Warn when named tuples resemble assignments (#21823)

This PR adds a warning for named tuples that look like assignment, such
as `(x = 1)`.

This is the first half to implement #21681. The second will be to add
warnings for named arguments to infix method calls (as a separate PR?).

Started during the Spree of October 21st.

Closes #21770.

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)

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 = ""

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 +3424,20 @@ 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 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 _ => ()
+
   /** 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/21681.check

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

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

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

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

tests/warn/21770.check

@@ -0,0 +1,7 @@
+-- [E203] Syntax Migration Warning: tests/warn/21770.scala:5:9 ---------------------------------------------------------
+5 |  f(i => (cache = Some(i))) // warn
+  |         ^^^^^^^^^^^^^^^^^
+  |         Ambiguous syntax: this is interpreted as a named tuple with one element,
+  |         not as an assignment.
+  |
+  |         To assign a value, use curly braces: `{cache = Some(i)}`.

tests/warn/21770.scala

@@ -0,0 +1,5 @@
+def f(g: Int  => Unit) = g(0)
+
+def test = 
+  var cache: Option[Int] = None
+  f(i => (cache = Some(i))) // warn