feat: add unsupported private member errors and fixture e2e tests

Add LWC1214 error for unsupported private member types (fields and
accessor methods). Update the forward transform to throw informative
errors instead of silently passing through. Replace affected unit tests
with error-expectation tests. Add fixture-based end-to-end tests that
run the full pipeline (forward transform, LWC plugin, class-properties
plugin, reverse transform) to verify private methods survive round-trip.

Made-with: Cursor

Author: a-chabot

packages/@lwc/babel-plugin-component/src/__tests__/fixtures/private-methods/multiple-private-methods/actual.js

@@ -0,0 +1,18 @@
+import { LightningElement } from 'lwc';
+export default class Test extends LightningElement {
+    #validate(input) {
+        return input != null;
+    }
+    #transform(input) {
+        return String(input).trim();
+    }
+    #process(input) {
+        if (this.#validate(input)) {
+            return this.#transform(input);
+        }
+        return null;
+    }
+    connectedCallback() {
+        this.#process('hello');
+    }
+}

packages/@lwc/babel-plugin-component/src/__tests__/fixtures/private-methods/multiple-private-methods/error.json

@@ -0,0 +1,26 @@
+import _tmpl from "./test.html";
+import { LightningElement, registerComponent as _registerComponent } from 'lwc';
+class Test extends LightningElement {
+  #validate(input) {
+    return input != null;
+  }
+  #transform(input) {
+    return String(input).trim();
+  }
+  #process(input) {
+    if (this.#validate(input)) {
+      return this.#transform(input);
+    }
+    return null;
+  }
+  connectedCallback() {
+    this.#process('hello');
+  }
+  /*LWC compiler vX.X.X*/
+}
+const __lwc_component_class_internal = _registerComponent(Test, {
+  tmpl: _tmpl,
+  sel: "lwc-test",
+  apiVersion: 9999999
+});
+export default __lwc_component_class_internal;

packages/@lwc/babel-plugin-component/src/__tests__/fixtures/private-methods/multiple-private-methods/expected.js

@@ -0,0 +1,13 @@
+import { LightningElement, api } from 'lwc';
+export default class Test extends LightningElement {
+    @api label = 'default';
+    count = 0;
+
+    #increment() {
+        this.count++;
+    }
+
+    handleClick() {
+        this.#increment();
+    }
+}

packages/@lwc/babel-plugin-component/src/__tests__/fixtures/private-methods/private-method-with-public-members/actual.js

@@ -0,0 +1,31 @@
+import { registerDecorators as _registerDecorators } from "lwc";
+import _tmpl from "./test.html";
+import { LightningElement, registerComponent as _registerComponent } from 'lwc';
+class Test extends LightningElement {
+  constructor(...args) {
+    super(...args);
+    this.label = 'default';
+    this.count = 0;
+  }
+  #increment() {
+    this.count++;
+  }
+  handleClick() {
+    this.#increment();
+  }
+  /*LWC compiler vX.X.X*/
+}
+_registerDecorators(Test, {
+  publicProps: {
+    label: {
+      config: 0
+    }
+  },
+  fields: ["count"]
+});
+const __lwc_component_class_internal = _registerComponent(Test, {
+  tmpl: _tmpl,
+  sel: "lwc-test",
+  apiVersion: 9999999
+});
+export default __lwc_component_class_internal;

packages/@lwc/babel-plugin-component/src/__tests__/fixtures/private-methods/private-method-with-public-members/error.json

@@ -0,0 +1,10 @@
+import { LightningElement } from 'lwc';
+export default class Test extends LightningElement {
+    #privateHelper() {
+        return 42;
+    }
+    connectedCallback() {
+        const val = this.#privateHelper();
+        console.log(val);
+    }
+}

packages/@lwc/babel-plugin-component/src/__tests__/fixtures/private-methods/private-method-with-public-members/expected.js

@@ -0,0 +1,18 @@
+import _tmpl from "./test.html";
+import { LightningElement, registerComponent as _registerComponent } from 'lwc';
+class Test extends LightningElement {
+  #privateHelper() {
+    return 42;
+  }
+  connectedCallback() {
+    const val = this.#privateHelper();
+    console.log(val);
+  }
+  /*LWC compiler vX.X.X*/
+}
+const __lwc_component_class_internal = _registerComponent(Test, {
+  tmpl: _tmpl,
+  sel: "lwc-test",
+  apiVersion: 9999999
+});
+export default __lwc_component_class_internal;

packages/@lwc/babel-plugin-component/src/__tests__/fixtures/private-methods/simple-private-method/actual.js

@@ -0,0 +1,11 @@
+import { LightningElement } from 'lwc';
+export default class Test extends LightningElement {
+    static async #fetchData(url) {
+        const response = await fetch(url);
+        return response.json();
+    }
+    async connectedCallback() {
+        const data = await Test.#fetchData('/api/data');
+        console.log(data);
+    }
+}