check defined in src for cache in companion

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

@@ -132,16 +132,24 @@ object Symbols {
     private[core] def defRunId: RunId =
       if (lastDenot == null) NoRunId else lastDenot.validFor.runId
 
+    private inline def associatedFileMatches(inline filter: AbstractFile => Boolean)(using Context): Boolean =
+      try
+        val file = associatedFile
+        file != null && filter(file)
+      catch case ex: StaleSymbol =>
+        // can happen for constructor proxy companions. Test case is pos-macros/i9484.
+        false
+
     /** Does this symbol come from a currently compiled source file? */
     final def isDefinedInCurrentRun(using Context): Boolean =
-      span.exists && defRunId == ctx.runId && {
-        try
-          val file = associatedFile
-          file != null && ctx.run.files.contains(file)
-        catch case ex: StaleSymbol =>
-          // can happen for constructor proxy companions. Test case is pos-macros/i9484.
-          false
-      }
+      span.exists && defRunId == ctx.runId && associatedFileMatches(ctx.run.files.contains)
+
+    /** 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 && associatedFileMatches(_.extension != "class")
 
     /** Is symbol valid in current run? */
     final def isValidInCurrentRun(using Context): Boolean =

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

@@ -111,18 +111,20 @@ 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 binary file, 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)