Invalidate base type caches only if runId changes

Phase-ids should not change base type caches. Instead we make sure
that class denotations that contain base type caches are copied
whenever base types might change.

Author: odersky

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

@@ -50,7 +50,7 @@ object DenotTransformers {
       }
     }
 
-    /** Denotations with a symbol where `mayChange` is false are guaranteed to be
+    /** Denotations with a symbol where `infoMayChange` is false are guaranteed to be
      *  unaffected by this transform, so `transformInfo` need not be run. This
      *  can save time, and more importantly, can help avoid forcing symbol completers.
      */

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

@@ -2641,7 +2641,11 @@ object SymDenotations {
     private var locked = false
     private var provisional = false
 
-    final def isValid(using Context): Boolean = valid && isValidAt(ctx.phase)
+    final def isValid(using Context): Boolean =
+      valid && createdAt.runId == ctx.runId
+        // Note: We rely on the fact that whenever base types of classes change,
+        // the affected classes will get new denotations with new basedata caches.
+        // So basedata caches can become invalid only if the run changes.
 
     def invalidate(): Unit =
       if (valid && !locked) {