Tighten check to cache Mirror.Sum in companion

cache the mirror in the companion if:
- the class is not scala 2 defined
  & the class has a companion
  & the class was either:
    - defined in source
    - from tasty and its companion is a subtype of Mirror.Sum

Author: bishabosha

compiler/src/dotty/tools/dotc/transform/SymUtils.scala

@@ -112,7 +112,14 @@ object SymUtils:
           self.isCoDefinedGiven(res.typeSymbol)
       self.isAllOf(Given | Method) && isCodefined(self.info)
 
-    def useCompanionAsMirror(using Context): Boolean = self.linkedClass.exists && !self.is(Scala2x)
+    def useCompanionAsSumMirror(using Context): Boolean =
+      self.linkedClass.exists
+      && !self.is(Scala2x)
+      && (
+        // self is from source, or companion is a subtype of Sum
+        self.isDefinedInCurrentRun
+        || self.linkedClass.isSubClass(defn.Mirror_SumClass)
+      )
 
     /** Is this a sealed class or trait for which a sum mirror is generated?
     *  It must satisfy the following conditions:
@@ -129,7 +136,7 @@ object SymUtils:
         s"it is not an abstract class"
       else {
         val children = self.children
-        val companionMirror = self.useCompanionAsMirror
+        val companionMirror = self.useCompanionAsSumMirror
         assert(!(companionMirror && (declScope ne self.linkedClass)))
         def problem(child: Symbol) = {
 
@@ -144,7 +151,7 @@ object SymUtils:
             val s = child.whyNotGenericProduct
             if (s.isEmpty) s
             else if (child.is(Sealed)) {
-              val s = child.whyNotGenericSum(if child.useCompanionAsMirror then child.linkedClass else ctx.owner)
+              val s = child.whyNotGenericSum(if child.useCompanionAsSumMirror then child.linkedClass else ctx.owner)
               if (s.isEmpty) s
               else i"its child $child is not a generic sum because $s"
             } else i"its child $child is not a generic product because $s"

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

@@ -271,7 +271,7 @@ class Synthesizer(typer: Typer)(using @constructorOnly c: Context):
 
   private def sumMirror(mirroredType: Type, formal: Type, span: Span)(using Context): Tree =
     val cls = mirroredType.classSymbol
-    val useCompanion = cls.useCompanionAsMirror
+    val useCompanion = cls.useCompanionAsSumMirror
 
     if cls.isGenericSum(if useCompanion then cls.linkedClass else ctx.owner) then
       val elemLabels = cls.children.map(c => ConstantType(Constant(c.name.toString)))