Try to type as an Assign

Author: mbovel

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

@@ -3398,7 +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)
-    checkAmbiguousNamedTuple(tree)
+    checkAmbiguousNamedTupleAssignment(tree)
     if tree1 ne tree then typed(tree1, pt)
     else
       val arity = tree.trees.length
@@ -3424,14 +3424,19 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
           val resTpe = TypeOps.nestedPairs(elemTpes)
           app1.cast(resTpe)
 
-  def checkAmbiguousNamedTuple(tree: untpd.Tuple)(using Context): Unit =
+ /** 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
+        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 _ =>
+      case _ => ()
 
   /** Retrieve symbol attached to given tree */
   protected def retrieveSym(tree: untpd.Tree)(using Context): Symbol = tree.removeAttachment(SymOfTree) match {

tests/warn/21681c.check

@@ -0,0 +1,7 @@
+-- [E203] Syntax Migration Warning: tests/warn/21681c.scala:6:2 --------------------------------------------------------
+6 |  (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 +1,7 @@
+
+object Test:
+  def age: Int = ???
+  def age_=(x: Int): Unit = ()
+  age = 29
+  (age = 29) // warn
+