Improved logic & error message.

Author: wwbakker

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

@@ -1762,13 +1762,14 @@ object desugar {
   def adaptPatternArgs(elems: List[Tree], pt: Type)(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)
-      if selNames.isEmpty then {
-        report.error(NamedElementsNotDefined(pt), elems.head.srcPos)
+      val isCaseClass = pt.classSymbol.is(CaseClass) && !defn.isTupleClass(pt.classSymbol)
+      if !pt.isNamedTupleType && !isCaseClass then
+        report.error(NamedPatternNotApplicable(pt), elems.head.srcPos)
         Nil
-      } else {
+      else
+        var selNames = pt.namedTupleElementTypes(false).map(_(0))
+        if selNames.isEmpty && isCaseClass 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)
@@ -1782,7 +1783,7 @@ object desugar {
             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

@@ -228,7 +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
+  case NamedPatternNotApplicableID // errorNumber: 215
 
   def errorNumber = ordinal - 1
 

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

@@ -3523,10 +3523,8 @@ final class IllegalContextBounds(using Context) extends SyntaxMsg(IllegalContext
 
   override protected def explain(using Context): String = ""
 
-final class NamedElementsNotDefined(selectorType: Type)(using Context) extends PatternMatchMsg(NamedElementsNotDefinedID):
+final class NamedPatternNotApplicable(selectorType: Type)(using Context) extends PatternMatchMsg(NamedPatternNotApplicableID):
   override protected def msg(using Context): String =
-    i"Selector type $selectorType does not define any named elements"
+    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 =
-    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
+  override protected def explain(using Context): String = ""

tests/neg/named-tuples.scala

@@ -58,3 +58,7 @@ object Test:
     // conversion to (name: Int, age: Int). This one would be harder to guard against.
 
   val typo: (name: ?, age: ?) = (name = "he", ag = 1) // error
+
+  val unnamedTuple: (String, Int) = ("text", 10)
+  unnamedTuple match
+    case (name = n, age = a) => // error