check defined in src for cache in companion

compiler/src/dotty/tools/dotc/core/Symbols.scala

@@ -143,6 +143,20 @@ object Symbols {
           false
       }
 
+    /** Is this symbol valid in the current run and has an associated file that is
+      * not a binary file. e.g. This will return true for
+      * symbols defined by the user in a prior run of the REPL, that are still valid.
+      */
+    final def isDefinedInSource(using Context): Boolean =
+      span.exists && isValidInCurrentRun && {
+        try
+          val file = associatedFile
+          file != null && file.extension != "class"
+        catch case ex: StaleSymbol =>
+          // can happen for constructor proxy companions. Test case is pos-macros/i9484.
+          false
+      }
+
     /** Is symbol valid in current run? */
     final def isValidInCurrentRun(using Context): Boolean =
       (lastDenot.validFor.runId == ctx.runId || stillValid(lastDenot)) &&

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

@@ -111,18 +111,19 @@ object SymUtils:
       self.isAllOf(Given | Method) && isCodefined(self.info)
 
     def useCompanionAsSumMirror(using Context): Boolean =
+      def companionExtendsSum(using Context): Boolean =
+        self.linkedClass.isSubClass(defn.Mirror_SumClass)
       self.linkedClass.exists
-      && !self.is(Scala2x)
-      && (
-        // If the sum type is compiled from source, and `self` is a "generic sum"
-        // then its companion object will become a sum mirror in `posttyper`. (This method
-        // can be called from `typer` when summoning a Mirror.)
-        // However if `self` is from a prior run then we should check that its companion subclasses `Mirror.Sum`.
-        // e.g. before Scala 3.1, hierarchical sum types were not considered "generic sums", so their
-        // companion would not cache the mirror. Companions from TASTy will already be typed as `Mirror.Sum`.
-        self.isDefinedInCurrentRun
-        || self.linkedClass.isSubClass(defn.Mirror_SumClass)
-      )
+        && !self.is(Scala2x)
+        && (
+          // If the sum type is compiled from source, and `self` is a "generic sum"
+          // then its companion object will become a sum mirror in `posttyper`. (This method
+          // can be called from `typer` when summoning a Mirror.)
+          // However if `self` is from a prior run then we should check that its companion subclasses `Mirror.Sum`.
+          // e.g. before Scala 3.1, hierarchical sum types were not considered "generic sums", so their
+          // companion would not cache the mirror. Companions from TASTy will already be typed as `Mirror.Sum`.
+          self.isDefinedInSource || companionExtendsSum
+        )
 
     /** Is this a sealed class or trait for which a sum mirror is generated?
     *  It must satisfy the following conditions:

compiler/test-resources/repl/i14540

@@ -0,0 +1,7 @@
+scala> enum I14540 { case A }
+// defined class I14540
+scala> summon[scala.deriving.Mirror.SumOf[I14540]] eq I14540
+val res0: Boolean = true
+scala> enum I14540B { case A }; summon[scala.deriving.Mirror.SumOf[I14540B]] eq I14540B
+// defined class I14540B
+val res1: Boolean = true

sbt-test/scala3-backcompat/hierarchical-mirrors/app/Main.scala

@@ -5,4 +5,5 @@ import scala.deriving.Mirror
 object Main:
   def main(args: Array[String]): Unit =
     val mirrorTop = summon[Mirror.SumOf[lib.Top]]
+    assert(mirrorTop ne lib.Top) // **NOT** cached in companion - previous run, pre-3.1 tasty dependency
     assert(mirrorTop.ordinal(lib.Middle()) == 0)

tests/run/i14540-priorRun/Lib_1.scala

@@ -0,0 +1,7 @@
+package lib
+
+sealed trait Top
+object Top // companion is necessary
+
+case class Middle() extends Top with Bottom
+sealed trait Bottom extends Top

tests/run/i14540-priorRun/Test_2.scala

@@ -0,0 +1,6 @@
+import scala.deriving.Mirror
+
+@main def Test =
+  val mirrorTop = summon[Mirror.SumOf[lib.Top]]
+  assert(mirrorTop eq lib.Top) // cached in companion - previous run, tasty dependency
+  assert(mirrorTop.ordinal(lib.Middle()) == 0)

tests/run/i14540-sameRun/Lib.scala

@@ -0,0 +1,7 @@
+package lib
+
+sealed trait Top
+object Top // companion is necessary
+
+case class Middle() extends Top with Bottom
+sealed trait Bottom extends Top

tests/run/i14540-sameRun/Test.scala

@@ -0,0 +1,6 @@
+import scala.deriving.Mirror
+
+@main def Test =
+  val mirrorTop = summon[Mirror.SumOf[lib.Top]]
+  assert(mirrorTop eq lib.Top) // cached in companion - same run, source dependency
+  assert(mirrorTop.ordinal(lib.Middle()) == 0)