diff --git a/compiler/src/dotty/tools/dotc/typer/Synthesizer.scala b/compiler/src/dotty/tools/dotc/typer/Synthesizer.scala
index 36addf639932..baa4ef57acd3 100644
--- a/compiler/src/dotty/tools/dotc/typer/Synthesizer.scala
+++ b/compiler/src/dotty/tools/dotc/typer/Synthesizer.scala
@@ -280,10 +280,9 @@ class Synthesizer(typer: Typer)(using @constructorOnly c: Context):
 
     def whyNotAcceptableType(tp: Type, cls: Symbol): String = tp match
       case tp: HKTypeLambda if tp.resultType.isInstanceOf[HKTypeLambda] =>
-        i"its subpart $tp is not a supported kind (either `*` or `* -> *`)"
+        i"its subpart `$tp` is not a supported kind (either `*` or `* -> *`)"
       case tp: TypeProxy    => whyNotAcceptableType(tp.underlying, cls)
-      case OrType(tp1, tp2) =>
-        Seq(tp1, tp2).map(whyNotAcceptableType(_, cls)).find(_.nonEmpty).getOrElse("")
+      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 $cls"
@@ -339,7 +338,7 @@ class Synthesizer(typer: Typer)(using @constructorOnly c: Context):
           if acceptableMsg.isEmpty then
             if cls.isGenericProduct then makeProductMirror(cls)
             else withErrors(i"$cls is not a generic product because ${cls.whyNotGenericProduct}")
-          else withErrors(i"type $mirroredType is not a generic product because $acceptableMsg")
+          else withErrors(i"type `$mirroredType` is not a generic product because $acceptableMsg")
   end productMirror
 
   private def sumMirror(mirroredType: Type, formal: Type, span: Span)(using Context): TreeWithErrors =
@@ -348,12 +347,11 @@ class Synthesizer(typer: Typer)(using @constructorOnly c: Context):
     val clsIsGenericSum = cls.isGenericSum
 
     def whyNotAcceptableType(tp: Type): String = tp match
-      case tp: TermRef => i"its subpart $tp is a term reference"
+      case tp: TermRef => i"its subpart `$tp` is a term reference"
       case tp: HKTypeLambda if tp.resultType.isInstanceOf[HKTypeLambda] =>
-        i"its subpart $tp is not a supported kind (either `*` or `* -> *`)"
+        i"its subpart `$tp` is not a supported kind (either `*` or `* -> *`)"
       case tp: TypeProxy => whyNotAcceptableType(tp.underlying)
-      case OrType(tp1, tp2) =>
-        Seq(tp1, tp2).map(whyNotAcceptableType).find(_.nonEmpty).getOrElse("")
+      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"
@@ -416,7 +414,7 @@ class Synthesizer(typer: Typer)(using @constructorOnly c: Context):
         else anonymousMirror(monoType, ExtendsSumMirror, span)
       withNoErrors(mirrorRef.cast(mirrorType))
     else if acceptableMsg.nonEmpty then
-      withErrors(i"type $mirroredType is not a generic sum because $acceptableMsg")
+      withErrors(i"type `$mirroredType` is not a generic sum because $acceptableMsg")
     else if !clsIsGenericSum then
       withErrors(i"$cls is not a generic sum because ${cls.whyNotGenericSum}")
     else
diff --git a/tests/neg/i14025.check b/tests/neg/i14025.check
index bd29136e3ab0..3c67b954297b 100644
--- a/tests/neg/i14025.check
+++ b/tests/neg/i14025.check
@@ -1,8 +1,8 @@
 -- Error: tests/neg/i14025.scala:1:88 ----------------------------------------------------------------------------------
 1 |val foo = summon[deriving.Mirror.Product { type MirroredType = [X] =>> [Y] =>> (X, Y) }] // error
   |                                                                                        ^
-  |No given instance of type deriving.Mirror.Product{MirroredType[X] = [Y] =>> (X, Y)} was found for parameter x of method summon in object Predef. Failed to synthesize an instance of type deriving.Mirror.Product{MirroredType[X] = [Y] =>> (X, Y)}: type [X] =>> [Y] =>> (X, Y) is not a generic product because its subpart [X] =>> [Y] =>> (X, Y) is not a supported kind (either `*` or `* -> *`)
