Use the suggested alternative fix

Author: Andarist

src/compiler/checker.ts

@@ -16270,7 +16270,7 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
     }
 
     // This function assumes the constituent type list is sorted and deduplicated.
-    function getUnionTypeFromSortedList(types: Type[], objectFlags: ObjectFlags, aliasSymbol?: Symbol, aliasTypeArguments?: readonly Type[], origin?: Type): Type {
+    function getUnionTypeFromSortedList(types: Type[], precomputedObjectFlags: ObjectFlags, aliasSymbol?: Symbol, aliasTypeArguments?: readonly Type[], origin?: Type): Type {
         if (types.length === 0) {
             return neverType;
         }
@@ -16285,7 +16285,7 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
         let type = unionTypes.get(id);
         if (!type) {
             type = createType(TypeFlags.Union) as UnionType;
-            type.objectFlags = (objectFlags | getPropagatingFlagsOfTypes(types, /*excludeKinds*/ TypeFlags.Nullable)) & ~(ObjectFlags.IsUnknownLikeUnionComputed | ObjectFlags.IsUnknownLikeUnion);
+            type.objectFlags = precomputedObjectFlags | getPropagatingFlagsOfTypes(types, /*excludeKinds*/ TypeFlags.Nullable);
             type.types = types;
             type.origin = origin;
             type.aliasSymbol = aliasSymbol;
@@ -25649,7 +25649,9 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
                     newOrigin = createOriginUnionOrIntersectionType(TypeFlags.Union, originFiltered);
                 }
             }
-            return getUnionTypeFromSortedList(filtered, (type as UnionType).objectFlags, /*aliasSymbol*/ undefined, /*aliasTypeArguments*/ undefined, newOrigin);
+            // filtering could remove intersections so `ContainsIntersections` might be forwarded "incorrectly"
+            // it is purely an optimization hint so there is no harm in accidentally forwarding it
+            return getUnionTypeFromSortedList(filtered, (type as UnionType).objectFlags & (ObjectFlags.PrimitiveUnion | ObjectFlags.ContainsIntersections), /*aliasSymbol*/ undefined, /*aliasTypeArguments*/ undefined, newOrigin);
         }
         return type.flags & TypeFlags.Never || f(type) ? type : neverType;
     }