Don't generate Mirror.Sum for simple enum cases

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

@@ -273,7 +273,11 @@ class Synthesizer(typer: Typer)(using @constructorOnly c: Context):
     val cls = mirroredType.classSymbol
     val useCompanion = cls.useCompanionAsSumMirror
 
-    if cls.isGenericSum(if useCompanion then cls.linkedClass else ctx.owner) then
+    def acceptable(tp: Type): Boolean = tp match
+      case OrType(tp1, tp2) => acceptable(tp1) && acceptable(tp2)
+      case _                => !tp.termSymbol.isEnumCase && (tp.classSymbol eq cls)
+
+    if acceptable(mirroredType) && cls.isGenericSum(if useCompanion then cls.linkedClass else ctx.owner) then
       val elemLabels = cls.children.map(c => ConstantType(Constant(c.name.toString)))
 
       def solve(sym: Symbol): Type = sym match

tests/neg/scala_enum_testing.scala

@@ -0,0 +1,28 @@
+import scala.util.NotGiven
+import scala.compiletime.*
+import scala.deriving.Mirror
+
+enum Color:
+  case Red, Green, Blue
+end Color
+
+enum Bar:
+  case A(i: Int)
+  case B(b: Boolean)
+  case C(s: String)
+
+object Singletons {
+  object A
+  object B
+}
+
+object Test {
+
+    def main(args: Array[String]): Unit = {
+      summon[Mirror.ProductOf[Color]] // error
+      summon[Mirror.SumOf[Color.Red.type]] // error
+      summon[Mirror.SumOf[Color.Red.type | Color.Green.type]] // error
+      summon[Mirror.SumOf[Bar.A | Bar.B]] // error
+      summon[Mirror.SumOf[Singletons.A.type | Singletons.B.type]] // error
+    }
+}

tests/run/scala_enum_testing.scala

@@ -0,0 +1,17 @@
+import scala.util.NotGiven
+import scala.compiletime.*
+import scala.deriving.Mirror
+
+enum Color:
+  case Red, Green, Blue
+end Color
+
+object Test {
+
+    def main(args: Array[String]): Unit = {
+      summon[Mirror.Of[Color.Red.type]]
+      summon[Mirror.Of[Color]]
+      summon[Mirror.ProductOf[Color.Red.type]]
+      summon[Mirror.SumOf[Color]]
+    }
+}