diff --git a/tests/ui/type-alias-impl-trait/coherence_generalization.rs b/tests/ui/type-alias-impl-trait/coherence_generalization.rs
index 5c9ad9498b6..679b2b0f188 100644
--- a/tests/ui/type-alias-impl-trait/coherence_generalization.rs
+++ b/tests/ui/type-alias-impl-trait/coherence_generalization.rs
@@ -1,5 +1,7 @@
 // check-pass
 
+// FIXME(type_alias_impl_trait): What does this test? This needs a comment
+// explaining what we're worried about here.
 #![feature(type_alias_impl_trait)]
 trait Trait {}
 type Opaque<T> = impl Sized;