optimize the cachability check

compiler/src/dotty/tools/dotc/cc/CapturingType.scala

@@ -48,6 +48,16 @@ object CapturingType:
       EventuallyCapturingType.unapply(tp)
     else None
 
+  /** Check whether a type is uncachable when computing `baseType`.
+    * - Avoid caching all the types during the setup phase, since at that point
+    *   the capture set variables are not fully installed yet.
+    * - Avoid caching capturing types when IgnoreCaptures mode is set, since the
+    *   capture sets may be thrown away in the computed base type.
+    */
+  def isUncachable(tp: Type)(using Context): Boolean =
+    ctx.phase == Phases.checkCapturesPhase &&
+      (Setup.isDuringSetup || ctx.mode.is(Mode.IgnoreCaptures) && tp.isEventuallyCapturingType)
+
 end CapturingType
 
 /** An extractor for types that will be capturing types at phase CheckCaptures. Also

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

@@ -2140,15 +2140,14 @@ object SymDenotations {
     final def baseTypeOf(tp: Type)(using Context): Type = {
       val btrCache = baseTypeCache
       def inCache(tp: Type) = tp match
-        case EventuallyCapturingType(_, _) => false
         case tp: CachedType => btrCache.contains(tp)
         case _ => false
       def record(tp: CachedType, baseTp: Type) = {
         if (Stats.monitored) {
           Stats.record("basetype cache entries")
           if (!baseTp.exists) Stats.record("basetype cache NoTypes")
         }
-        if (!tp.isProvisional && !tp.isEventuallyCapturingType && !Setup.isDuringSetup)
+        if (!tp.isProvisional && !CapturingType.isUncachable(tp))
           btrCache(tp) = baseTp
         else
           btrCache.remove(tp) // Remove any potential sentinel value