fix(es/compat): Fix handling of private members in optional chaining pass (#7610)

**Related issue:**

 - Closes #7561.

Author: komyg

crates/swc_ecma_transforms_compat/src/es2020/optional_chaining.rs

@@ -1,6 +1,7 @@
 use std::mem;
 
 use serde::Deserialize;
+use swc_atoms::js_word;
 use swc_common::{util::take::Take, DUMMY_SP};
 use swc_ecma_ast::*;
 use swc_ecma_utils::{
@@ -181,7 +182,13 @@ impl OptChaining {
                     next = m.obj.take();
                     m.prop.visit_mut_with(self);
                     chain.push(if optional {
-                        Gathering::OptMember(m.take(), self.memoize(&next))
+                        match *next {
+                            Expr::This(_) => Gathering::OptMember(
+                                m.take(),
+                                Ident::new(js_word!("this"), DUMMY_SP),
+                            ),
+                            _ => Gathering::OptMember(m.take(), self.memoize(&next)),
+                        }
                     } else {
                         Gathering::Member(m.take())
                     });
@@ -299,7 +306,11 @@ impl OptChaining {
                 Gathering::OptMember(mut m, memo) => {
                     committed_cond.push(CondExpr {
                         span: DUMMY_SP,
-                        test: init_and_eq_null_or_undefined(&memo, current, no_document_all),
+                        test: if memo.sym == js_word!("this") {
+                            eq_null_or_undefined(&memo, no_document_all)
+                        } else {
+                            init_and_eq_null_or_undefined(&memo, current, no_document_all)
+                        },
                         cons: if is_delete {
                             true.into()
                         } else {
@@ -409,3 +420,37 @@ fn init_and_eq_null_or_undefined(i: &Ident, init: Expr, no_document_all: bool) -
         right: void_cmp,
     }))
 }
+
+fn eq_null_or_undefined(i: &Ident, no_document_all: bool) -> Box<Expr> {
+    let lhs = Box::new(Expr::Ident(i.clone()));
+
+    if no_document_all {
+        return Box::new(Expr::Bin(BinExpr {
+            span: DUMMY_SP,
+            left: lhs,
+            op: op!("=="),
+            right: Box::new(Expr::Lit(Lit::Null(Null { span: DUMMY_SP }))),
+        }));
+    }
+
+    let null_cmp = Box::new(Expr::Bin(BinExpr {
+        span: DUMMY_SP,
+        left: lhs,
+        op: op!("==="),
+        right: Box::new(Expr::Lit(Lit::Null(Null { span: DUMMY_SP }))),
+    }));
+
+    let void_cmp = Box::new(Expr::Bin(BinExpr {
+        span: DUMMY_SP,
+        left: Box::new(Expr::Ident(i.clone())),
+        op: op!("==="),
+        right: undefined(DUMMY_SP),
+    }));
+
+    Box::new(Expr::Bin(BinExpr {
+        span: DUMMY_SP,
+        left: null_cmp,
+        op: op!("||"),
+        right: void_cmp,
+    }))
+}

crates/swc_ecma_transforms_compat/tests/optional-chaining/issue-7561/input.js

@@ -0,0 +1,7 @@
+class Foo {
+    #x;
+
+    test() {
+        this?.y.#x;
+    }
+}

crates/swc_ecma_transforms_compat/tests/optional-chaining/issue-7561/output.js

@@ -0,0 +1,6 @@
+class Foo {
+    #x;
+    test() {
+        this === null || this === void 0 ? void 0 : this.y.#x;
+    }
+}