+  |No given instance of type deriving.Mirror.Product{MirroredType[X] = [Y] =>> (X, Y)} was found for parameter x of method summon in object Predef. Failed to synthesize an instance of type deriving.Mirror.Product{MirroredType[X] = [Y] =>> (X, Y)}: type `[X] =>> [Y] =>> (X, Y)` is not a generic product because its subpart `[X] =>> [Y] =>> (X, Y)` is not a supported kind (either `*` or `* -> *`)
 -- Error: tests/neg/i14025.scala:2:90 ----------------------------------------------------------------------------------
 2 |val bar = summon[deriving.Mirror.Sum { type MirroredType = [X] =>> [Y] =>> List[(X, Y)] }] // error
   |                                                                                          ^
-  |No given instance of type deriving.Mirror.Sum{MirroredType[X] = [Y] =>> List[(X, Y)]} was found for parameter x of method summon in object Predef. Failed to synthesize an instance of type deriving.Mirror.Sum{MirroredType[X] = [Y] =>> List[(X, Y)]}: type [X] =>> [Y] =>> List[(X, Y)] is not a generic sum because its subpart [X] =>> [Y] =>> List[(X, Y)] is not a supported kind (either `*` or `* -> *`)
+  |No given instance of type deriving.Mirror.Sum{MirroredType[X] = [Y] =>> List[(X, Y)]} was found for parameter x of method summon in object Predef. Failed to synthesize an instance of type deriving.Mirror.Sum{MirroredType[X] = [Y] =>> List[(X, Y)]}: type `[X] =>> [Y] =>> List[(X, Y)]` is not a generic sum because its subpart `[X] =>> [Y] =>> List[(X, Y)]` is not a supported kind (either `*` or `* -> *`)
diff --git a/tests/neg/i14823.check b/tests/neg/i14823.check
index 1da3c0560d9c..4d5a64680882 100644
--- a/tests/neg/i14823.check
+++ b/tests/neg/i14823.check
@@ -2,5 +2,5 @@
 8 |val baz = summon[Mirror.Of[SubA[Int] | SubB[Int]]] // error
   |                                                  ^
   |No given instance of type deriving.Mirror.Of[SubA[Int] | SubB[Int]] was found for parameter x of method summon in object Predef. Failed to synthesize an instance of type deriving.Mirror.Of[SubA[Int] | SubB[Int]]: 
-  |	* type SubA[Int] | SubB[Int] is not a generic product because a subpart reduces to the more precise class SubA, expected class Cov
-  |	* type SubA[Int] | SubB[Int] is not a generic sum because a subpart reduces to the more precise class SubA, expected class Cov
+  |	* type `SubA[Int] | SubB[Int]` is not a generic product because its subpart `SubA[Int] | SubB[Int]` is a top-level union type.
+  |	* type `SubA[Int] | SubB[Int]` is not a generic sum because its subpart `SubA[Int] | SubB[Int]` is a top-level union type.
diff --git a/tests/neg/i14823a.check b/tests/neg/i14823a.check
new file mode 100644
index 000000000000..9c917548d9bf
--- /dev/null
+++ b/tests/neg/i14823a.check
@@ -0,0 +1,24 @@
+-- Error: tests/neg/i14823a.scala:16:48 --------------------------------------------------------------------------------
+16 |val foo = summon[Mirror.Of[Box[Int] | Box[Int]]] // error
+   |                                                ^
+   |No given instance of type deriving.Mirror.Of[Box[Int] | Box[Int]] was found for parameter x of method summon in object Predef. Failed to synthesize an instance of type deriving.Mirror.Of[Box[Int] | Box[Int]]: 
+   |	* type `Box[Int] | Box[Int]` is not a generic product because its subpart `Box[Int] | Box[Int]` is a top-level union type.
+   |	* type `Box[Int] | Box[Int]` is not a generic sum because its subpart `Box[Int] | Box[Int]` is a top-level union type.
