Our progress from the scala-compiler spree:

- Added an error message when named matching was tried, while element is not a case class or named tuple.

Author: wwbakker

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

@@ -1765,19 +1765,24 @@ object desugar {
       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 $pt", arg.srcPos)
-      reordered.toList
+      if selNames.isEmpty then {
+        report.error(NamedElementsNotDefined(pt), elems.head.srcPos)
+        Nil
+      } else {
+        val nameToIdx = selNames.zipWithIndex.toMap
+        val reordered = Array.fill[untpd.Tree](selNames.length):
+          untpd.Ident(nme.WILDCARD).withSpan(wildcardSpan)
+        for case arg@NamedArg(name: TermName, _) <- elems do
+          nameToIdx.get(name) match
+            case Some(idx) =>
+              if reordered(idx).isInstanceOf[Ident] then
+                reordered(idx) = arg
+              else
+                report.error(em"Duplicate named pattern", arg.srcPos)
+            case _ =>
+              report.error(em"No element named `$name` is defined in selector type $pt", arg.srcPos)
+        reordered.toList
+      }
 
     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 NamedElementsNotDefinedID // errorNumber: 215
 
   def errorNumber = ordinal - 1
 

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

@@ -3523,4 +3523,10 @@ final class IllegalContextBounds(using Context) extends SyntaxMsg(IllegalContext
 
   override protected def explain(using Context): String = ""
 
-end IllegalContextBounds
+final class NamedElementsNotDefined(selectorType: Type)(using Context) extends PatternMatchMsg(NamedElementsNotDefinedID):
+  override protected def msg(using Context): String =
+    i"Selector type $selectorType does not define any named elements"
+
+  override protected def explain(using Context): String =
+    i"""|$selectorType is not a valid selector type for the use with named patterns.
+        |Only named tuples and case classes can be used with named patterns.""".stripMargin