Unhelpful error message when trying to use named extraction, when not matching case class or named tuple

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

@@ -1719,7 +1719,7 @@ object desugar {
    */
   def tuple(tree: Tuple, pt: Type)(using Context): Tree =
     var elems = checkWellFormedTupleElems(tree.trees)
-    if ctx.mode.is(Mode.Pattern) then elems = adaptPatternArgs(elems, pt)
+    if ctx.mode.is(Mode.Pattern) then elems = adaptPatternArgs(elems, pt, tree.srcPos)
     val elemValues = elems.mapConserve(stripNamedArg)
     val tup =
       val arity = elems.length
@@ -1759,25 +1759,31 @@ object desugar {
    *   - If `elems` are named pattern elements, rearrange them to match `pt`.
    *     This requires all names in `elems` to be also present in `pt`.
    */
-  def adaptPatternArgs(elems: List[Tree], pt: Type)(using Context): List[Tree] =
+  def adaptPatternArgs(elems: List[Tree], pt: Type, pos: SrcPos)(using Context): List[Tree] =
 
     def reorderedNamedArgs(wildcardSpan: Span): List[untpd.Tree] =
-      var selNames = pt.namedTupleElementTypes(false).map(_(0))
-      if selNames.isEmpty && pt.classSymbol.is(CaseClass) then
-        selNames = pt.classSymbol.caseAccessors.map(_.name.asTermName)
-      val nameToIdx = selNames.zipWithIndex.toMap
-      val reordered = Array.fill[untpd.Tree](selNames.length):
-        untpd.Ident(nme.WILDCARD).withSpan(wildcardSpan)
-      for case arg @ NamedArg(name: TermName, _) <- elems do
-        nameToIdx.get(name) match
-          case Some(idx) =>
-            if reordered(idx).isInstanceOf[Ident] then
-              reordered(idx) = arg
-            else
-              report.error(em"Duplicate named pattern", arg.srcPos)
-          case _ =>
-            report.error(em"No element named `$name` is defined in selector type $pt", arg.srcPos)
-      reordered.toList
+      inline def isCaseClass = pt.classSymbol.is(CaseClass) && !defn.isTupleClass(pt.classSymbol)
+      if !isCaseClass && !pt.isNamedTupleType then
+        report.error(NamedPatternNotApplicable(pt), pos)
+        Nil
+      else
+        var selNames = pt.namedTupleElementTypes(false).map(_(0))
+        if isCaseClass && selNames.isEmpty then
+          selNames = pt.classSymbol.caseAccessors.map(_.name.asTermName)
+        val nameToIdx = selNames.zipWithIndex.toMap
+        val reordered = Array.fill[untpd.Tree](selNames.length):
+          untpd.Ident(nme.WILDCARD).withSpan(wildcardSpan)
+        for case arg @ NamedArg(name: TermName, _) <- elems do
+          nameToIdx.get(name) match
+            case Some(idx) =>
+              if reordered(idx).isInstanceOf[Ident] then
+                reordered(idx) = arg
+              else
+                report.error(em"Duplicate named pattern", arg.srcPos)
+            case _ =>
+              report.error(em"No element named `$name` is defined in selector type $pt", arg.srcPos)
+        reordered.toList
+      end if
 
     elems match
       case (first @ NamedArg(_, _)) :: _ => reorderedNamedArgs(first.span.startPos)

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

@@ -228,6 +228,7 @@ enum ErrorMessageID(val isActive: Boolean = true) extends java.lang.Enum[ErrorMe
   case OnlyFullyDependentAppliedConstructorTypeID // errorNumber: 212
   case PointlessAppliedConstructorTypeID // errorNumber: 213
   case IllegalContextBoundsID // errorNumber: 214
+  case NamedPatternNotApplicableID // errorNumber: 215
 
   def errorNumber = ordinal - 1
 

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

@@ -3534,4 +3534,8 @@ final class IllegalContextBounds(using Context) extends SyntaxMsg(IllegalContext
 
   override protected def explain(using Context): String = ""
 
