpe = TypeOps.nestedPairs(elemTpes)
           app1.cast(resTpe)
 
+ /** Checks if `tree` is a named tuple with one element that could be
+  *  interpreted as an assignment, such as `(x = 1)`. If so, issues a warning.
+  */
+  def checkAmbiguousNamedTupleAssignment(tree: untpd.Tuple)(using Context): Unit =
+    tree.trees match
+      case List(NamedArg(name, value)) =>
+        val typedName = typedIdent(untpd.Ident(name), WildcardType)
+        val sym = typedName.symbol
+        if sym.exists && (sym.is(Flags.Mutable) || sym.setter.exists) then
+          report.migrationWarning(AmbiguousNamedTupleAssignment(name, value), tree.srcPos)
+      case _ => ()
+
   /** Retrieve symbol attached to given tree */
   protected def retrieveSym(tree: untpd.Tree)(using Context): Symbol = tree.removeAttachment(SymOfTree) match {
     case Some(sym) =>

tests/pos/21681d.scala

@@ -0,0 +1,16 @@
+def test1() =
+  class Person:
+    def age: Int = ???
+    def age_=(x: Int): Unit = ???
+
+  val person = Person()
+
+  (person.age = 29) // no warn (interpreted as `person.age_=(29)`)
+
+def test2() =
+  class Person:
+    var age: Int = 28
+
+  val person = Person()
+
+  (person.age = 29) // no warn (interpreted as `person.age_=(29)`)

tests/warn/21681b.check

@@ -0,0 +1,7 @@
+-- [E203] Syntax Migration Warning: tests/warn/21681b.scala:3:2 --------------------------------------------------------
+3 |  (age = 29) // warn
+  |  ^^^^^^^^^^
+  |  Ambiguous syntax: this is interpreted as a named tuple with one element,
+  |  not as an assignment.
+  |
+  |  To assign a value, use curly braces: `{age = 29}`.

tests/warn/21681b.scala

@@ -0,0 +1,3 @@
+object Test:
+  var age: Int = 28
+  (age = 29) // warn