fix(transformer): exponentiation transform: do not replace object when private property (#6313)

Fix exponentiation operator transform to bail out early if a private class property.

We can't transform this:

```js
class C {
    #p;
    method() {
        this.#p **= 2;
    }
}
```

But we should at least leave it alone. Previously `get_obj_ref` called `ast.move_expression(expr)` on the member expression's object before bailing out, so example above was transformed to:

```js
class C {
    #p;
    method() {
        null.#p **= 2; // <-- `null`
    }
}
```

This PR makes it spot this case early and bail out *before* calling `ast.move_expression(expr)`.

crates/oxc_transformer/src/es2016/exponentiation_operator.rs

@@ -171,10 +171,10 @@ impl<'a, 'ctx> ExponentiationOperator<'a, 'ctx> {
                 (ident, obj)
             }
             match_member_expression!(AssignmentTarget) => {
-                let obj = self.get_obj_ref(node, nodes, ctx).unwrap();
+                let obj = self.get_obj_ref(node, nodes, ctx)?;
                 let member_expr = node.to_member_expression_mut();
                 let computed = member_expr.is_computed();
-                let prop = self.get_prop_ref(member_expr, nodes, ctx)?;
+                let prop = self.get_prop_ref(member_expr, nodes, ctx);
                 let optional = false;
                 let obj_clone = Self::clone_expression(&obj, ctx);
                 let (reference, uid) = match &prop {
@@ -248,7 +248,11 @@ impl<'a, 'ctx> ExponentiationOperator<'a, 'ctx> {
                 let expr = match node.to_member_expression_mut() {
                     MemberExpression::ComputedMemberExpression(e) => &mut e.object,
                     MemberExpression::StaticMemberExpression(e) => &mut e.object,
-                    MemberExpression::PrivateFieldExpression(e) => &mut e.object,
+                    // From Babel: "We can't generate property ref for private name, please install
+                    // `@babel/plugin-transform-class-properties`".
+                    // TODO: Ensure this plugin interacts correctly with class private properties
+                    // transform, so the property is transformed before this transform.
+                    MemberExpression::PrivateFieldExpression(_) => return None,
                 };
                 let expr = ctx.ast.move_expression(expr);
                 // the object reference that we need to save is locally declared
@@ -277,24 +281,21 @@ impl<'a, 'ctx> ExponentiationOperator<'a, 'ctx> {
         node: &mut MemberExpression<'a>,
         nodes: &mut Vec<'a, Expression<'a>>,
         ctx: &mut TraverseCtx<'a>,
-    ) -> Option<Expression<'a>> {
-        let expr = match node {
+    ) -> Expression<'a> {
+        match node {
             MemberExpression::ComputedMemberExpression(expr) => {
                 let expr = ctx.ast.move_expression(&mut expr.expression);
                 if expr.is_literal() {
-                    return Some(expr);
+                    return expr;
                 }
-                expr
+                self.add_new_reference(expr, nodes, ctx)
             }
             MemberExpression::StaticMemberExpression(expr) => {
-                return Some(ctx.ast.expression_string_literal(SPAN, expr.property.name.clone()));
+                ctx.ast.expression_string_literal(SPAN, expr.property.name.clone())
             }
-            MemberExpression::PrivateFieldExpression(_) => {
-                // From babel: "We can't generate property ref for private name, please install `@babel/plugin-transform-class-properties`"
-                return None;
-            }
-        };
-        Some(self.add_new_reference(expr, nodes, ctx))
+            // This possibility is ruled out in earlier call to `get_obj_ref`
+            MemberExpression::PrivateFieldExpression(_) => unreachable!(),
+        }
     }
 
     fn add_new_reference(

tasks/transform_conformance/snapshots/oxc.snap.md

@@ -1,6 +1,6 @@
 commit: 3bcfee23
 
-Passed: 56/65
+Passed: 57/66
 
 # All Passed:
 * babel-plugin-transform-nullish-coalescing-operator

tasks/transform_conformance/tests/babel-plugin-transform-exponentiation-operator/test/fixtures/bail-private-properties/input.js

@@ -0,0 +1,9 @@
+class C {
+    #p = 1;
+
+    method(obj) {
+        obj.x **= 2; // Transform
+        obj['y'] **= 3; // Transform
+        obj.#p **= 4; // Bail
+    }
+}

tasks/transform_conformance/tests/babel-plugin-transform-exponentiation-operator/test/fixtures/bail-private-properties/output.js

@@ -0,0 +1,9 @@
+class C {
+    #p = 1;
+
+    method(obj) {
+        obj["x"] = Math.pow(obj["x"], 2);
+        obj["y"] = Math.pow(obj["y"], 3);
+        obj.#p **= 4;
+    }
+}