From 8d03e8c9d64a596fb6e0de4b00228ab1d2a1947c Mon Sep 17 00:00:00 2001
From: Dale Wijnand <dale.wijnand@gmail.com>
Date: Fri, 23 Feb 2024 17:22:37 +0000
Subject: [PATCH] Avoid losing the symbols denotation on update

---
 compiler/src/dotty/tools/dotc/core/Symbols.scala | 15 +++++++++++----
 1 file changed, 11 insertions(+), 4 deletions(-)

diff --git a/compiler/src/dotty/tools/dotc/core/Symbols.scala b/compiler/src/dotty/tools/dotc/core/Symbols.scala
index 7cad8bd0e250..90108a15a816 100644
--- a/compiler/src/dotty/tools/dotc/core/Symbols.scala
+++ b/compiler/src/dotty/tools/dotc/core/Symbols.scala
@@ -110,16 +110,23 @@ object Symbols {
     private def computeDenot(lastd: SymDenotation)(using Context): SymDenotation = {
       util.Stats.record("Symbol.computeDenot")
       val now = ctx.period
+      val prev = checkedPeriod
       checkedPeriod = now
-      if (lastd.validFor contains now) lastd else recomputeDenot(lastd)
+      if lastd.validFor.contains(now) then
+        lastd
+      else
+        val newd = recomputeDenot(lastd)
+        if newd.exists then
+          lastDenot = newd
+        else
+          checkedPeriod = prev
+        newd
     }
 
     /** Overridden in NoSymbol */
     protected def recomputeDenot(lastd: SymDenotation)(using Context): SymDenotation = {
       util.Stats.record("Symbol.recomputeDenot")
-      val newd = lastd.current.asInstanceOf[SymDenotation]
-      lastDenot = newd
-      newd
+      lastd.current.asSymDenotation
     }
 
     /** The original denotation of this symbol, without forcing anything */