Warn for named tuples that look like assignments

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

@@ -1605,9 +1605,10 @@ object desugar {
 
   /** Translate tuple expressions
    *
-   *     ()             ==>   ()
-   *     (t)            ==>   t
-   *     (t1, ..., tN)  ==>   TupleN(t1, ..., tN)
+   *     ()                       ==>   ()
+   *     (t)                      ==>   t
+   *     (t1, ..., tN)            ==>   TupleN(t1, ..., tN)
+   *     (n1 = t1, ..., nN = tN)  ==>   NamedTuple.build[(n1, ..., nN)]()(TupleN(t1, ..., tN))
    */
   def tuple(tree: Tuple, pt: Type)(using Context): Tree =
     var elems = checkWellFormedTupleElems(tree.trees)
@@ -1638,6 +1639,8 @@ 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/reporting/ErrorMessageID.scala

@@ -216,6 +216,7 @@ enum ErrorMessageID(val isActive: Boolean = true) extends java.lang.Enum[ErrorMe
   case FinalLocalDefID // errorNumber: 200
   case NonNamedArgumentInJavaAnnotationID // errorNumber: 201
   case QuotedTypeMissingID // errorNumber: 202
+  case AmbiguousNamedTupleAssignmentID // errorNumber: 203
 
   def errorNumber = ordinal - 1
 

compiler/src/dotty/tools/dotc/reporting/messages.scala

@@ -3343,3 +3343,12 @@ final class QuotedTypeMissing(tpe: Type)(using Context) extends StagingMessage(Q
         |"""
 
 end QuotedTypeMissing
+
+final class AmbiguousNamedTupleAssignment(key: Name, value: untpd.Tree)(using Context) extends SyntaxMsg(AmbiguousNamedTupleAssignmentID):
+  override protected def msg(using Context): String =
+    i"""Ambiguous syntax: this is interpreted as a named tuple with one element,
+      |not as an assignment.
+      |
+      |To assign a value, use curly braces: `{${key} = ${value}}`."""
+
+  override protected def explain(using Context): String = ""

tests/warn/21681.check

@@ -0,0 +1,7 @@
+-- [E203] Syntax Migration Warning: tests/warn/21681.scala:4:2 ---------------------------------------------------------
+4 |  (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/21681.scala

@@ -0,0 +1,5 @@
+
+def main() =
+  var age: Int = 28
+  (age = 29) // warn
+