fix(set): coerce set elements without duplicating originals

src/structs/types.ts

@@ -418,7 +418,7 @@ export function set<T>(Element?: Struct<T>): any {
     schema: null,
     *entries(value) {
       if (Element && value instanceof Set) {
-        for (const v of value) {
+        for (const v of [...value]) {
           yield [v as string, v, Element]
         }
       }

src/utils.ts

@@ -153,6 +153,11 @@ export function* run<T, S>(
     yield [failure, undefined]
   }
 
+  // `Set.add()` can only insert members, never replace one positionally, so a
+  // coerced element would accumulate alongside its original. Clear the set once
+  // on the first coerced write-back so only coerced members remain.
+  let clearedSet = false
+
   for (let [k, v, s] of struct.entries(value, ctx)) {
     const ts = run(v, s as Struct, {
       path: k === undefined ? path : [...path, k],
@@ -174,6 +179,10 @@ export function* run<T, S>(
         } else if (value instanceof Map) {
           value.set(k, v)
         } else if (value instanceof Set) {
+          if (!clearedSet) {
+            value.clear()
+            clearedSet = true
+          }
           value.add(v)
         } else if (isObject(value)) {
           if (v !== undefined || k in value) value[k] = v

test/validation/set/coerce-element.ts

@@ -0,0 +1,11 @@
+import { coerce, number, set, string } from '../../../src'
+
+const toNumber = coerce(number(), string(), (v) => parseFloat(v))
+
+export const Struct = set(toNumber)
+
+export const data = new Set(['1', '2'])
+
+export const output = new Set([1, 2])
+
+export const create = true