src/TraitExpression.ts

@@ -63,10 +63,12 @@ export class TraitExpression<
 export const emptyTraitExpression = new TraitExpression(TraitExpressionNullSuperclass, [] as const)
 
 interface PrependTraitSupertraitsFn extends Fn {
-    return: [
+    return: this["arg0"] extends Trait<any, any, any>
+        ? [
             ...Call<TraitSupertraitsFn, this["arg0"]>,
             this["arg0"],
         ]
+        : never
 }
 export interface TraitExpressionTraitsFn extends Fn {
     return: this["arg0"] extends TraitExpression<any, infer Traits>

src/tests.ts

@@ -1,4 +1,5 @@
-import { Implements, abstract, expression, trait } from "."
+import { Call } from "hotscript"
+import { Implements, TraitExpressionTraitsFn, abstract, expression, trait } from "."
 
 
 const PrintsHelloOnNew = trait(
@@ -54,10 +55,11 @@ const builder = expression
         PrintsHelloOnNew,
         Identifiable<bigint>(),
         // Identifiable<number>(),
+        StatefulSubscription,
     )
 
 const exp = builder.get()
-type Abs = Implements<typeof exp>
+type Abs = Call<TraitExpressionTraitsFn, typeof exp>
 
 class User extends exp.extends implements Implements<typeof exp> {
     id: bigint = -1n