+-- Error: tests/neg/i14823a.scala:17:58 --------------------------------------------------------------------------------
+17 |val bar = summon[MirrorK1.Of[[X] =>> Box[Int] | Box[Int]]] // error
+   |                                                          ^
+   |No given instance of type MirrorK1.Of[[X] =>> Box[Int] | Box[Int]] was found for parameter x of method summon in object Predef. Failed to synthesize an instance of type MirrorK1.Of[[X] =>> Box[Int] | Box[Int]]: 
+   |	* type `[A] =>> Box[Int] | Box[Int]` is not a generic product because its subpart `Box[Int] | Box[Int]` is a top-level union type.
+   |	* type `[A] =>> Box[Int] | Box[Int]` is not a generic sum because its subpart `Box[Int] | Box[Int]` is a top-level union type.
+-- Error: tests/neg/i14823a.scala:18:63 --------------------------------------------------------------------------------
+18 |def baz = summon[deriving.Mirror.Of[Foo[String] | Foo[String]]] // error
+   |                                                               ^
+   |No given instance of type deriving.Mirror.Of[Foo[String] | Foo[String]] was found for parameter x of method summon in object Predef. Failed to synthesize an instance of type deriving.Mirror.Of[Foo[String] | Foo[String]]: 
+   |	* type `Foo[String] | Foo[String]` is not a generic product because its subpart `Foo[String] | Foo[String]` is a top-level union type.
+   |	* type `Foo[String] | Foo[String]` is not a generic sum because its subpart `Foo[String] | Foo[String]` is a top-level union type.
+-- Error: tests/neg/i14823a.scala:20:66 --------------------------------------------------------------------------------
+20 |def qux = summon[deriving.Mirror.Of[Option[Int] | Option[String]]] // error
+   |                                                                  ^
+   |No given instance of type deriving.Mirror.Of[Option[Int] | Option[String]] was found for parameter x of method summon in object Predef. Failed to synthesize an instance of type deriving.Mirror.Of[Option[Int] | Option[String]]: 
+   |	* type `Option[Int] | Option[String]` is not a generic product because its subpart `Option[Int] | Option[String]` is a top-level union type.
+   |	* type `Option[Int] | Option[String]` is not a generic sum because its subpart `Option[Int] | Option[String]` is a top-level union type.
diff --git a/tests/neg/i14823a.scala b/tests/neg/i14823a.scala
new file mode 100644
index 000000000000..b80f8e5062e3
--- /dev/null
+++ b/tests/neg/i14823a.scala
@@ -0,0 +1,20 @@
+import deriving.Mirror
+
+object MirrorK1:
+  type Of[F[_]] = Mirror { type MirroredType[A] = F[A] }
+
+sealed trait Box[T]
+object Box
+
+case class Child[T]() extends Box[T]
+
+sealed abstract class Foo[T]
+object Foo {
+  case class A[T]() extends Foo[T]
+}
+
+val foo = summon[Mirror.Of[Box[Int] | Box[Int]]] // error
+val bar = summon[MirrorK1.Of[[X] =>> Box[Int] | Box[Int]]] // error
+def baz = summon[deriving.Mirror.Of[Foo[String] | Foo[String]]] // error
+
+def qux = summon[deriving.Mirror.Of[Option[Int] | Option[String]]] // error
diff --git a/tests/pos/i14823.scala b/tests/pos/i14823.scala
deleted file mode 100644
index 01b8a76095d5..000000000000
--- a/tests/pos/i14823.scala
+++ /dev/null
@@ -1,18 +0,0 @@
-import deriving.Mirror
-
-object MirrorK1:
-  type Of[F[_]] = Mirror { type MirroredType[A] = F[A] }
-
-sealed trait Box[T]
-object Box
-
-case class Child[T]() extends Box[T]
-
-sealed abstract class Foo[T]
-object Foo {
-  case class A[T]() extends Foo[T]
-}
-
-val foo = summon[Mirror.Of[Box[Int] | Box[Int]]]
-val bar = summon[MirrorK1.Of[[X] =>> Box[Int] | Box[Int]]]
-def baz = summon[deriving.Mirror.Of[Foo[String] | Foo[String]]]