diff --git a/crates/fayalite/src/ty.rs b/crates/fayalite/src/ty.rs
index 69080c9..55c7e8f 100644
--- a/crates/fayalite/src/ty.rs
+++ b/crates/fayalite/src/ty.rs
@@ -11,6 +11,7 @@ use crate::{
     intern::{Intern, Interned},
     reset::{AsyncReset, Reset, SyncReset},
     source_location::SourceLocation,
+    util::ConstUsize,
 };
 use std::{fmt, hash::Hash, iter::FusedIterator, ops::Index};
 
@@ -166,7 +167,7 @@ impl<T: 'static + Send + Sync> MatchVariantAndInactiveScope for MatchVariantWith
 }
 
 pub trait FillInDefaultedGenerics {
-    type Type: Type;
+    type Type;
     fn fill_in_defaulted_generics(self) -> Self::Type;
 }
 
@@ -178,6 +179,22 @@ impl<T: Type> FillInDefaultedGenerics for T {
     }
 }
 
+impl FillInDefaultedGenerics for usize {
+    type Type = usize;
+
+    fn fill_in_defaulted_generics(self) -> Self::Type {
+        self
+    }
+}
+
+impl<const V: usize> FillInDefaultedGenerics for ConstUsize<V> {
+    type Type = ConstUsize<V>;
+
+    fn fill_in_defaulted_generics(self) -> Self::Type {
+        self
+    }
+}
+
 mod sealed {
     pub trait TypeOrDefaultSealed {}
     pub trait BaseTypeSealed {}