use const type parameter for Config.withDefault (#2460)

Author: tim-smart

.changeset/thirty-rules-remember.md

@@ -0,0 +1,7 @@
+---
+"effect": patch
+---
+
+use const type parameter for Config.withDefault
+
+Which ensures that the fallback value type is not widened for literals.

packages/effect/src/Config.ts

@@ -437,8 +437,8 @@ export const validate: {
  * @category utils
  */
 export const withDefault: {
-  <A2>(def: A2): <A>(self: Config<A>) => Config<A2 | A>
-  <A, A2>(self: Config<A>, def: A2): Config<A | A2>
+  <const A2>(def: A2): <A>(self: Config<A>) => Config<A2 | A>
+  <A, const A2>(self: Config<A>, def: A2): Config<A | A2>
 } = internal.withDefault
 
 /**

packages/effect/src/internal/config.ts

@@ -582,8 +582,8 @@ export const validate = dual<
 
 /** @internal */
 export const withDefault = dual<
-  <A2>(def: A2) => <A>(self: Config.Config<A>) => Config.Config<A | A2>,
-  <A, A2>(self: Config.Config<A>, def: A2) => Config.Config<A | A2>
+  <const A2>(def: A2) => <A>(self: Config.Config<A>) => Config.Config<A | A2>,
+  <A, const A2>(self: Config.Config<A>, def: A2) => Config.Config<A | A2>
 >(2, (self, def) =>
   orElseIf(self, {
     orElse: () => succeed(def),