-end IllegalContextBounds
+final class NamedPatternNotApplicable(selectorType: Type)(using Context) extends PatternMatchMsg(NamedPatternNotApplicableID):
+  override protected def msg(using Context): String =
+    i"Named patterns cannot be used with $selectorType, because it is not a named tuple or case class"
+
+  override protected def explain(using Context): String = ""

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

@@ -207,10 +207,10 @@ object Applications {
         case List(defn.NamedTuple(_, _))=>
           // if the product types list is a singleton named tuple, autotupling might be applied, so don't fail eagerly
           tryEither[Option[List[untpd.Tree]]]
-            (Some(desugar.adaptPatternArgs(elems, pt)))
+            (Some(desugar.adaptPatternArgs(elems, pt, pos)))
             ((_, _) => None)
         case pts =>
-          Some(desugar.adaptPatternArgs(elems, pt))
+          Some(desugar.adaptPatternArgs(elems, pt, pos))
 
     private def getUnapplySelectors(tp: Type)(using Context): List[Type] =
       // We treat patterns as product elements if

tests/neg/i22903.check

@@ -0,0 +1,4 @@
+-- [E215] Pattern Match Error: tests/neg/i22903.scala:18:21 ------------------------------------------------------------
+18 |    case ProductMatch(someName = x) => println (x) // error
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |         Named patterns cannot be used with CustomProduct, because it is not a named tuple or case class

tests/neg/i22903.scala

@@ -0,0 +1,20 @@
+class CustomProduct(x: Int) extends Product {
+  def _1 = someName
+  def _2 = blub
+
+  val someName = x + 5
+  val blub = "blub"
+
+  override def canEqual(that: Any): Boolean = ???
+}
+
+object ProductMatch {
+  def unapply(x: Int): CustomProduct = new CustomProduct(x)
+}
+
+@main
+def run = {
+  3 match {
+    case ProductMatch(someName = x) => println (x) // error
+  }
+}

tests/neg/named-tuples.check

@@ -47,14 +47,10 @@
 26 |  val (name = x, agee = y) = person // error
    |                 ^^^^^^^^
    |                 No element named `agee` is defined in selector type (name : String, age : Int)
--- Error: tests/neg/named-tuples.scala:29:10 ---------------------------------------------------------------------------
-29 |    case (name = n, age = a) => () // error // error
-   |          ^^^^^^^^
-   |          No element named `name` is defined in selector type (String, Int)
--- Error: tests/neg/named-tuples.scala:29:20 ---------------------------------------------------------------------------
-29 |    case (name = n, age = a) => () // error // error
-   |                    ^^^^^^^
-   |                    No element named `age` is defined in selector type (String, Int)
+-- [E215] Pattern Match Error: tests/neg/named-tuples.scala:29:9 -------------------------------------------------------
+29 |    case (name = n, age = a) => () // error
+   |         ^^^^^^^^^^^^^^^^^^^
+   |         Named patterns cannot be used with (String, Int), because it is not a named tuple or case class
 -- [E172] Type Error: tests/neg/named-tuples.scala:31:27 ---------------------------------------------------------------
 31 |  val pp = person ++ (1, 2)  // error
    |                           ^
@@ -75,10 +71,10 @@
 41 |    case (name, age = a) => () // error
    |                ^^^^^^^
    |                Illegal combination of named and unnamed tuple elements
--- Error: tests/neg/named-tuples.scala:44:10 ---------------------------------------------------------------------------
+-- [E215] Pattern Match Error: tests/neg/named-tuples.scala:44:9 -------------------------------------------------------
 44 |    case (age = x) => // error
-   |          ^^^^^^^
-   |          No element named `age` is defined in selector type Tuple
+   |         ^^^^^^^^^
+   |         Named patterns cannot be used with Tuple, because it is not a named tuple or case class
 -- [E172] Type Error: tests/neg/named-tuples.scala:46:27 ---------------------------------------------------------------
 46 |  val p2 = person ++ person // error
    |                           ^

tests/neg/named-tuples.scala

@@ -26,7 +26,7 @@ object Test:
   val (name = x, agee = y) = person // error
 
   ("Ives", 2) match
-    case (name = n, age = a) => () // error // error
+    case (name = n, age = a) => () // error
 
   val pp = person ++ (1, 2)  // error
   val qq = ("a", true) ++ (1, 2)