diff --git a/compiler/rustc_middle/src/ty/mod.rs b/compiler/rustc_middle/src/ty/mod.rs
index 56a7abfac6635..89a8a45b81914 100644
--- a/compiler/rustc_middle/src/ty/mod.rs
+++ b/compiler/rustc_middle/src/ty/mod.rs
@@ -338,7 +338,9 @@ impl TyCtxt<'_> {
     }
 
     pub fn is_descendant_of(self, mut descendant: DefId, ancestor: DefId) -> bool {
-        if descendant.krate != ancestor.krate {
+        // Def-ids from different crates are always unordered, and def-ids of parent nodes
+        // are always created before def-ids of child nodes.
+        if descendant.krate != ancestor.krate || descendant.index < ancestor.index {
             return false;
         }