Try to type as an Assign

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

@@ -3430,9 +3430,11 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
   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
+        val tmpCtx = ctx.fresh.setNewTyperState()
+        typedAssign(untpd.Assign(untpd.Ident(name), value), WildcardType)(using tmpCtx)
+        if !tmpCtx.reporter.hasErrors then
+          // If there are no errors typing the above, then the named tuple is
+          // ambiguous and we issue a warning.
           report.migrationWarning(AmbiguousNamedTupleAssignment(name, value), tree.srcPos)
       case _ => ()
 

tests/warn/21681c.check

@@ -0,0 +1,7 @@
+-- [E203] Syntax Migration Warning: tests/warn/21681c.scala:5:2 --------------------------------------------------------
+5 |  (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/21681c.scala

@@ -0,0 +1,5 @@
+object Test:
+  def age: Int = ???
+  def age_=(x: Int): Unit = ()
+  age = 29
+  (age = 29) // warn