src/TraitExpressionBuilder.ts

@@ -76,7 +76,10 @@ export class TraitExpressionBuilder<
         >[]
     >(
         ...traits: T
-    ) {
+    ): TraitExpressionBuilder<
+        Superclass,
+        TraitExpressionBuilder.ExpressesReturnTypeTraits<Traits, T>
+    > {
         return new this.constructor(
             this.expressionSuperclass,
 
@@ -99,7 +102,10 @@ export class TraitExpressionBuilder<
         >[]
     >(
         ...traits: T
-    ) {
+    ): TraitExpressionBuilder<
+        Superclass,
+        TraitExpressionBuilder.ExpressesFirstReturnTypeTraits<Traits, T>
+    > {
         return new this.constructor(
             this.expressionSuperclass,
 
@@ -166,6 +172,26 @@ export namespace TraitExpressionBuilder {
             : false
     )
 
+    export type ExpressesReturnTypeTraits<
+        Traits extends readonly Trait<any, any, any, any>[],
+        T      extends readonly Trait<any, any, any, any>[],
+    > = (
+        TraitExpressionBuilder.TraitsUniq<[
+            ...Traits,
+            ...TraitExpressionBuilder.SpreadSupertraits<T>,
+        ]>
+    )
+
+    export type ExpressesFirstReturnTypeTraits<
+        Traits extends readonly Trait<any, any, any, any>[],
+        T      extends readonly Trait<any, any, any, any>[],
+    > = (
+        TraitExpressionBuilder.TraitsUniq<[
+            ...TraitExpressionBuilder.SpreadSupertraits<T>,
+            ...Traits,
+        ]>
+    )
+
     export type BuildTraitExpression<
         Superclass extends AbstractClass<object>,
         Traits     extends Trait<any, any, any, any>[],