[scanner] Combine surrogate pairs at start when scanning private names

Bug: v8:12523
Change-Id: Ic3779fe6f20965d177d99d0a570a735df72e4fde
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3366994Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Shu-yu Guo <syg@chromium.org>
Cr-Commit-Position: refs/heads/main@{#78493}

src/parsing/scanner.cc

@@ -497,15 +497,17 @@ Token::Value Scanner::ScanPrivateName() {
   next().literal_chars.Start();
   DCHECK_EQ(c0_, '#');
   DCHECK(!IsIdentifierStart(kEndOfInput));
-  if (!IsIdentifierStart(Peek())) {
-    ReportScannerError(source_pos(),
-                       MessageTemplate::kInvalidOrUnexpectedToken);
-    return Token::ILLEGAL;
+  Advance();
+  if (IsIdentifierStart(c0_) ||
+      (CombineSurrogatePair() && IsIdentifierStart(c0_))) {
+    AddLiteralChar('#');
+    Token::Value token = ScanIdentifierOrKeywordInner();
+    return token == Token::ILLEGAL ? Token::ILLEGAL : Token::PRIVATE_NAME;
   }
 
-  AddLiteralCharAdvance();
-  Token::Value token = ScanIdentifierOrKeywordInner();
-  return token == Token::ILLEGAL ? Token::ILLEGAL : Token::PRIVATE_NAME;
+  PushBack('#');  // Undo Advance()
+  ReportScannerError(source_pos(), MessageTemplate::kInvalidOrUnexpectedToken);
+  return Token::ILLEGAL;
 }
 
 Token::Value Scanner::ScanTemplateSpan() {

test/mjsunit/harmony/private-name-surrogate-pair.js

+// Copyright 2022 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+class C1 {
+  #𖥸 = 42;
+  m() { return this.#𖥸; }
+}
+
+assertEquals((new C1).m(), 42);
+
+class C2 {
+  #𖥸() { return 42; }
+  m() { return this.#𖥸(); }
+}
+
+assertEquals((new C2).m(), 42);