Disallow shadowing.

src/structy/export/BaseStruct.js

@@ -147,7 +147,9 @@ export class BaseStruct {
       const disallowed = [];
 
       for (const name of leftovers) {
-        if (this._impl_allowExtraProperty(name)) {
+        if (Reflect.has(this, name)) {
+          throw new Error(`Extra property would shadow pre-existing property: ${name}`);
+        } else if (this._impl_allowExtraProperty(name)) {
           const value = this._impl_extraProperty(name, rawObject[name]);
           if (value === undefined) {
             throw new Error(`Extra property checker \`_impl_extraProperty()\` did not return a value. Maybe missing a \`return\`?`);

src/structy/tests/BaseStruct.test.js

@@ -330,6 +330,10 @@ describe('using a subclass which allows extra properties', () => {
   class SomeStruct extends BaseStruct {
     // @defaultConstructor
 
+    get yesNo() {
+      return 'maybe';
+    }
+
     _impl_allowExtraProperty(name) {
       return name.startsWith('yes');
     }
@@ -370,5 +374,10 @@ describe('using a subclass which allows extra properties', () => {
       const arg = { yesUndefined: 123 };
       expect(() => new SomeStruct(arg)).toThrow(/did not return/);
     });
+
+    test('throws given an extra property which would shadow a pre-existing property', () => {
+      const arg = { yesNo: 12345 };
+      expect(() => new SomeStruct(arg)).toThrow(/would shadow/);
+    });
   });
 });