address review comments

Author: bishabosha

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

@@ -408,11 +408,8 @@ class Synthesizer(typer: Typer)(using @constructorOnly c: Context):
       }
       val mirrorRef =
         if cls.useCompanionAsProductMirror then companionPath(mirroredType, span)
-        else
-          if defn.isTupleClass(cls) then // add `|| cls == defn.PairClass` when we support TupleXXL
-            newTupleMirror(arity = typeElems.size)
-          else
-            anonymousMirror(monoType, ExtendsProductMirror, span)
+        else if defn.isTupleClass(cls) then newTupleMirror(typeElems.size) // TODO: cls == defn.PairClass when > 22
+        else anonymousMirror(monoType, ExtendsProductMirror, span)
       withNoErrors(mirrorRef.cast(mirrorType))
     end makeProductMirror
 

library/src/scala/runtime/TupleMirror.scala

@@ -4,13 +4,11 @@ package scala.runtime
  *  as we do not need to synthesize an anonymous Mirror class at every callsite.
  */
 final class TupleMirror(arity: Int) extends scala.deriving.Mirror.Product with Serializable:
-  assert(arity > 0) // EmptyTuple is not a valid `MirroredType` for TupleMirror
+  assert(arity >= 0) // technically could be used for EmptyTuple also, but it has its own singleton mirror.
 
-  override type MirroredMonoType <: NonEmptyTuple
+  override type MirroredMonoType <: Tuple
 
   final def fromProduct(product: Product): MirroredMonoType =
     if product.productArity != arity then
       throw IllegalArgumentException(s"expected Product with $arity elements, got ${product.productArity}")
     runtime.Tuples.fromProduct(product).asInstanceOf[MirroredMonoType]
-
-  override final def toString: String = s"<tuple-mirror@${Integer.toHexString(hashCode).nn.take(6)}>"