improve error message

Author: bishabosha

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

@@ -302,7 +302,7 @@ class Synthesizer(typer: Typer)(using @constructorOnly c: Context):
       case ClassSymbol(cls) => cls
       case GenericTuple(arity, _) =>
         if arity <= Definitions.MaxTupleArity then defn.TupleType(arity).nn.classSymbol
-        else defn.TupleXXLClass
+        else defn.PairClass
 
   object MirrorSource:
     def tuple(tps: List[Type]): MirrorSource.GenericTuple = MirrorSource.GenericTuple(tps.size, tps)
@@ -340,21 +340,24 @@ class Synthesizer(typer: Typer)(using @constructorOnly c: Context):
       else None
 
     /** do all parts match the class symbol? */
-    def whyNotAcceptableType(tp: Type, msrc: MirrorSource): String = tp match
-      case tp: HKTypeLambda if tp.resultType.isInstanceOf[HKTypeLambda] =>
-        i"its subpart `$tp` is not a supported kind (either `*` or `* -> *`)"
-      case OrType(tp1, tp2) => i"its subpart `$tp` is a top-level union type."
-      case GenericTupleType(args) if args.size <= Definitions.MaxTupleArity =>
-        val tup = MirrorSource.tuple(args)
-        if tup.equiv(msrc) then ""
-        else i"a subpart reduces to the unrelated tuple ${tup.asClass}, expected ${msrc.asClass}"
-      case tp: TypeProxy    => whyNotAcceptableType(tp.underlying, msrc)
-      case _ =>
-        mirrorSource(tp) match
-          case Some(msrc2) =>
-            if msrc2.equiv(msrc) then ""
-            else i"a subpart reduces to the more precise ${msrc2.asClass}, expected ${msrc.asClass}"
-          case _ => "???" // caught early by initial `mirrorSource` that made `msrc`
+    def whyNotAcceptableType(tp: Type, msrc: MirrorSource): String =
+      def compareMirrorSrc(msrc1: MirrorSource): String =
+        if msrc1.equiv(msrc) then ""
+        else i"a subpart reduces to the unrelated ${msrc1.asClass}, expected ${msrc.asClass}"
+      tp match
+        case tp: HKTypeLambda if tp.resultType.isInstanceOf[HKTypeLambda] =>
+          i"its subpart $tp is not a supported kind (either `*` or `* -> *`)"
+        case OrType(tp1, tp2) => i"its subpart `$tp` is a top-level union type."
+        case GenericTupleType(args) =>
+          val maxArity = Definitions.MaxTupleArity
+          val tup = MirrorSource.tuple(args)
+          if tup.arity <= maxArity then compareMirrorSrc(tup)
+          else i"a subpart `$tp` reduces to a tuple with arity ${tup.arity}, expected arity <= $maxArity"
+        case tp: TypeProxy    => whyNotAcceptableType(tp.underlying, msrc)
+        case _                =>
+          mirrorSource(tp) match
+            case Some(msrc2) => compareMirrorSrc(msrc2)
+            case _ => i"a subpart `$tp` does not reduce to a class or generic tuple type"
 
     /** widen TermRef to see if they are an alias to an enum singleton */
     def isEnumSingletonRef(tp: Type)(using Context): Boolean = tp match
@@ -418,7 +421,7 @@ class Synthesizer(typer: Typer)(using @constructorOnly c: Context):
                   withErrors(i"${msrc.asClass} is not a generic product because ${msrc.asClass.whyNotGenericProduct}")
               else withErrors(i"type `$mirroredType` is not a generic product because $acceptableMsg")
             case None =>
-              withErrors(i"type `$mirroredType` does not reduce to a class or generic tuple type")
+              withErrors(i"type $mirroredType does not reduce to a class or generic tuple type")
   end productMirror
 
   private def sumMirror(mirroredType: Type, formal: Type, span: Span)(using Context): TreeWithErrors =
@@ -434,7 +437,7 @@ class Synthesizer(typer: Typer)(using @constructorOnly c: Context):
       case OrType(tp1, tp2) => i"its subpart `$tp` is a top-level union type."
       case _ =>
         if tp.classSymbol eq cls then ""
-        else i"a subpart reduces to the more precise ${tp.classSymbol}, expected $cls"
+        else i"a subpart reduces to the unrelated ${tp.classSymbol}, expected $cls"
 
 
     val acceptableMsg = whyNotAcceptableType(mirroredType)

tests/neg/i14127.check

@@ -0,0 +1,10 @@
+-- Error: tests/neg/i14127.scala:6:55 ----------------------------------------------------------------------------------
+6 |    *: Int *: Int *: Int *: Int *: Int *: EmptyTuple)]] // error
+  |                                                       ^
+  |No given instance of type deriving.Mirror.Of[(Int, Int, Int, Int, Int, Int, Int, Int, Int, Int, Int, Int, Int, Int, Int, Int, Int, Int, Int, Int, 
+  |  Int
+  |, Int, Int)] was found for parameter x of method summon in object Predef. Failed to synthesize an instance of type deriving.Mirror.Of[(Int, Int, Int, Int, Int, Int, Int, Int, Int, Int, Int, Int, Int, Int, Int, Int, Int, Int, Int, Int, 
+  |  Int
+  |, Int, Int)]: 
+  |	* type (Int, Int, Int, Int, Int, Int, Int, Int, Int, Int, Int, Int, Int, Int, Int, Int, Int, Int, Int, Int, Int, Int, Int) is not a generic product because a subpart `(Int, Int, Int, Int, Int, Int, Int, Int, Int, Int, Int, Int, Int, Int, Int, Int, Int, Int, Int, Int, Int, Int, Int)` reduces to a tuple with arity 23, expected arity <= 22
+  |	* class *: is not a generic sum because it does not have subclasses