Reuse the readOnlyError helper for private methods

packages/babel-helper-create-class-features-plugin/src/fields.js

@@ -294,7 +294,9 @@ const privateNameHandlerSpec = {
           value,
         ]);
       }
-      return t.callExpression(file.addHelper("classPrivateMethodSet"), []);
+      return t.callExpression(file.addHelper("readOnlyError"), [
+        t.stringLiteral(`#${name}`),
+      ]);
     }
     return t.callExpression(file.addHelper("classPrivateFieldSet"), [
       this.receiver(member),

packages/babel-helpers/src/helpers.js

@@ -2092,11 +2092,14 @@ helpers.classPrivateMethodGet = helper("7.1.6")`
   }
 `;
 
-helpers.classPrivateMethodSet = helper("7.1.6")`
-  export default function _classPrivateMethodSet() {
-    throw new TypeError("attempted to reassign private method");
-  }
-`;
+if (!process.env.BABEL_8_BREAKING) {
+  // Use readOnlyError instead
+  helpers.classPrivateMethodSet = helper("7.1.6")`
+    export default function _classPrivateMethodSet() {
+      throw new TypeError("attempted to reassign private method");
+    }
+  `;
+}
 
 helpers.wrapRegExp = helper("7.2.6")`
   import wrapNativeSuper from "wrapNativeSuper";

packages/babel-plugin-proposal-private-methods/test/fixtures/accessors/set-only-getter/output.js

@@ -14,7 +14,7 @@ class Cl {
       value: 0
     });
 
-    babelHelpers.classPrivateMethodSet();
+    babelHelpers.readOnlyError("#privateFieldValue");
   }
 
 }

packages/babel-plugin-proposal-private-methods/test/fixtures/private-method/read-only/exec.js

@@ -0,0 +1,9 @@
+class A {
+  #method() {}
+
+  run() {
+    this.#method = 2;
+  }
+}
+
+expect(() => new A().run()).toThrow(TypeError);

packages/babel-plugin-proposal-private-methods/test/fixtures/private-method/read-only/input.js

@@ -0,0 +1,7 @@
+class A {
+  #method() {}
+
+  run() {
+    this.#method = 2;
+  }
+}

packages/babel-plugin-proposal-private-methods/test/fixtures/private-method/read-only/output.js

@@ -0,0 +1,14 @@
+var _method = new WeakSet();
+
+class A {
+  constructor() {
+    _method.add(this);
+  }
+
+  run() {
+    babelHelpers.readOnlyError("#method");
+  }
+
+}
+
+var _method2 = function _method2() {};