Allow data classes to be branded with an additional type parameter

.changeset/witty-balloons-shout.md

@@ -0,0 +1,26 @@
+---
+"effect": minor
+---
+
+Allow data classes to be branded with an additional type parameter
+
+```ts
+import { Data, HashMap } from "effect"
+
+export class T extends Data.Class<
+  { id: string },
+  { readonly _brand: unique symbol }
+> {}
+
+// type error
+export const hashMap1: HashMap.HashMap<T, string> = HashMap.empty().pipe(
+  HashMap.set({ id: "one" }, "one"),
+  HashMap.set({ id: "one" }, "two")
+)
+
+// no type error
+export const hashMap2: HashMap.HashMap<T, string> = HashMap.empty().pipe(
+  HashMap.set(new T({ id: "one" }), "one"),
+  HashMap.set(new T({ id: "one" }), "two")
+)
+```

packages/effect/src/Data.ts

@@ -183,10 +183,14 @@ export const tagged = <A extends { readonly _tag: string }>(
  * @since 2.0.0
  * @category constructors
  */
-export const Class: new<A extends Record<string, any> = {}>(
-  args: Types.Equals<A, {}> extends true ? void
-    : { readonly [P in keyof A]: A[P] }
-) => Readonly<A> = internal.Structural as any
+export const Class: {
+  new<A extends Record<string, any> = {}>(
+    args: Types.Equals<A, {}> extends true ? void : { readonly [P in keyof A]: A[P] }
+  ): Readonly<A>
+  new<A extends Record<string, any> = {}, Brand = {}>(
+    args: Types.Equals<A, {}> extends true ? void : { readonly [P in keyof A]: A[P] }
+  ): Readonly<A> & Brand
+} = internal.Structural as any
 
 /**
  * Provides a Tagged constructor for a Case Class.